Binder.ValueChecks.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

#nullable disable

using System;
using System.Collections.Generic;
using System.Collections.Immutable;
using System.Diagnostics;
using Microsoft.CodeAnalysis.CSharp.CodeGen;
using Microsoft.CodeAnalysis.CSharp.Symbols;
using Microsoft.CodeAnalysis.CSharp.Syntax;
using Microsoft.CodeAnalysis.PooledObjects;
using Microsoft.CodeAnalysis.Text;
using Roslyn.Utilities;

namespace Microsoft.CodeAnalysis.CSharp
{
    internal partial class Binder
    {
        /// <summary>
        /// For the purpose of escape verification we operate with the depth of local scopes.
        /// The depth is a uint, with smaller number representing shallower/wider scopes.
        /// The 0 and 1 are special scopes - 
        /// 0 is the "external" or "return" scope that is outside of the containing method/lambda. 
        ///   If something can escape to scope 0, it can escape to any scope in a given method or can be returned.
        /// 1 is the "parameter" or "top" scope that is just inside the containing method/lambda. 
        ///   If something can escape to scope 1, it can escape to any scope in a given method, but cannot be returned.
        /// n + 1 corresponds to scopes immediately inside a scope of depth n. 
        ///   Since sibling scopes do not intersect and a value cannot escape from one to another without 
        ///   escaping to a wider scope, we can use simple depth numbering without ambiguity.
        /// </summary>
        internal const uint ExternalScope = 0;
        internal const uint TopLevelScope = 1;

        // Some value kinds are semantically the same and the only distinction is how errors are reported
        // for those purposes we reserve lowest 2 bits
        private const int ValueKindInsignificantBits = 2;
        private const BindValueKind ValueKindSignificantBitsMask = unchecked((BindValueKind)~((1 << ValueKindInsignificantBits) - 1));

        /// <summary>
        /// Expression capabilities and requirements.
        /// </summary>
        [Flags]
        internal enum BindValueKind : ushort
        {
            ///////////////////
            // All expressions can be classified according to the following 4 capabilities:
            //

            /// <summary>
            /// Expression can be an RHS of an assignment operation.
            /// </summary>
            /// <remarks>
            /// The following are rvalues: values, variables, null literals, properties
            /// and indexers with getters, events. 
            /// 
            /// The following are not rvalues:
            /// namespaces, types, method groups, anonymous functions.
            /// </remarks>
            RValue = 1 << ValueKindInsignificantBits,

            /// <summary>
            /// Expression can be the LHS of a simple assignment operation.
            /// Example: 
            ///   property with a setter
            /// </summary>
            Assignable = 2 << ValueKindInsignificantBits,

            /// <summary>
            /// Expression represents a location. Often referred as a "variable"
            /// Examples:
            ///  local variable, parameter, field
            /// </summary>
            RefersToLocation = 4 << ValueKindInsignificantBits,

            /// <summary>
            /// Expression can be the LHS of a ref-assign operation.
            /// Example:
            ///  ref local, ref parameter, out parameter
            /// </summary>
            RefAssignable = 8 << ValueKindInsignificantBits,

            ///////////////////
            // The rest are just combinations of the above.
            //

            /// <summary>
            /// Expression is the RHS of an assignment operation
            /// and may be a method group.
            /// Basically an RValue, but could be treated differently for the purpose of error reporting
            /// </summary>
            RValueOrMethodGroup = RValue + 1,

            /// <summary>
            /// Expression can be an LHS of a compound assignment
            /// operation (such as +=).
            /// </summary>
            CompoundAssignment = RValue | Assignable,

            /// <summary>
            /// Expression can be the operand of an increment or decrement operation.
            /// Same as CompoundAssignment, the distinction is really just for error reporting.
            /// </summary>
            IncrementDecrement = CompoundAssignment + 1,

            /// <summary>
            /// Expression is a r/o reference.
            /// </summary>
            ReadonlyRef = RefersToLocation | RValue,

            /// <summary>
            /// Expression can be the operand of an address-of operation (&amp;).
            /// Same as ReadonlyRef. The difference is just for error reporting.
            /// </summary>
            AddressOf = ReadonlyRef + 1,

            /// <summary>
            /// Expression is the receiver of a fixed buffer field access
            /// Same as ReadonlyRef. The difference is just for error reporting.
            /// </summary>
            FixedReceiver = ReadonlyRef + 2,

            /// <summary>
            /// Expression is passed as a ref or out parameter or assigned to a byref variable.
            /// </summary>
            RefOrOut = RefersToLocation | RValue | Assignable,

            /// <summary>
            /// Expression is returned by an ordinary r/w reference.
            /// Same as RefOrOut. The difference is just for error reporting.
            /// </summary>
            RefReturn = RefOrOut + 1,
        }

        private static bool RequiresRValueOnly(BindValueKind kind)
        {
            return (kind & ValueKindSignificantBitsMask) == BindValueKind.RValue;
        }

        private static bool RequiresAssignmentOnly(BindValueKind kind)
        {
            return (kind & ValueKindSignificantBitsMask) == BindValueKind.Assignable;
        }

        private static bool RequiresVariable(BindValueKind kind)
        {
            return !RequiresRValueOnly(kind);
        }

        private static bool RequiresReferenceToLocation(BindValueKind kind)
        {
            return (kind & BindValueKind.RefersToLocation) != 0;
        }

        private static bool RequiresAssignableVariable(BindValueKind kind)
        {
            return (kind & BindValueKind.Assignable) != 0;
        }

        private static bool RequiresRefAssignableVariable(BindValueKind kind)
        {
            return (kind & BindValueKind.RefAssignable) != 0;
        }

        private static bool RequiresRefOrOut(BindValueKind kind)
        {
            return (kind & BindValueKind.RefOrOut) == BindValueKind.RefOrOut;
        }

#nullable enable

        private BoundIndexerAccess BindIndexerDefaultArguments(BoundIndexerAccess indexerAccess, BindValueKind valueKind, DiagnosticBag diagnostics)
        {
            var useSetAccessor = valueKind == BindValueKind.Assignable && !indexerAccess.Indexer.ReturnsByRef;
            var accessorForDefaultArguments = useSetAccessor
                ? indexerAccess.Indexer.GetOwnOrInheritedSetMethod()
                : indexerAccess.Indexer.GetOwnOrInheritedGetMethod();
            if (accessorForDefaultArguments is not null)
            {
                var argumentsBuilder = ArrayBuilder<BoundExpression>.GetInstance(accessorForDefaultArguments.ParameterCount);
                argumentsBuilder.AddRange(indexerAccess.Arguments);

                ArrayBuilder<RefKind>? refKindsBuilderOpt;
                if (!indexerAccess.ArgumentRefKindsOpt.IsDefaultOrEmpty)
                {
                    refKindsBuilderOpt = ArrayBuilder<RefKind>.GetInstance(accessorForDefaultArguments.ParameterCount);
                    refKindsBuilderOpt.AddRange(indexerAccess.ArgumentRefKindsOpt);
                }
                else
                {
                    refKindsBuilderOpt = null;
                }
                var argsToParams = indexerAccess.ArgsToParamsOpt;

                // It is possible for the indexer 'value' parameter from metadata to have a default value, but the compiler will not use it.
                // However, we may still use any default values from the preceding parameters.
                var parameters = accessorForDefaultArguments.Parameters;
                if (useSetAccessor)
                {
                    parameters = parameters.RemoveAt(parameters.Length - 1);
                }

                BitVector defaultArguments = default;
                Debug.Assert(parameters.Length == indexerAccess.Indexer.Parameters.Length);

                // If OriginalIndexersOpt is set, there was an overload resolution failure, and we don't want to make guesses about the default
                // arguments that will end up being reflected in the SemanticModel/IOperation
                if (indexerAccess.OriginalIndexersOpt.IsDefault)
                {
                    BindDefaultArguments(indexerAccess.Syntax, parameters, argumentsBuilder, refKindsBuilderOpt, ref argsToParams, out defaultArguments, indexerAccess.Expanded, enableCallerInfo: true, diagnostics);
                }

                indexerAccess = indexerAccess.Update(
                    indexerAccess.ReceiverOpt,
                    indexerAccess.Indexer,
                    argumentsBuilder.ToImmutableAndFree(),
                    indexerAccess.ArgumentNamesOpt,
                    refKindsBuilderOpt?.ToImmutableOrNull() ?? default,
                    indexerAccess.Expanded,
                    argsToParams,
                    defaultArguments,
                    indexerAccess.Type);

                refKindsBuilderOpt?.Free();
            }

            return indexerAccess;
        }

#nullable disable

        /// <summary>
        /// Check the expression is of the required lvalue and rvalue specified by valueKind.
        /// The method returns the original expression if the expression is of the required
        /// type. Otherwise, an appropriate error is added to the diagnostics bag and the
        /// method returns a BoundBadExpression node. The method returns the original
        /// expression without generating any error if the expression has errors.
        /// </summary>
        private BoundExpression CheckValue(BoundExpression expr, BindValueKind valueKind, DiagnosticBag diagnostics)
        {
            switch (expr.Kind)
            {
                case BoundKind.PropertyGroup:
                    expr = BindIndexedPropertyAccess((BoundPropertyGroup)expr, mustHaveAllOptionalParameters: false, diagnostics: diagnostics);
                    if (expr is BoundIndexerAccess indexerAccess)
                    {
                        expr = BindIndexerDefaultArguments(indexerAccess, valueKind, diagnostics);
                    }
                    break;

                case BoundKind.Local:
                    Debug.Assert(expr.Syntax.Kind() != SyntaxKind.Argument || valueKind == BindValueKind.RefOrOut);
                    break;

                case BoundKind.OutVariablePendingInference:
                case BoundKind.OutDeconstructVarPendingInference:
                    Debug.Assert(valueKind == BindValueKind.RefOrOut);
                    return expr;

                case BoundKind.DiscardExpression:
                    Debug.Assert(valueKind == BindValueKind.Assignable || valueKind == BindValueKind.RefOrOut || diagnostics.HasAnyResolvedErrors());
                    return expr;

                case BoundKind.IndexerAccess:
                    expr = BindIndexerDefaultArguments((BoundIndexerAccess)expr, valueKind, diagnostics);
                    break;

                case BoundKind.UnconvertedObjectCreationExpression:
                    if (valueKind == BindValueKind.RValue)
                    {
                        return expr;
                    }
                    break;
            }

            bool hasResolutionErrors = false;

            // If this a MethodGroup where an rvalue is not expected or where the caller will not explicitly handle
            // (and resolve) MethodGroups (in short, cases where valueKind != BindValueKind.RValueOrMethodGroup),
            // resolve the MethodGroup here to generate the appropriate errors, otherwise resolution errors (such as
            // "member is inaccessible") will be dropped.
            if (expr.Kind == BoundKind.MethodGroup && valueKind != BindValueKind.RValueOrMethodGroup)
            {
                var methodGroup = (BoundMethodGroup)expr;
                HashSet<DiagnosticInfo> useSiteDiagnostics = null;
                var resolution = this.ResolveMethodGroup(methodGroup, analyzedArguments: null, isMethodGroupConversion: false, useSiteDiagnostics: ref useSiteDiagnostics);
                diagnostics.Add(expr.Syntax, useSiteDiagnostics);
                Symbol otherSymbol = null;
                bool resolvedToMethodGroup = resolution.MethodGroup != null;
                if (!expr.HasAnyErrors) diagnostics.AddRange(resolution.Diagnostics); // Suppress cascading.
                hasResolutionErrors = resolution.HasAnyErrors;
                if (hasResolutionErrors)
                {
                    otherSymbol = resolution.OtherSymbol;
                }
                resolution.Free();

                // It's possible the method group is not a method group at all, but simply a
                // delayed lookup that resolved to a non-method member (perhaps an inaccessible
                // field or property), or nothing at all. In those cases, the member should not be exposed as a
                // method group, not even within a BoundBadExpression. Instead, the
                // BoundBadExpression simply refers to the receiver and the resolved symbol (if any).
                if (!resolvedToMethodGroup)
                {
                    Debug.Assert(methodGroup.ResultKind != LookupResultKind.Viable);
                    var receiver = methodGroup.ReceiverOpt;
                    if ((object)otherSymbol != null && receiver?.Kind == BoundKind.TypeOrValueExpression)
                    {
                        // Since we're not accessing a method, this can't be a Color Color case, so TypeOrValueExpression should not have been used.
                        // CAVEAT: otherSymbol could be invalid in some way (e.g. inaccessible), in which case we would have fallen back on a
                        // method group lookup (to allow for extension methods), which would have required a TypeOrValueExpression.
                        Debug.Assert(methodGroup.LookupError != null);

                        // Since we have a concrete member in hand, we can resolve the receiver.
                        var typeOrValue = (BoundTypeOrValueExpression)receiver;
                        receiver = otherSymbol.RequiresInstanceReceiver()
                            ? typeOrValue.Data.ValueExpression
                            : null; // no receiver required
                    }
                    return new BoundBadExpression(
                        expr.Syntax,
                        methodGroup.ResultKind,
                        (object)otherSymbol == null ? ImmutableArray<Symbol>.Empty : ImmutableArray.Create(otherSymbol),
                        receiver == null ? ImmutableArray<BoundExpression>.Empty : ImmutableArray.Create(receiver),
                        GetNonMethodMemberType(otherSymbol));
                }
            }

            if (!hasResolutionErrors && CheckValueKind(expr.Syntax, expr, valueKind, checkingReceiver: false, diagnostics: diagnostics) ||
                expr.HasAnyErrors && valueKind == BindValueKind.RValueOrMethodGroup)
            {
                return expr;
            }

            var resultKind = (valueKind == BindValueKind.RValue || valueKind == BindValueKind.RValueOrMethodGroup) ?
                LookupResultKind.NotAValue :
                LookupResultKind.NotAVariable;

            return ToBadExpression(expr, resultKind);
        }

        internal static bool IsTypeOrValueExpression(BoundExpression expression)
        {
            switch (expression?.Kind)
            {
                case BoundKind.TypeOrValueExpression:
                case BoundKind.QueryClause when ((BoundQueryClause)expression).Value.Kind == BoundKind.TypeOrValueExpression:
                    return true;
                default:
                    return false;
            }
        }

        /// <summary>
        /// The purpose of this method is to determine if the expression satisfies desired capabilities. 
        /// If it is not then this code gives an appropriate error message.
        ///
        /// To determine the appropriate error message we need to know two things:
        ///
        /// (1) What capabilities we need - increment it, assign, return as a readonly reference, . . . ?
        ///
        /// (2) Are we trying to determine if the left hand side of a dot is a variable in order
        ///     to determine if the field or property on the right hand side of a dot is assignable?
        ///     
        /// (3) The syntax of the expression that started the analysis. (for error reporting purposes).
        /// </summary>
        internal bool CheckValueKind(SyntaxNode node, BoundExpression expr, BindValueKind valueKind, bool checkingReceiver, DiagnosticBag diagnostics)
        {
            Debug.Assert(!checkingReceiver || expr.Type.IsValueType || expr.Type.IsTypeParameter());

            if (expr.HasAnyErrors)
            {
                return false;
            }

            switch (expr.Kind)
            {
                // we need to handle properties and event in a special way even in an RValue case because of getters
                case BoundKind.PropertyAccess:
                case BoundKind.IndexerAccess:
                    return CheckPropertyValueKind(node, expr, valueKind, checkingReceiver, diagnostics);

                case BoundKind.IndexOrRangePatternIndexerAccess:
                    var patternIndexer = ((BoundIndexOrRangePatternIndexerAccess)expr);
                    if (patternIndexer.PatternSymbol.Kind == SymbolKind.Property)
                    {
                        // If this is an Index indexer, PatternSymbol should be a property, pointing to the
                        // pattern indexer. If it's a Range access, it will be a method, pointing to a Slice method
                        // and it's handled below as part of invocations.
                        return CheckPropertyValueKind(node, expr, valueKind, checkingReceiver, diagnostics);
                    }
                    Debug.Assert(patternIndexer.PatternSymbol.Kind == SymbolKind.Method);
                    break;

                case BoundKind.EventAccess:
                    return CheckEventValueKind((BoundEventAccess)expr, valueKind, diagnostics);
            }

            // easy out for a very common RValue case.
            if (RequiresRValueOnly(valueKind))
            {
                return CheckNotNamespaceOrType(expr, diagnostics);
            }

            // constants/literals are strictly RValues
            // void is not even an RValue
            if ((expr.ConstantValue != null) || (expr.Type.GetSpecialTypeSafe() == SpecialType.System_Void))
            {
                Error(diagnostics, GetStandardLvalueError(valueKind), node);
                return false;
            }

            switch (expr.Kind)
            {
                case BoundKind.NamespaceExpression:
                    var ns = (BoundNamespaceExpression)expr;
                    Error(diagnostics, ErrorCode.ERR_BadSKknown, node, ns.NamespaceSymbol, MessageID.IDS_SK_NAMESPACE.Localize(), MessageID.IDS_SK_VARIABLE.Localize());
                    return false;

                case BoundKind.TypeExpression:
                    var type = (BoundTypeExpression)expr;
                    Error(diagnostics, ErrorCode.ERR_BadSKknown, node, type.Type, MessageID.IDS_SK_TYPE.Localize(), MessageID.IDS_SK_VARIABLE.Localize());
                    return false;

                case BoundKind.Lambda:
                case BoundKind.UnboundLambda:
                    // lambdas can only be used as RValues
                    Error(diagnostics, GetStandardLvalueError(valueKind), node);
                    return false;

                case BoundKind.UnconvertedAddressOfOperator:
                    var unconvertedAddressOf = (BoundUnconvertedAddressOfOperator)expr;
                    Error(diagnostics, GetMethodGroupOrFunctionPointerLvalueError(valueKind), node, unconvertedAddressOf.Operand.Name, MessageID.IDS_AddressOfMethodGroup.Localize());
                    return false;

                case BoundKind.MethodGroup when valueKind == BindValueKind.AddressOf:
                    // If the addressof operator is used not as an rvalue, that will get flagged when CheckValue
                    // is called on the parent BoundUnconvertedAddressOf node.
                    return true;

                case BoundKind.MethodGroup:
                    // method groups can only be used as RValues except when taking the address of one
                    var methodGroup = (BoundMethodGroup)expr;
                    Error(diagnostics, GetMethodGroupOrFunctionPointerLvalueError(valueKind), node, methodGroup.Name, MessageID.IDS_MethodGroup.Localize());
                    return false;

                case BoundKind.RangeVariable:
                    // range variables can only be used as RValues
                    var queryref = (BoundRangeVariable)expr;
                    Error(diagnostics, GetRangeLvalueError(valueKind), node, queryref.RangeVariableSymbol.Name);
                    return false;

                case BoundKind.Conversion:
                    var conversion = (BoundConversion)expr;
                    // conversions are strict RValues, but unboxing has a specific error
                    if (conversion.ConversionKind == ConversionKind.Unboxing)
                    {
                        Error(diagnostics, ErrorCode.ERR_UnboxNotLValue, node);
                        return false;
                    }
                    break;

                // array access is readwrite variable if the indexing expression is not System.Range
                case BoundKind.ArrayAccess:
                    {
                        if (RequiresRefAssignableVariable(valueKind))
                        {
                            Error(diagnostics, ErrorCode.ERR_RefLocalOrParamExpected, node);
                            return false;
                        }

                        var boundAccess = (BoundArrayAccess)expr;
                        if (boundAccess.Indices.Length == 1 &&
                            TypeSymbol.Equals(
                                boundAccess.Indices[0].Type,
                                Compilation.GetWellKnownType(WellKnownType.System_Range),
                                TypeCompareKind.ConsiderEverything))
                        {
                            // Range indexer is an rvalue
                            Error(diagnostics, GetStandardLvalueError(valueKind), node);
                            return false;
                        }
                        return true;
                    }

                // pointer dereferencing is a readwrite variable
                case BoundKind.PointerIndirectionOperator:
                // The undocumented __refvalue(tr, T) expression results in a variable of type T.
                case BoundKind.RefValueOperator:
                // dynamic expressions are readwrite, and can even be passed by ref (which is implemented via a temp)
                case BoundKind.DynamicMemberAccess:
                case BoundKind.DynamicIndexerAccess:
                    {
                        if (RequiresRefAssignableVariable(valueKind))
                        {
                            Error(diagnostics, ErrorCode.ERR_RefLocalOrParamExpected, node);
                            return false;
                        }

                        // These are readwrite variables
                        return true;
                    }

                case BoundKind.PointerElementAccess:
                    {
                        if (RequiresRefAssignableVariable(valueKind))
                        {
                            Error(diagnostics, ErrorCode.ERR_RefLocalOrParamExpected, node);
                            return false;
                        }

                        var receiver = ((BoundPointerElementAccess)expr).Expression;
                        if (receiver is BoundFieldAccess fieldAccess && fieldAccess.FieldSymbol.IsFixedSizeBuffer)
                        {
                            return CheckValueKind(node, fieldAccess.ReceiverOpt, valueKind, checkingReceiver: true, diagnostics);
                        }

                        return true;
                    }

                case BoundKind.Parameter:
                    var parameter = (BoundParameter)expr;
                    return CheckParameterValueKind(node, parameter, valueKind, checkingReceiver, diagnostics);

                case BoundKind.Local:
                    var local = (BoundLocal)expr;
                    return CheckLocalValueKind(node, local, valueKind, checkingReceiver, diagnostics);

                case BoundKind.ThisReference:
                    // `this` is never ref assignable
                    if (RequiresRefAssignableVariable(valueKind))
                    {
                        Error(diagnostics, ErrorCode.ERR_RefLocalOrParamExpected, node);
                        return false;
                    }

                    // We will already have given an error for "this" used outside of a constructor, 
                    // instance method, or instance accessor. Assume that "this" is a variable if it is in a struct.

                    // SPEC: when this is used in a primary-expression within an instance constructor of a struct, 
                    // SPEC: it is classified as a variable. 

                    // SPEC: When this is used in a primary-expression within an instance method or instance accessor
                    // SPEC: of a struct, it is classified as a variable. 

                    // Note: RValueOnly is checked at the beginning of this method. Since we are here we need more than readable.
                    // "this" is readonly in members marked "readonly" and in members of readonly structs, unless we are in a constructor.
                    var isValueType = ((BoundThisReference)expr).Type.IsValueType;
                    if (!isValueType || (RequiresAssignableVariable(valueKind) && (this.ContainingMemberOrLambda as MethodSymbol)?.IsEffectivelyReadOnly == true))
                    {
                        Error(diagnostics, GetThisLvalueError(valueKind, isValueType), node, node);
                        return false;
                    }

                    return true;

                case BoundKind.ImplicitReceiver:
                case BoundKind.ObjectOrCollectionValuePlaceholder:
                    Debug.Assert(!RequiresRefAssignableVariable(valueKind));
                    return true;

                case BoundKind.Call:
                    var call = (BoundCall)expr;
                    return CheckCallValueKind(call, node, valueKind, checkingReceiver, diagnostics);

                case BoundKind.FunctionPointerInvocation:
                    return CheckMethodReturnValueKind(((BoundFunctionPointerInvocation)expr).FunctionPointer.Signature,
                        expr.Syntax,
                        node,
                        valueKind,
                        checkingReceiver,
                        diagnostics);

                case BoundKind.IndexOrRangePatternIndexerAccess:
                    var patternIndexer = (BoundIndexOrRangePatternIndexerAccess)expr;
                    // If we got here this should be a pattern indexer taking a Range,
                    // meaning that the pattern symbol must be a method (either Slice or Substring)
                    return CheckMethodReturnValueKind(
                        (MethodSymbol)patternIndexer.PatternSymbol,
                        patternIndexer.Syntax,
                        node,
                        valueKind,
                        checkingReceiver,
                        diagnostics);

                case BoundKind.ConditionalOperator:
                    var conditional = (BoundConditionalOperator)expr;

                    // byref conditional defers to its operands
                    if (conditional.IsRef &&
                        (CheckValueKind(conditional.Consequence.Syntax, conditional.Consequence, valueKind, checkingReceiver: false, diagnostics: diagnostics) &
                        CheckValueKind(conditional.Alternative.Syntax, conditional.Alternative, valueKind, checkingReceiver: false, diagnostics: diagnostics)))
                    {
                        return true;
                    }

                    // report standard lvalue error
                    break;

                case BoundKind.FieldAccess:
                    {
                        var fieldAccess = (BoundFieldAccess)expr;
                        return CheckFieldValueKind(node, fieldAccess, valueKind, checkingReceiver, diagnostics);
                    }

                case BoundKind.AssignmentOperator:
                    var assignment = (BoundAssignmentOperator)expr;
                    return CheckSimpleAssignmentValueKind(node, assignment, valueKind, diagnostics);
            }

            // At this point we should have covered all the possible cases for anything that is not a strict RValue.
            Error(diagnostics, GetStandardLvalueError(valueKind), node);
            return false;
        }

        private static bool CheckNotNamespaceOrType(BoundExpression expr, DiagnosticBag diagnostics)
        {
            switch (expr.Kind)
            {
                case BoundKind.NamespaceExpression:
                    Error(diagnostics, ErrorCode.ERR_BadSKknown, expr.Syntax, ((BoundNamespaceExpression)expr).NamespaceSymbol, MessageID.IDS_SK_NAMESPACE.Localize(), MessageID.IDS_SK_VARIABLE.Localize());
                    return false;
                case BoundKind.TypeExpression:
                    Error(diagnostics, ErrorCode.ERR_BadSKunknown, expr.Syntax, expr.Type, MessageID.IDS_SK_TYPE.Localize());
                    return false;
                default:
                    return true;
            }
        }

        private bool CheckLocalValueKind(SyntaxNode node, BoundLocal local, BindValueKind valueKind, bool checkingReceiver, DiagnosticBag diagnostics)
        {
            // Local constants are never variables. Local variables are sometimes
            // not to be treated as variables, if they are fixed, declared in a using, 
            // or declared in a foreach.

            LocalSymbol localSymbol = local.LocalSymbol;
            if (RequiresAssignableVariable(valueKind))
            {
                if (this.LockedOrDisposedVariables.Contains(localSymbol))
                {
                    diagnostics.Add(ErrorCode.WRN_AssignmentToLockOrDispose, local.Syntax.Location, localSymbol);
                }

                // IsWritable means the variable is writable. If this is a ref variable, IsWritable
                // does not imply anything about the storage location
                if (localSymbol.RefKind == RefKind.RefReadOnly ||
                    (localSymbol.RefKind == RefKind.None && !localSymbol.IsWritableVariable))
                {
                    ReportReadonlyLocalError(node, localSymbol, valueKind, checkingReceiver, diagnostics);
                    return false;
                }
            }
            else if (RequiresRefAssignableVariable(valueKind))
            {
                if (localSymbol.RefKind == RefKind.None)
                {
                    diagnostics.Add(ErrorCode.ERR_RefLocalOrParamExpected, node.Location, localSymbol);
                    return false;
                }
                else if (!localSymbol.IsWritableVariable)
                {
                    ReportReadonlyLocalError(node, localSymbol, valueKind, checkingReceiver, diagnostics);
                    return false;
                }
            }

            return true;
        }

        private static bool CheckLocalRefEscape(SyntaxNode node, BoundLocal local, uint escapeTo, bool checkingReceiver, DiagnosticBag diagnostics)
        {
            LocalSymbol localSymbol = local.LocalSymbol;

            // if local symbol can escape to the same or wider/shallower scope then escapeTo
            // then it is all ok, otherwise it is an error.
            if (localSymbol.RefEscapeScope <= escapeTo)
            {
                return true;
            }

            if (escapeTo == Binder.ExternalScope)
            {
                if (localSymbol.RefKind == RefKind.None)
                {
                    if (checkingReceiver)
                    {
                        Error(diagnostics, ErrorCode.ERR_RefReturnLocal2, local.Syntax, localSymbol);
                    }
                    else
                    {
                        Error(diagnostics, ErrorCode.ERR_RefReturnLocal, node, localSymbol);
                    }
                    return false;
                }

                if (checkingReceiver)
                {
                    Error(diagnostics, ErrorCode.ERR_RefReturnNonreturnableLocal2, local.Syntax, localSymbol);
                }
                else
                {
                    Error(diagnostics, ErrorCode.ERR_RefReturnNonreturnableLocal, node, localSymbol);
                }
                return false;
            }

            Error(diagnostics, ErrorCode.ERR_EscapeLocal, node, localSymbol);
            return false;
        }

        private bool CheckParameterValueKind(SyntaxNode node, BoundParameter parameter, BindValueKind valueKind, bool checkingReceiver, DiagnosticBag diagnostics)
        {
            ParameterSymbol parameterSymbol = parameter.ParameterSymbol;

            // all parameters can be passed by ref/out or assigned to
            // except "in" parameters, which are readonly
            if (parameterSymbol.RefKind == RefKind.In && RequiresAssignableVariable(valueKind))
            {
                ReportReadOnlyError(parameterSymbol, node, valueKind, checkingReceiver, diagnostics);
                return false;
            }
            else if (parameterSymbol.RefKind == RefKind.None && RequiresRefAssignableVariable(valueKind))
            {
                Error(diagnostics, ErrorCode.ERR_RefLocalOrParamExpected, node);
                return false;
            }

            if (this.LockedOrDisposedVariables.Contains(parameterSymbol))
            {
                // Consider: It would be more conventional to pass "symbol" rather than "symbol.Name".
                // The issue is that the error SymbolDisplayFormat doesn't display parameter
                // names - only their types - which works great in signatures, but not at all
                // at the top level.
                diagnostics.Add(ErrorCode.WRN_AssignmentToLockOrDispose, parameter.Syntax.Location, parameterSymbol.Name);
            }

            return true;
        }

        private static bool CheckParameterRefEscape(SyntaxNode node, BoundParameter parameter, uint escapeTo, bool checkingReceiver, DiagnosticBag diagnostics)
        {
            ParameterSymbol parameterSymbol = parameter.ParameterSymbol;

            // byval parameters can escape to method's top level. Others can escape further.
            // NOTE: "method" here means nearest containing method, lambda or local function.
            if (escapeTo == Binder.ExternalScope && parameterSymbol.RefKind == RefKind.None)
            {
                if (checkingReceiver)
                {
                    Error(diagnostics, ErrorCode.ERR_RefReturnParameter2, parameter.Syntax, parameterSymbol.Name);
                }
                else
                {
                    Error(diagnostics, ErrorCode.ERR_RefReturnParameter, node, parameterSymbol.Name);
                }
                return false;
            }

            // can ref-escape to any scope otherwise
            return true;
        }

        private bool CheckFieldValueKind(SyntaxNode node, BoundFieldAccess fieldAccess, BindValueKind valueKind, bool checkingReceiver, DiagnosticBag diagnostics)
        {
            var fieldSymbol = fieldAccess.FieldSymbol;
            var fieldIsStatic = fieldSymbol.IsStatic;

            if (RequiresAssignableVariable(valueKind))
            {
                // A field is writeable unless 
                // (1) it is readonly and we are not in a constructor or field initializer
                // (2) the receiver of the field is of value type and is not a variable or object creation expression.
                // For example, if you have a class C with readonly field f of type S, and
                // S has a mutable field x, then c.f.x is not a variable because c.f is not
                // writable.

                if (fieldSymbol.IsReadOnly)
                {
                    var canModifyReadonly = false;

                    Symbol containing = this.ContainingMemberOrLambda;
                    if ((object)containing != null &&
                        fieldIsStatic == containing.IsStatic &&
                        (fieldIsStatic || fieldAccess.ReceiverOpt.Kind == BoundKind.ThisReference) &&
                        (Compilation.FeatureStrictEnabled
                            ? TypeSymbol.Equals(fieldSymbol.ContainingType, containing.ContainingType, TypeCompareKind.ConsiderEverything2)
                            // We duplicate a bug in the native compiler for compatibility in non-strict mode
                            : TypeSymbol.Equals(fieldSymbol.ContainingType.OriginalDefinition, containing.ContainingType.OriginalDefinition, TypeCompareKind.ConsiderEverything2)))
                    {
                        if (containing.Kind == SymbolKind.Method)
                        {
                            MethodSymbol containingMethod = (MethodSymbol)containing;
                            MethodKind desiredMethodKind = fieldIsStatic ? MethodKind.StaticConstructor : MethodKind.Constructor;
                            canModifyReadonly = (containingMethod.MethodKind == desiredMethodKind) ||
                                isAssignedFromInitOnlySetterOnThis(fieldAccess.ReceiverOpt);
                        }
                        else if (containing.Kind == SymbolKind.Field)
                        {
                            canModifyReadonly = true;
                        }
                    }

                    if (!canModifyReadonly)
                    {
                        ReportReadOnlyFieldError(fieldSymbol, node, valueKind, checkingReceiver, diagnostics);
                        return false;
                    }
                }

                if (fieldSymbol.IsFixedSizeBuffer)
                {
                    Error(diagnostics, GetStandardLvalueError(valueKind), node);
                    return false;
                }
            }

            if (RequiresRefAssignableVariable(valueKind))
            {
                Error(diagnostics, ErrorCode.ERR_RefLocalOrParamExpected, node);
                return false;
            }

            // r/w fields that are static or belong to reference types are writeable and returnable
            if (fieldIsStatic || fieldSymbol.ContainingType.IsReferenceType)
            {
                return true;
            }

            // for other fields defer to the receiver.
            return CheckIsValidReceiverForVariable(node, fieldAccess.ReceiverOpt, valueKind, diagnostics);

            bool isAssignedFromInitOnlySetterOnThis(BoundExpression receiver)
            {
                // bad: other.readonlyField = ...
                // bad: base.readonlyField = ...
                if (!(receiver is BoundThisReference))
                {
                    return false;
                }

                if (!(ContainingMemberOrLambda is MethodSymbol method))
                {
                    return false;
                }

                return method.IsInitOnly;
            }
        }

        private bool CheckSimpleAssignmentValueKind(SyntaxNode node, BoundAssignmentOperator assignment, BindValueKind valueKind, DiagnosticBag diagnostics)
        {
            // Only ref-assigns produce LValues
            if (assignment.IsRef)
            {
                return CheckValueKind(node, assignment.Left, valueKind, checkingReceiver: false, diagnostics);
            }

            Error(diagnostics, GetStandardLvalueError(valueKind), node);
            return false;
        }

        private static bool CheckFieldRefEscape(SyntaxNode node, BoundFieldAccess fieldAccess, uint escapeFrom, uint escapeTo, DiagnosticBag diagnostics)
        {
            var fieldSymbol = fieldAccess.FieldSymbol;
            // fields that are static or belong to reference types can ref escape anywhere
            if (fieldSymbol.IsStatic || fieldSymbol.ContainingType.IsReferenceType)
            {
                return true;
            }

            // for other fields defer to the receiver.
            return CheckRefEscape(node, fieldAccess.ReceiverOpt, escapeFrom, escapeTo, checkingReceiver: true, diagnostics: diagnostics);
        }

        private static bool CheckFieldLikeEventRefEscape(SyntaxNode node, BoundEventAccess eventAccess, uint escapeFrom, uint escapeTo, DiagnosticBag diagnostics)
        {
            var eventSymbol = eventAccess.EventSymbol;

            // field-like events that are static or belong to reference types can ref escape anywhere
            if (eventSymbol.IsStatic || eventSymbol.ContainingType.IsReferenceType)
            {
                return true;
            }

            // for other events defer to the receiver.
            return CheckRefEscape(node, eventAccess.ReceiverOpt, escapeFrom, escapeTo, checkingReceiver: true, diagnostics: diagnostics);
        }

        private bool CheckEventValueKind(BoundEventAccess boundEvent, BindValueKind valueKind, DiagnosticBag diagnostics)
        {
            // Compound assignment (actually "event assignment") is allowed "everywhere", subject to the restrictions of
            // accessibility, use site errors, and receiver variable-ness (for structs).
            // Other operations are allowed only for field-like events and only where the backing field is accessible
            // (i.e. in the declaring type) - subject to use site errors and receiver variable-ness.

            BoundExpression receiver = boundEvent.ReceiverOpt;
            SyntaxNode eventSyntax = GetEventName(boundEvent); //does not include receiver
            EventSymbol eventSymbol = boundEvent.EventSymbol;

            if (valueKind == BindValueKind.CompoundAssignment)
            {
                // NOTE: accessibility has already been checked by lookup.
                // NOTE: availability of well-known members is checked in BindEventAssignment because
                // we don't have the context to determine whether addition or subtraction is being performed.

                if (ReportUseSiteDiagnostics(eventSymbol, diagnostics, eventSyntax))
                {
                    // NOTE: BindEventAssignment checks use site errors on the specific accessor 
                    // (since we don't know which is being used).
                    return false;
                }

                Debug.Assert(!RequiresVariableReceiver(receiver, eventSymbol));
                return true;
            }
            else
            {
                if (!boundEvent.IsUsableAsField)
                {
                    // Dev10 reports this in addition to ERR_BadAccess, but we won't even reach this point if the event isn't accessible (caught by lookup).
                    Error(diagnostics, GetBadEventUsageDiagnosticInfo(eventSymbol), eventSyntax);
                    return false;
                }
                else if (ReportUseSiteDiagnostics(eventSymbol, diagnostics, eventSyntax))
                {
                    if (!CheckIsValidReceiverForVariable(eventSyntax, receiver, BindValueKind.Assignable, diagnostics))
                    {
                        return false;
                    }
                }
                else if (RequiresVariable(valueKind))
                {
                    if (eventSymbol.IsWindowsRuntimeEvent && valueKind != BindValueKind.Assignable)
                    {
                        // NOTE: Dev11 reports ERR_RefProperty, as if this were a property access (since that's how it will be lowered).
                        // Roslyn reports a new, more specific, error code.
                        ErrorCode errorCode = valueKind == BindValueKind.RefOrOut ? ErrorCode.ERR_WinRtEventPassedByRef : GetStandardLvalueError(valueKind);
                        Error(diagnostics, errorCode, eventSyntax, eventSymbol);

                        return false;
                    }
                    else if (RequiresVariableReceiver(receiver, eventSymbol.AssociatedField) && // NOTE: using field, not event
                        !CheckIsValidReceiverForVariable(eventSyntax, receiver, valueKind, diagnostics))
                    {
                        return false;
                    }
                }

                return true;
            }
        }

        private bool CheckIsValidReceiverForVariable(SyntaxNode node, BoundExpression receiver, BindValueKind kind, DiagnosticBag diagnostics)
        {
            Debug.Assert(receiver != null);
            return Flags.Includes(BinderFlags.ObjectInitializerMember) && receiver.Kind == BoundKind.ObjectOrCollectionValuePlaceholder ||
                CheckValueKind(node, receiver, kind, true, diagnostics);
        }

        /// <summary>
        /// SPEC: When a property or indexer declared in a struct-type is the target of an 
        /// SPEC: assignment, the instance expression associated with the property or indexer 
        /// SPEC: access must be classified as a variable. If the instance expression is 
        /// SPEC: classified as a value, a compile-time error occurs. Because of 7.6.4, 
        /// SPEC: the same rule also applies to fields.
        /// </summary>
        /// <remarks>
        /// NOTE: The spec fails to impose the restriction that the event receiver must be classified
        /// as a variable (unlike for properties - 7.17.1).  This seems like a bug, but we have
        /// production code that won't build with the restriction in place (see DevDiv #15674).
        /// </remarks>
        private static bool RequiresVariableReceiver(BoundExpression receiver, Symbol symbol)
        {
            return symbol.RequiresInstanceReceiver()
                && symbol.Kind != SymbolKind.Event
                && receiver?.Type?.IsValueType == true;
        }

        private bool CheckCallValueKind(BoundCall call, SyntaxNode node, BindValueKind valueKind, bool checkingReceiver, DiagnosticBag diagnostics)
            => CheckMethodReturnValueKind(call.Method, call.Syntax, node, valueKind, checkingReceiver, diagnostics);

        protected bool CheckMethodReturnValueKind(
            MethodSymbol methodSymbol,
            SyntaxNode callSyntaxOpt,
            SyntaxNode node,
            BindValueKind valueKind,
            bool checkingReceiver,
            DiagnosticBag diagnostics)
        {
            // A call can only be a variable if it returns by reference. If this is the case,
            // whether or not it is a valid variable depends on whether or not the call is the
            // RHS of a return or an assign by reference:
            // - If call is used in a context demanding ref-returnable reference all of its ref
            //   inputs must be ref-returnable

            if (RequiresVariable(valueKind) && methodSymbol.RefKind == RefKind.None)
            {
                if (checkingReceiver)
                {
                    // Error is associated with expression, not node which may be distinct.
                    Error(diagnostics, ErrorCode.ERR_ReturnNotLValue, callSyntaxOpt, methodSymbol);
                }
                else
                {
                    Error(diagnostics, GetStandardLvalueError(valueKind), node);
                }

                return false;
            }

            if (RequiresAssignableVariable(valueKind) && methodSymbol.RefKind == RefKind.RefReadOnly)
            {
                ReportReadOnlyError(methodSymbol, node, valueKind, checkingReceiver, diagnostics);
                return false;
            }

            if (RequiresRefAssignableVariable(valueKind))
            {
                Error(diagnostics, ErrorCode.ERR_RefLocalOrParamExpected, node);
                return false;
            }

            return true;

        }

        private bool CheckPropertyValueKind(SyntaxNode node, BoundExpression expr, BindValueKind valueKind, bool checkingReceiver, DiagnosticBag diagnostics)
        {
            // SPEC: If the left operand is a property or indexer access, the property or indexer must
            // SPEC: have a set accessor. If this is not the case, a compile-time error occurs.

            // Addendum: Assignment is also allowed for get-only autoprops in their constructor

            BoundExpression receiver;
            SyntaxNode propertySyntax;
            var propertySymbol = GetPropertySymbol(expr, out receiver, out propertySyntax);

            Debug.Assert((object)propertySymbol != null);
            Debug.Assert(propertySyntax != null);

            if ((RequiresReferenceToLocation(valueKind) || checkingReceiver) &&
                propertySymbol.RefKind == RefKind.None)
            {
                if (checkingReceiver)
                {
                    // Error is associated with expression, not node which may be distinct.
                    // This error is reported for all values types. That is a breaking
                    // change from Dev10 which reports this error for struct types only,
                    // not for type parameters constrained to "struct".

                    Debug.Assert(propertySymbol.TypeWithAnnotations.HasType);
                    Error(diagnostics, ErrorCode.ERR_ReturnNotLValue, expr.Syntax, propertySymbol);
                }
                else
                {
                    Error(diagnostics, valueKind == BindValueKind.RefOrOut ? ErrorCode.ERR_RefProperty : GetStandardLvalueError(valueKind), node, propertySymbol);
                }

                return false;
            }

            if (RequiresAssignableVariable(valueKind) && propertySymbol.RefKind == RefKind.RefReadOnly)
            {
                ReportReadOnlyError(propertySymbol, node, valueKind, checkingReceiver, diagnostics);
                return false;
            }

            var requiresSet = RequiresAssignableVariable(valueKind) && propertySymbol.RefKind == RefKind.None;
            if (requiresSet)
            {
                var setMethod = propertySymbol.GetOwnOrInheritedSetMethod();

                if (setMethod is null)
                {
                    var containing = this.ContainingMemberOrLambda;
                    if (!AccessingAutoPropertyFromConstructor(receiver, propertySymbol, containing))
                    {
                        Error(diagnostics, ErrorCode.ERR_AssgReadonlyProp, node, propertySymbol);
                        return false;
                    }
                }
                else
                {
                    if (setMethod.IsInitOnly)
                    {
                        if (!isAllowedInitOnlySet(receiver))
                        {
                            Error(diagnostics, ErrorCode.ERR_AssignmentInitOnly, node, propertySymbol);
                            return false;
                        }

                        if (setMethod.DeclaringCompilation != this.Compilation)
                        {
                            // an error would have already been reported on declaring an init-only setter
                            CheckFeatureAvailability(node, MessageID.IDS_FeatureInitOnlySetters, diagnostics);
                        }
                    }

                    var accessThroughType = this.GetAccessThroughType(receiver);
                    bool failedThroughTypeCheck;
                    HashSet<DiagnosticInfo> useSiteDiagnostics = null;
                    bool isAccessible = this.IsAccessible(setMethod, accessThroughType, out failedThroughTypeCheck, ref useSiteDiagnostics);
                    diagnostics.Add(node, useSiteDiagnostics);

                    if (!isAccessible)
                    {
                        if (failedThroughTypeCheck)
                        {
                            Error(diagnostics, ErrorCode.ERR_BadProtectedAccess, node, propertySymbol, accessThroughType, this.ContainingType);
                        }
                        else
                        {
                            Error(diagnostics, ErrorCode.ERR_InaccessibleSetter, node, propertySymbol);
                        }
                        return false;
                    }

                    ReportDiagnosticsIfObsolete(diagnostics, setMethod, node, receiver?.Kind == BoundKind.BaseReference);

                    var setValueKind = setMethod.IsEffectivelyReadOnly ? BindValueKind.RValue : BindValueKind.Assignable;
                    if (RequiresVariableReceiver(receiver, setMethod) && !CheckIsValidReceiverForVariable(node, receiver, setValueKind, diagnostics))
                    {
                        return false;
                    }

                    if (IsBadBaseAccess(node, receiver, setMethod, diagnostics, propertySymbol) ||
                        (!object.Equals(setMethod.GetUseSiteDiagnostic(), propertySymbol.GetUseSiteDiagnostic()) && ReportUseSiteDiagnostics(setMethod, diagnostics, propertySyntax)))
                    {
                        return false;
                    }

                    CheckRuntimeSupportForSymbolAccess(node, receiver, setMethod, diagnostics);
                }
            }

            var requiresGet = !RequiresAssignmentOnly(valueKind) || propertySymbol.RefKind != RefKind.None;
            if (requiresGet)
            {
                var getMethod = propertySymbol.GetOwnOrInheritedGetMethod();

                if ((object)getMethod == null)
                {
                    Error(diagnostics, ErrorCode.ERR_PropertyLacksGet, node, propertySymbol);
                    return false;
                }
                else
                {
                    var accessThroughType = this.GetAccessThroughType(receiver);
                    bool failedThroughTypeCheck;
                    HashSet<DiagnosticInfo> useSiteDiagnostics = null;
                    bool isAccessible = this.IsAccessible(getMethod, accessThroughType, out failedThroughTypeCheck, ref useSiteDiagnostics);
                    diagnostics.Add(node, useSiteDiagnostics);

                    if (!isAccessible)
                    {
                        if (failedThroughTypeCheck)
                        {
                            Error(diagnostics, ErrorCode.ERR_BadProtectedAccess, node, propertySymbol, accessThroughType, this.ContainingType);
                        }
                        else
                        {
                            Error(diagnostics, ErrorCode.ERR_InaccessibleGetter, node, propertySymbol);
                        }
                        return false;
                    }

                    CheckImplicitThisCopyInReadOnlyMember(receiver, getMethod, diagnostics);
                    ReportDiagnosticsIfObsolete(diagnostics, getMethod, node, receiver?.Kind == BoundKind.BaseReference);

                    if (IsBadBaseAccess(node, receiver, getMethod, diagnostics, propertySymbol) ||
                        (!object.Equals(getMethod.GetUseSiteDiagnostic(), propertySymbol.GetUseSiteDiagnostic()) && ReportUseSiteDiagnostics(getMethod, diagnostics, propertySyntax)))
                    {
                        return false;
                    }

                    CheckRuntimeSupportForSymbolAccess(node, receiver, getMethod, diagnostics);
                }
            }

            if (RequiresRefAssignableVariable(valueKind))
            {
                Error(diagnostics, ErrorCode.ERR_RefLocalOrParamExpected, node);
                return false;
            }

            return true;

            bool isAllowedInitOnlySet(BoundExpression receiver)
            {
                // ok: new C() { InitOnlyProperty = ... }
                // bad: { ... = { InitOnlyProperty = ... } }
                if (receiver is BoundObjectOrCollectionValuePlaceholder placeholder)
                {
                    return placeholder.IsNewInstance;
                }

                // bad: other.InitOnlyProperty = ...
                if (!(receiver is BoundThisReference || receiver is BoundBaseReference))
                {
                    return false;
                }

                var containingMember = ContainingMemberOrLambda;
                if (!(containingMember is MethodSymbol method))
                {
                    return false;
                }

                if (method.MethodKind == MethodKind.Constructor || method.IsInitOnly)
                {
                    // ok: setting on `this` or `base` from an instance constructor or init-only setter
                    return true;
                }

                return false;
            }
        }

        private bool IsBadBaseAccess(SyntaxNode node, BoundExpression receiverOpt, Symbol member, DiagnosticBag diagnostics,
                                     Symbol propertyOrEventSymbolOpt = null)
        {
            Debug.Assert(member.Kind != SymbolKind.Property);
            Debug.Assert(member.Kind != SymbolKind.Event);

            if (receiverOpt?.Kind == BoundKind.BaseReference && member.IsAbstract)
            {
                Error(diagnostics, ErrorCode.ERR_AbstractBaseCall, node, propertyOrEventSymbolOpt ?? member);
                return true;
            }

            return false;
        }

        /// <summary>
        /// Computes the scope to which the given invocation can escape
        /// NOTE: the escape scope for ref and val escapes is the same for invocations except for trivial cases (ordinary type returned by val) 
        ///       where escape is known otherwise. Therefore we do not vave two ref/val variants of this.
        ///       
        /// NOTE: we need scopeOfTheContainingExpression as some expressions such as optional <c>in</c> parameters or <c>ref dynamic</c> behave as 
        ///       local variables declared at the scope of the invocation.
        /// </summary>
        internal static uint GetInvocationEscapeScope(
            Symbol symbol,
            BoundExpression receiverOpt,
            ImmutableArray<ParameterSymbol> parameters,
            ImmutableArray<BoundExpression> argsOpt,
            ImmutableArray<RefKind> argRefKindsOpt,
            ImmutableArray<int> argsToParamsOpt,
            uint scopeOfTheContainingExpression,
            bool isRefEscape
        )
        {
            // SPEC: (also applies to the CheckInvocationEscape counterpart)
            //
            //            An lvalue resulting from a ref-returning method invocation e1.M(e2, ...) is ref-safe - to - escape the smallest of the following scopes:
            //•	The entire enclosing method
            //•	the ref-safe-to-escape of all ref/out/in argument expressions(excluding the receiver)
            //•	the safe-to - escape of all argument expressions(including the receiver)
            //
            //            An rvalue resulting from a method invocation e1.M(e2, ...) is safe - to - escape from the smallest of the following scopes:
            //•	The entire enclosing method
            //•	the safe-to-escape of all argument expressions(including the receiver)
            //

            if (!symbol.RequiresInstanceReceiver())
            {
                // ignore receiver when symbol is static
                receiverOpt = null;
            }

            //by default it is safe to escape
            uint escapeScope = Binder.ExternalScope;

            ArrayBuilder<bool> inParametersMatchedWithArgs = null;

            if (!argsOpt.IsDefault)
            {
moreArguments:
                for (var argIndex = 0; argIndex < argsOpt.Length; argIndex++)
                {
                    var argument = argsOpt[argIndex];
                    if (argument.Kind == BoundKind.ArgListOperator)
                    {
                        Debug.Assert(argIndex == argsOpt.Length - 1, "vararg must be the last");
                        var argList = (BoundArgListOperator)argument;

                        // unwrap varargs and process as more arguments
                        argsOpt = argList.Arguments;
                        // ref kinds of varargs are not interesting here. 
                        // __refvalue is not ref-returnable, so ref varargs can't come back from a call
                        argRefKindsOpt = default;
                        parameters = ImmutableArray<ParameterSymbol>.Empty;
                        argsToParamsOpt = default;

                        goto moreArguments;
                    }

                    RefKind effectiveRefKind = GetEffectiveRefKindAndMarkMatchedInParameter(argIndex, argRefKindsOpt, parameters, argsToParamsOpt, ref inParametersMatchedWithArgs);

                    // ref escape scope is the narrowest of 
                    // - ref escape of all byref arguments
                    // - val escape of all byval arguments  (ref-like values can be unwrapped into refs, so treat val escape of values as possible ref escape of the result)
                    //
                    // val escape scope is the narrowest of 
                    // - val escape of all byval arguments  (refs cannot be wrapped into values, so their ref escape is irrelevant, only use val escapes)

                    var argEscape = effectiveRefKind != RefKind.None && isRefEscape ?
                                        GetRefEscape(argument, scopeOfTheContainingExpression) :
                                        GetValEscape(argument, scopeOfTheContainingExpression);

                    escapeScope = Math.Max(escapeScope, argEscape);

                    if (escapeScope >= scopeOfTheContainingExpression)
                    {
                        // no longer needed
                        inParametersMatchedWithArgs?.Free();

                        // can't get any worse
                        return escapeScope;
                    }
                }
            }

            // handle omitted optional "in" parameters if there are any
            ParameterSymbol unmatchedInParameter = TryGetunmatchedInParameterAndFreeMatchedArgs(parameters, ref inParametersMatchedWithArgs);

            // unmatched "in" parameter is the same as a literal, its ref escape is scopeOfTheContainingExpression  (can't get any worse)
            //                                                    its val escape is ExternalScope                   (does not affect overall result)
            if (unmatchedInParameter != null && isRefEscape)
            {
                return scopeOfTheContainingExpression;
            }

            // check receiver if ref-like
            if (receiverOpt?.Type?.IsRefLikeType == true)
            {
                escapeScope = Math.Max(escapeScope, GetValEscape(receiverOpt, scopeOfTheContainingExpression));
            }

            return escapeScope;
        }

        /// <summary>
        /// Validates whether given invocation can allow its results to escape from <paramref name="escapeFrom"/> level to <paramref name="escapeTo"/> level.
        /// The result indicates whether the escape is possible. 
        /// Additionally, the method emits diagnostics (possibly more than one, recursively) that would help identify the cause for the failure.
        /// 
        /// NOTE: we need scopeOfTheContainingExpression as some expressions such as optional <c>in</c> parameters or <c>ref dynamic</c> behave as 
        ///       local variables declared at the scope of the invocation.
        /// </summary>
        private static bool CheckInvocationEscape(
            SyntaxNode syntax,
            Symbol symbol,
            BoundExpression receiverOpt,
            ImmutableArray<ParameterSymbol> parameters,
            ImmutableArray<BoundExpression> argsOpt,
            ImmutableArray<RefKind> argRefKindsOpt,
            ImmutableArray<int> argsToParamsOpt,
            bool checkingReceiver,
            uint escapeFrom,
            uint escapeTo,
            DiagnosticBag diagnostics,
            bool isRefEscape
        )
        {
            // SPEC: 
            //            In a method invocation, the following constraints apply:
            //•	If there is a ref or out argument to a ref struct type (including the receiver), with safe-to-escape E1, then
            //  o no ref or out argument(excluding the receiver and arguments of ref-like types) may have a narrower ref-safe-to-escape than E1; and
            //  o   no argument(including the receiver) may have a narrower safe-to-escape than E1.

            if (!symbol.RequiresInstanceReceiver())
            {
                // ignore receiver when symbol is static
                receiverOpt = null;
            }

            ArrayBuilder<bool> inParametersMatchedWithArgs = null;

            if (!argsOpt.IsDefault)
            {

moreArguments:
                for (var argIndex = 0; argIndex < argsOpt.Length; argIndex++)
                {
                    var argument = argsOpt[argIndex];
                    if (argument.Kind == BoundKind.ArgListOperator)
                    {
                        Debug.Assert(argIndex == argsOpt.Length - 1, "vararg must be the last");
                        var argList = (BoundArgListOperator)argument;

                        // unwrap varargs and process as more arguments
                        argsOpt = argList.Arguments;
                        // ref kinds of varargs are not interesting here. 
                        // __refvalue is not ref-returnable, so ref varargs can't come back from a call
                        argRefKindsOpt = default;
                        parameters = ImmutableArray<ParameterSymbol>.Empty;
                        argsToParamsOpt = default;

                        goto moreArguments;
                    }

                    RefKind effectiveRefKind = GetEffectiveRefKindAndMarkMatchedInParameter(argIndex, argRefKindsOpt, parameters, argsToParamsOpt, ref inParametersMatchedWithArgs);

                    // ref escape scope is the narrowest of 
                    // - ref escape of all byref arguments
                    // - val escape of all byval arguments  (ref-like values can be unwrapped into refs, so treat val escape of values as possible ref escape of the result)
                    //
                    // val escape scope is the narrowest of 
                    // - val escape of all byval arguments  (refs cannot be wrapped into values, so their ref escape is irrelevant, only use val escapes)
                    var valid = effectiveRefKind != RefKind.None && isRefEscape ?
                                        CheckRefEscape(argument.Syntax, argument, escapeFrom, escapeTo, false, diagnostics) :
                                        CheckValEscape(argument.Syntax, argument, escapeFrom, escapeTo, false, diagnostics);

                    if (!valid)
                    {
                        // no longer needed
                        inParametersMatchedWithArgs?.Free();

                        ErrorCode errorCode = GetStandardCallEscapeError(checkingReceiver);

                        string parameterName;
                        if (parameters.Length > 0)
                        {
                            var paramIndex = argsToParamsOpt.IsDefault ? argIndex : argsToParamsOpt[argIndex];
                            parameterName = parameters[paramIndex].Name;

                            if (string.IsNullOrEmpty(parameterName))
                            {
                                parameterName = paramIndex.ToString();
                            }
                        }
                        else
                        {
                            parameterName = "__arglist";
                        }

                        Error(diagnostics, errorCode, syntax, symbol, parameterName);
                        return false;
                    }
                }
            }

            // handle omitted optional "in" parameters if there are any
            ParameterSymbol unmatchedInParameter = TryGetunmatchedInParameterAndFreeMatchedArgs(parameters, ref inParametersMatchedWithArgs);

            // unmatched "in" parameter is the same as a literal, its ref escape is scopeOfTheContainingExpression  (can't get any worse)
            //                                                    its val escape is ExternalScope                   (does not affect overall result)
            if (unmatchedInParameter != null && isRefEscape)
            {
                var parameterName = unmatchedInParameter.Name;
                if (string.IsNullOrEmpty(parameterName))
                {
                    parameterName = unmatchedInParameter.Ordinal.ToString();
                }
                Error(diagnostics, GetStandardCallEscapeError(checkingReceiver), syntax, symbol, parameterName);
                return false;
            }

            // check receiver if ref-like
            if (receiverOpt?.Type?.IsRefLikeType == true)
            {
                return CheckValEscape(receiverOpt.Syntax, receiverOpt, escapeFrom, escapeTo, false, diagnostics);
            }

            return true;
        }

        /// <summary>
        /// Validates whether the invocation is valid per no-mixing rules.
        /// Returns <see langword="false"/> when it is not valid and produces diagnostics (possibly more than one recursively) that helps to figure the reason.
        /// </summary>
        private static bool CheckInvocationArgMixing(
            SyntaxNode syntax,
            Symbol symbol,
            BoundExpression receiverOpt,
            ImmutableArray<ParameterSymbol> parameters,
            ImmutableArray<BoundExpression> argsOpt,
            ImmutableArray<int> argsToParamsOpt,
            uint scopeOfTheContainingExpression,
            DiagnosticBag diagnostics)
        {
            // SPEC:
            // In a method invocation, the following constraints apply:
            // - If there is a ref or out argument of a ref struct type (including the receiver), with safe-to-escape E1, then
            // - no argument (including the receiver) may have a narrower safe-to-escape than E1.

            if (!symbol.RequiresInstanceReceiver())
            {
                // ignore receiver when symbol is static
                receiverOpt = null;
            }

            // widest possible escape via writeable ref-like receiver or ref/out argument.
            uint escapeTo = scopeOfTheContainingExpression;

            // collect all writeable ref-like arguments, including receiver
            var receiverType = receiverOpt?.Type;
            if (receiverType?.IsRefLikeType == true && !isReceiverRefReadOnly(symbol))
            {
                escapeTo = GetValEscape(receiverOpt, scopeOfTheContainingExpression);
            }

            if (!argsOpt.IsDefault)
            {
                BoundArgListOperator argList = null;
                for (var argIndex = 0; argIndex < argsOpt.Length; argIndex++)
                {
                    var argument = argsOpt[argIndex];
                    if (argument.Kind == BoundKind.ArgListOperator)
                    {
                        Debug.Assert(argIndex == argsOpt.Length - 1, "vararg must be the last");
                        argList = (BoundArgListOperator)argument;
                        break;
                    }

                    var paramIndex = argsToParamsOpt.IsDefault ? argIndex : argsToParamsOpt[argIndex];
                    if (parameters[paramIndex].RefKind.IsWritableReference() && argument.Type?.IsRefLikeType == true)
                    {
                        escapeTo = Math.Min(escapeTo, GetValEscape(argument, scopeOfTheContainingExpression));
                    }
                }

                if (argList != null)
                {
                    var argListArgs = argList.Arguments;
                    var argListRefKindsOpt = argList.ArgumentRefKindsOpt;

                    for (var argIndex = 0; argIndex < argListArgs.Length; argIndex++)
                    {
                        var argument = argListArgs[argIndex];
                        var refKind = argListRefKindsOpt.IsDefault ? RefKind.None : argListRefKindsOpt[argIndex];
                        if (refKind.IsWritableReference() && argument.Type?.IsRefLikeType == true)
                        {
                            escapeTo = Math.Min(escapeTo, GetValEscape(argument, scopeOfTheContainingExpression));
                        }
                    }
                }
            }

            if (escapeTo == scopeOfTheContainingExpression)
            {
                // cannot fail. common case.
                return true;
            }

            if (!argsOpt.IsDefault)
            {
moreArguments:
                for (var argIndex = 0; argIndex < argsOpt.Length; argIndex++)
                {
                    // check val escape of all arguments
                    var argument = argsOpt[argIndex];
                    if (argument.Kind == BoundKind.ArgListOperator)
                    {
                        Debug.Assert(argIndex == argsOpt.Length - 1, "vararg must be the last");
                        var argList = (BoundArgListOperator)argument;

                        // unwrap varargs and process as more arguments
                        argsOpt = argList.Arguments;
                        parameters = ImmutableArray<ParameterSymbol>.Empty;
                        argsToParamsOpt = default;

                        goto moreArguments;
                    }

                    var valid = CheckValEscape(argument.Syntax, argument, scopeOfTheContainingExpression, escapeTo, false, diagnostics);

                    if (!valid)
                    {
                        string parameterName;
                        if (parameters.Length > 0)
                        {
                            var paramIndex = argsToParamsOpt.IsDefault ? argIndex : argsToParamsOpt[argIndex];
                            parameterName = parameters[paramIndex].Name;
                        }
                        else
                        {
                            parameterName = "__arglist";
                        }

                        Error(diagnostics, ErrorCode.ERR_CallArgMixing, syntax, symbol, parameterName);
                        return false;
                    }
                }
            }

            //NB: we do not care about unmatched "in" parameters here. 
            //    They have "outer" val escape, so cannot be worse than escapeTo.

            // check val escape of receiver if ref-like
            if (receiverOpt?.Type?.IsRefLikeType == true)
            {
                return CheckValEscape(receiverOpt.Syntax, receiverOpt, scopeOfTheContainingExpression, escapeTo, false, diagnostics);
            }

            return true;

            static bool isReceiverRefReadOnly(Symbol methodOrPropertySymbol) => methodOrPropertySymbol switch
            {
                MethodSymbol m => m.IsEffectivelyReadOnly,
                // TODO: val escape checks should be skipped for property accesses when
                // we can determine the only accessors being called are readonly.
                // For now we are pessimistic and check escape if any accessor is non-readonly.
                // Tracking in https://github.com/dotnet/roslyn/issues/35606
                PropertySymbol p => p.GetMethod?.IsEffectivelyReadOnly != false && p.SetMethod?.IsEffectivelyReadOnly != false,
                _ => throw ExceptionUtilities.UnexpectedValue(methodOrPropertySymbol)
            };
        }

        /// <summary>
        /// Gets "effective" ref kind of an argument. 
        /// If the ref kind is 'in', marks that that corresponding parameter was matched with a value
        /// We need that to detect when there were optional 'in' parameters for which values were not supplied.
        /// 
        /// NOTE: Generally we know if a formal argument is passed as ref/out/in by looking at the call site. 
        /// However, 'in' may also be passed as an ordinary val argument so we need to take a look at corresponding parameter, if such exists. 
        /// There are cases like params/vararg, when a corresponding parameter may not exist, then val cannot become 'in'.
        /// </summary>
        private static RefKind GetEffectiveRefKindAndMarkMatchedInParameter(
            int argIndex,
            ImmutableArray<RefKind> argRefKindsOpt,
            ImmutableArray<ParameterSymbol> parameters,
            ImmutableArray<int> argsToParamsOpt,
            ref ArrayBuilder<bool> inParametersMatchedWithArgs)
        {
            var effectiveRefKind = argRefKindsOpt.IsDefault ? RefKind.None : argRefKindsOpt[argIndex];
            if ((effectiveRefKind == RefKind.None || effectiveRefKind == RefKind.In) && argIndex < parameters.Length)
            {
                var paramIndex = argsToParamsOpt.IsDefault ? argIndex : argsToParamsOpt[argIndex];

                if (parameters[paramIndex].RefKind == RefKind.In)
                {
                    effectiveRefKind = RefKind.In;
                    inParametersMatchedWithArgs = inParametersMatchedWithArgs ?? ArrayBuilder<bool>.GetInstance(parameters.Length, fillWithValue: false);
                    inParametersMatchedWithArgs[paramIndex] = true;
                }
            }

            return effectiveRefKind;
        }

        /// <summary>
        /// Gets a "in" parameter for which there is no argument supplied, if such exists. 
        /// That indicates an optional "in" parameter. We treat it as an RValue passed by reference via a temporary.
        /// The effective scope of such variable is the immediately containing scope.
        /// </summary>
        private static ParameterSymbol TryGetunmatchedInParameterAndFreeMatchedArgs(ImmutableArray<ParameterSymbol> parameters, ref ArrayBuilder<bool> inParametersMatchedWithArgs)
        {
            try
            {
                if (!parameters.IsDefault)
                {
                    for (int i = 0; i < parameters.Length; i++)
                    {
                        var parameter = parameters[i];
                        if (parameter.IsParams)
                        {
                            break;
                        }

                        if (parameter.RefKind == RefKind.In &&
                            inParametersMatchedWithArgs?[i] != true &&
                            parameter.Type.IsRefLikeType == false)
                        {
                            return parameter;
                        }
                    }
                }

                return null;
            }
            finally
            {
                inParametersMatchedWithArgs?.Free();
                // make sure noone uses it after.
                inParametersMatchedWithArgs = null;
            }
        }

        private static ErrorCode GetStandardCallEscapeError(bool checkingReceiver)
        {
            return checkingReceiver ? ErrorCode.ERR_EscapeCall2 : ErrorCode.ERR_EscapeCall;
        }

        private static void ReportReadonlyLocalError(SyntaxNode node, LocalSymbol local, BindValueKind kind, bool checkingReceiver, DiagnosticBag diagnostics)
        {
            Debug.Assert((object)local != null);
            Debug.Assert(kind != BindValueKind.RValue);

            MessageID cause;
            if (local.IsForEach)
            {
                cause = MessageID.IDS_FOREACHLOCAL;
            }
            else if (local.IsUsing)
            {
                cause = MessageID.IDS_USINGLOCAL;
            }
            else if (local.IsFixed)
            {
                cause = MessageID.IDS_FIXEDLOCAL;
            }
            else
            {
                Error(diagnostics, GetStandardLvalueError(kind), node);
                return;
            }

            ErrorCode[] ReadOnlyLocalErrors =
            {
                ErrorCode.ERR_RefReadonlyLocalCause,
                ErrorCode.ERR_AssgReadonlyLocalCause,

                ErrorCode.ERR_RefReadonlyLocal2Cause,
                ErrorCode.ERR_AssgReadonlyLocal2Cause
            };

            int index = (checkingReceiver ? 2 : 0) + (RequiresRefOrOut(kind) ? 0 : 1);

            Error(diagnostics, ReadOnlyLocalErrors[index], node, local, cause.Localize());
        }

        private static ErrorCode GetThisLvalueError(BindValueKind kind, bool isValueType)
        {
            switch (kind)
            {
                case BindValueKind.CompoundAssignment:
                case BindValueKind.Assignable:
                    return ErrorCode.ERR_AssgReadonlyLocal;

                case BindValueKind.RefOrOut:
                    return ErrorCode.ERR_RefReadonlyLocal;

                case BindValueKind.AddressOf:
                    return ErrorCode.ERR_InvalidAddrOp;

                case BindValueKind.IncrementDecrement:
                    return isValueType ? ErrorCode.ERR_AssgReadonlyLocal : ErrorCode.ERR_IncrementLvalueExpected;

                case BindValueKind.RefReturn:
                case BindValueKind.ReadonlyRef:
                    return ErrorCode.ERR_RefReturnThis;

                case BindValueKind.RefAssignable:
                    return ErrorCode.ERR_RefLocalOrParamExpected;
            }

            if (RequiresReferenceToLocation(kind))
            {
                return ErrorCode.ERR_RefLvalueExpected;
            }

            throw ExceptionUtilities.UnexpectedValue(kind);
        }

        private static ErrorCode GetRangeLvalueError(BindValueKind kind)
        {
            switch (kind)
            {
                case BindValueKind.Assignable:
                case BindValueKind.CompoundAssignment:
                case BindValueKind.IncrementDecrement:
                    return ErrorCode.ERR_QueryRangeVariableReadOnly;

                case BindValueKind.AddressOf:
                    return ErrorCode.ERR_InvalidAddrOp;

                case BindValueKind.RefReturn:
                case BindValueKind.ReadonlyRef:
                    return ErrorCode.ERR_RefReturnRangeVariable;

                case BindValueKind.RefAssignable:
                    return ErrorCode.ERR_RefLocalOrParamExpected;
            }

            if (RequiresReferenceToLocation(kind))
            {
                return ErrorCode.ERR_QueryOutRefRangeVariable;
            }

            throw ExceptionUtilities.UnexpectedValue(kind);
        }

        private static ErrorCode GetMethodGroupOrFunctionPointerLvalueError(BindValueKind valueKind)
        {
            if (RequiresReferenceToLocation(valueKind))
            {
                return ErrorCode.ERR_RefReadonlyLocalCause;
            }

            // Cannot assign to 'W' because it is a 'method group'
            return ErrorCode.ERR_AssgReadonlyLocalCause;
        }

        private static ErrorCode GetStandardLvalueError(BindValueKind kind)
        {
            switch (kind)
            {
                case BindValueKind.CompoundAssignment:
                case BindValueKind.Assignable:
                    return ErrorCode.ERR_AssgLvalueExpected;

                case BindValueKind.AddressOf:
                    return ErrorCode.ERR_InvalidAddrOp;

                case BindValueKind.IncrementDecrement:
                    return ErrorCode.ERR_IncrementLvalueExpected;

                case BindValueKind.FixedReceiver:
                    return ErrorCode.ERR_FixedNeedsLvalue;

                case BindValueKind.RefReturn:
                case BindValueKind.ReadonlyRef:
                    return ErrorCode.ERR_RefReturnLvalueExpected;

                case BindValueKind.RefAssignable:
                    return ErrorCode.ERR_RefLocalOrParamExpected;
            }

            if (RequiresReferenceToLocation(kind))
            {
                return ErrorCode.ERR_RefLvalueExpected;
            }

            throw ExceptionUtilities.UnexpectedValue(kind);
        }

        private static ErrorCode GetStandardRValueRefEscapeError(uint escapeTo)
        {
            if (escapeTo == Binder.ExternalScope)
            {
                return ErrorCode.ERR_RefReturnLvalueExpected;
            }

            return ErrorCode.ERR_EscapeOther;
        }

        private static void ReportReadOnlyFieldError(FieldSymbol field, SyntaxNode node, BindValueKind kind, bool checkingReceiver, DiagnosticBag diagnostics)
        {
            Debug.Assert((object)field != null);
            Debug.Assert(RequiresAssignableVariable(kind));
            Debug.Assert(field.Type != (object)null);

            // It's clearer to say that the address can't be taken than to say that the field can't be modified
            // (even though the latter message gives more explanation of why).
            if (kind == BindValueKind.AddressOf)
            {
                Error(diagnostics, ErrorCode.ERR_InvalidAddrOp, node);
                return;
            }

            ErrorCode[] ReadOnlyErrors =
            {
                ErrorCode.ERR_RefReturnReadonly,
                ErrorCode.ERR_RefReadonly,
                ErrorCode.ERR_AssgReadonly,
                ErrorCode.ERR_RefReturnReadonlyStatic,
                ErrorCode.ERR_RefReadonlyStatic,
                ErrorCode.ERR_AssgReadonlyStatic,
                ErrorCode.ERR_RefReturnReadonly2,
                ErrorCode.ERR_RefReadonly2,
                ErrorCode.ERR_AssgReadonly2,
                ErrorCode.ERR_RefReturnReadonlyStatic2,
                ErrorCode.ERR_RefReadonlyStatic2,
                ErrorCode.ERR_AssgReadonlyStatic2
            };
            int index = (checkingReceiver ? 6 : 0) + (field.IsStatic ? 3 : 0) + (kind == BindValueKind.RefReturn ? 0 : (RequiresRefOrOut(kind) ? 1 : 2));
            if (checkingReceiver)
            {
                Error(diagnostics, ReadOnlyErrors[index], node, field);
            }
            else
            {
                Error(diagnostics, ReadOnlyErrors[index], node);
            }
        }

        private static void ReportReadOnlyError(Symbol symbol, SyntaxNode node, BindValueKind kind, bool checkingReceiver, DiagnosticBag diagnostics)
        {
            Debug.Assert((object)symbol != null);
            Debug.Assert(RequiresAssignableVariable(kind));

            // It's clearer to say that the address can't be taken than to say that the parameter can't be modified
            // (even though the latter message gives more explanation of why).
            if (kind == BindValueKind.AddressOf)
            {
                Error(diagnostics, ErrorCode.ERR_InvalidAddrOp, node);
                return;
            }

            var symbolKind = symbol.Kind.Localize();

            ErrorCode[] ReadOnlyErrors =
            {
                ErrorCode.ERR_RefReturnReadonlyNotField,
                ErrorCode.ERR_RefReadonlyNotField,
                ErrorCode.ERR_AssignReadonlyNotField,
                ErrorCode.ERR_RefReturnReadonlyNotField2,
                ErrorCode.ERR_RefReadonlyNotField2,
                ErrorCode.ERR_AssignReadonlyNotField2,
            };

            int index = (checkingReceiver ? 3 : 0) + (kind == BindValueKind.RefReturn ? 0 : (RequiresRefOrOut(kind) ? 1 : 2));
            Error(diagnostics, ReadOnlyErrors[index], node, symbolKind, symbol);
        }

        /// <summary>
        /// Checks whether given expression can escape from the current scope to the <paramref name="escapeTo"/>
        /// In a case if it cannot a bad expression is returned and diagnostics is produced.
        /// </summary>
        internal BoundExpression ValidateEscape(BoundExpression expr, uint escapeTo, bool isByRef, DiagnosticBag diagnostics)
        {
            if (isByRef)
            {
                if (CheckRefEscape(expr.Syntax, expr, this.LocalScopeDepth, escapeTo, checkingReceiver: false, diagnostics: diagnostics))
                {
                    return expr;
                }
            }
            else
            {
                if (CheckValEscape(expr.Syntax, expr, this.LocalScopeDepth, escapeTo, checkingReceiver: false, diagnostics: diagnostics))
                {
                    return expr;
                }
            }

            return ToBadExpression(expr);
        }

        /// <summary>
        /// Computes the widest scope depth to which the given expression can escape by reference.
        /// 
        /// NOTE: in a case if expression cannot be passed by an alias (RValue and similar), the ref-escape is scopeOfTheContainingExpression
        ///       There are few cases where RValues are permitted to be passed by reference which implies that a temporary local proxy is passed instead.
        ///       We reflect such behavior by constraining the escape value to the narrowest scope possible. 
        /// </summary>
        internal static uint GetRefEscape(BoundExpression expr, uint scopeOfTheContainingExpression)
        {
            // cannot infer anything from errors
            if (expr.HasAnyErrors)
            {
                return Binder.ExternalScope;
            }

            // cannot infer anything from Void (broken code)
            if (expr.Type?.GetSpecialTypeSafe() == SpecialType.System_Void)
            {
                return Binder.ExternalScope;
            }

            // constants/literals cannot ref-escape current scope
            if (expr.ConstantValue != null)
            {
                return scopeOfTheContainingExpression;
            }

            // cover case that cannot refer to local state
            // otherwise default to current scope (RValues, etc)
            switch (expr.Kind)
            {
                case BoundKind.ArrayAccess:
                case BoundKind.PointerIndirectionOperator:
                case BoundKind.PointerElementAccess:
                    // array elements and pointer dereferencing are readwrite variables
                    return Binder.ExternalScope;

                case BoundKind.RefValueOperator:
                    // The undocumented __refvalue(tr, T) expression results in an lvalue of type T.
                    // for compat reasons it is not ref-returnable (since TypedReference is not val-returnable)
                    // it can, however, ref-escape to any other level (since TypedReference can val-escape to any other level)
                    return Binder.TopLevelScope;

                case BoundKind.DiscardExpression:
                    // same as write-only byval local
                    break;

                case BoundKind.DynamicMemberAccess:
                case BoundKind.DynamicIndexerAccess:
                    // dynamic expressions can be read and written to
                    // can even be passed by reference (which is implemented via a temp)
                    // it is not valid to escape them by reference though, so treat them as RValues here
                    break;

                case BoundKind.Parameter:
                    var parameter = ((BoundParameter)expr).ParameterSymbol;

                    // byval parameters can escape to method's top level. Others can escape further.
                    // NOTE: "method" here means nearest containing method, lambda or local function.
                    return parameter.RefKind == RefKind.None ? Binder.TopLevelScope : Binder.ExternalScope;

                case BoundKind.Local:
                    return ((BoundLocal)expr).LocalSymbol.RefEscapeScope;

                case BoundKind.ThisReference:
                    var thisref = (BoundThisReference)expr;

                    // "this" is an RValue, unless in a struct.
                    if (!thisref.Type.IsValueType)
                    {
                        break;
                    }

                    //"this" is not returnable by reference in a struct.
                    // can ref escape to any other level
                    return Binder.TopLevelScope;

                case BoundKind.ConditionalOperator:
                    var conditional = (BoundConditionalOperator)expr;

                    if (conditional.IsRef)
                    {
                        // ref conditional defers to its operands
                        return Math.Max(GetRefEscape(conditional.Consequence, scopeOfTheContainingExpression),
                                        GetRefEscape(conditional.Alternative, scopeOfTheContainingExpression));
                    }

                    // otherwise it is an RValue
                    break;

                case BoundKind.FieldAccess:
                    var fieldAccess = (BoundFieldAccess)expr;
                    var fieldSymbol = fieldAccess.FieldSymbol;

                    // fields that are static or belong to reference types can ref escape anywhere
                    if (fieldSymbol.IsStatic || fieldSymbol.ContainingType.IsReferenceType)
                    {
                        return Binder.ExternalScope;
                    }

                    // for other fields defer to the receiver.
                    return GetRefEscape(fieldAccess.ReceiverOpt, scopeOfTheContainingExpression);

                case BoundKind.EventAccess:
                    var eventAccess = (BoundEventAccess)expr;
                    if (!eventAccess.IsUsableAsField)
                    {
                        // not field-like events are RValues
                        break;
                    }

                    var eventSymbol = eventAccess.EventSymbol;

                    // field-like events that are static or belong to reference types can ref escape anywhere
                    if (eventSymbol.IsStatic || eventSymbol.ContainingType.IsReferenceType)
                    {
                        return Binder.ExternalScope;
                    }

                    // for other events defer to the receiver.
                    return GetRefEscape(eventAccess.ReceiverOpt, scopeOfTheContainingExpression);

                case BoundKind.Call:
                    var call = (BoundCall)expr;

                    var methodSymbol = call.Method;
                    if (methodSymbol.RefKind == RefKind.None)
                    {
                        break;
                    }

                    return GetInvocationEscapeScope(
                        call.Method,
                        call.ReceiverOpt,
                        methodSymbol.Parameters,
                        call.Arguments,
                        call.ArgumentRefKindsOpt,
                        call.ArgsToParamsOpt,
                        scopeOfTheContainingExpression,
                        isRefEscape: true);

                case BoundKind.FunctionPointerInvocation:
                    var ptrInvocation = (BoundFunctionPointerInvocation)expr;

                    methodSymbol = ptrInvocation.FunctionPointer.Signature;
                    if (methodSymbol.RefKind == RefKind.None)
                    {
                        break;
                    }

                    return GetInvocationEscapeScope(
                        methodSymbol,
                        receiverOpt: null,
                        methodSymbol.Parameters,
                        ptrInvocation.Arguments,
                        ptrInvocation.ArgumentRefKindsOpt,
                        argsToParamsOpt: default,
                        scopeOfTheContainingExpression,
                        isRefEscape: true);

                case BoundKind.IndexerAccess:
                    var indexerAccess = (BoundIndexerAccess)expr;
                    var indexerSymbol = indexerAccess.Indexer;

                    return GetInvocationEscapeScope(
                        indexerSymbol,
                        indexerAccess.ReceiverOpt,
                        indexerSymbol.Parameters,
                        indexerAccess.Arguments,
                        indexerAccess.ArgumentRefKindsOpt,
                        indexerAccess.ArgsToParamsOpt,
                        scopeOfTheContainingExpression,
                        isRefEscape: true);

                case BoundKind.PropertyAccess:
                    var propertyAccess = (BoundPropertyAccess)expr;

                    // not passing any arguments/parameters
                    return GetInvocationEscapeScope(
                        propertyAccess.PropertySymbol,
                        propertyAccess.ReceiverOpt,
                        default,
                        default,
                        default,
                        default,
                        scopeOfTheContainingExpression,
                        isRefEscape: true);

                case BoundKind.AssignmentOperator:
                    var assignment = (BoundAssignmentOperator)expr;

                    if (!assignment.IsRef)
                    {
                        // non-ref assignments are RValues
                        break;
                    }

                    return GetRefEscape(assignment.Left, scopeOfTheContainingExpression);
            }

            // At this point we should have covered all the possible cases for anything that is not a strict RValue.
            return scopeOfTheContainingExpression;
        }

        /// <summary>
        /// A counterpart to the GetRefEscape, which validates if given escape demand can be met by the expression.
        /// The result indicates whether the escape is possible. 
        /// Additionally, the method emits diagnostics (possibly more than one, recursively) that would help identify the cause for the failure.
        /// </summary>
        internal static bool CheckRefEscape(SyntaxNode node, BoundExpression expr, uint escapeFrom, uint escapeTo, bool checkingReceiver, DiagnosticBag diagnostics)
        {
            Debug.Assert(!checkingReceiver || expr.Type.IsValueType || expr.Type.IsTypeParameter());

            if (escapeTo >= escapeFrom)
            {
                // escaping to same or narrower scope is ok.
                return true;
            }

            if (expr.HasAnyErrors)
            {
                // already an error
                return true;
            }

            // void references cannot escape (error should be reported somewhere)
            if (expr.Type?.GetSpecialTypeSafe() == SpecialType.System_Void)
            {
                return true;
            }

            // references to constants/literals cannot escape higher.
            if (expr.ConstantValue != null)
            {
                Error(diagnostics, GetStandardRValueRefEscapeError(escapeTo), node);
                return false;
            }

            switch (expr.Kind)
            {
                case BoundKind.ArrayAccess:
                case BoundKind.PointerIndirectionOperator:
                case BoundKind.PointerElementAccess:
                    // array elements and pointer dereferencing are readwrite variables
                    return true;

                case BoundKind.RefValueOperator:
                    // The undocumented __refvalue(tr, T) expression results in an lvalue of type T.
                    // for compat reasons it is not ref-returnable (since TypedReference is not val-returnable)
                    if (escapeTo == Binder.ExternalScope)
                    {
                        break;
                    }

                    // it can, however, ref-escape to any other level (since TypedReference can val-escape to any other level)
                    return true;

                case BoundKind.DiscardExpression:
                    // same as write-only byval local
                    break;

                case BoundKind.DynamicMemberAccess:
                case BoundKind.DynamicIndexerAccess:
                    // dynamic expressions can be read and written to
                    // can even be passed by reference (which is implemented via a temp)
                    // it is not valid to escape them by reference though.
                    break;

                case BoundKind.Parameter:
                    var parameter = (BoundParameter)expr;
                    return CheckParameterRefEscape(node, parameter, escapeTo, checkingReceiver, diagnostics);

                case BoundKind.Local:
                    var local = (BoundLocal)expr;
                    return CheckLocalRefEscape(node, local, escapeTo, checkingReceiver, diagnostics);

                case BoundKind.ThisReference:
                    var thisref = (BoundThisReference)expr;

                    // "this" is an RValue, unless in a struct.
                    if (!thisref.Type.IsValueType)
                    {
                        break;
                    }

                    //"this" is not returnable by reference in a struct.
                    if (escapeTo == Binder.ExternalScope)
                    {
                        Error(diagnostics, ErrorCode.ERR_RefReturnStructThis, node, ThisParameterSymbol.SymbolName);
                        return false;
                    }

                    // can ref escape to any other level
                    return true;

                case BoundKind.ConditionalOperator:
                    var conditional = (BoundConditionalOperator)expr;

                    if (conditional.IsRef)
                    {
                        return CheckRefEscape(conditional.Consequence.Syntax, conditional.Consequence, escapeFrom, escapeTo, checkingReceiver: false, diagnostics: diagnostics) &&
                               CheckRefEscape(conditional.Alternative.Syntax, conditional.Alternative, escapeFrom, escapeTo, checkingReceiver: false, diagnostics: diagnostics);
                    }

                    // report standard lvalue error
                    break;

                case BoundKind.FieldAccess:
                    var fieldAccess = (BoundFieldAccess)expr;
                    return CheckFieldRefEscape(node, fieldAccess, escapeFrom, escapeTo, diagnostics);

                case BoundKind.EventAccess:
                    var eventAccess = (BoundEventAccess)expr;
                    if (!eventAccess.IsUsableAsField)
                    {
                        // not field-like events are RValues
                        break;
                    }

                    return CheckFieldLikeEventRefEscape(node, eventAccess, escapeFrom, escapeTo, diagnostics);

                case BoundKind.Call:
                    var call = (BoundCall)expr;

                    var methodSymbol = call.Method;
                    if (methodSymbol.RefKind == RefKind.None)
                    {
                        break;
                    }

                    return CheckInvocationEscape(
                        call.Syntax,
                        methodSymbol,
                        call.ReceiverOpt,
                        methodSymbol.Parameters,
                        call.Arguments,
                        call.ArgumentRefKindsOpt,
                        call.ArgsToParamsOpt,
                        checkingReceiver,
                        escapeFrom,
                        escapeTo,
                        diagnostics,
                        isRefEscape: true);

                case BoundKind.IndexerAccess:
                    var indexerAccess = (BoundIndexerAccess)expr;
                    var indexerSymbol = indexerAccess.Indexer;

                    if (indexerSymbol.RefKind == RefKind.None)
                    {
                        break;
                    }

                    return CheckInvocationEscape(
                        indexerAccess.Syntax,
                        indexerSymbol,
                        indexerAccess.ReceiverOpt,
                        indexerSymbol.Parameters,
                        indexerAccess.Arguments,
                        indexerAccess.ArgumentRefKindsOpt,
                        indexerAccess.ArgsToParamsOpt,
                        checkingReceiver,
                        escapeFrom,
                        escapeTo,
                        diagnostics,
                        isRefEscape: true);

                case BoundKind.IndexOrRangePatternIndexerAccess:
                    var patternIndexer = (BoundIndexOrRangePatternIndexerAccess)expr;
                    RefKind refKind;
                    ImmutableArray<ParameterSymbol> parameters;

                    switch (patternIndexer.PatternSymbol)
                    {
                        case PropertySymbol p:
                            refKind = p.RefKind;
                            parameters = p.Parameters;
                            break;
                        case MethodSymbol m:
                            refKind = m.RefKind;
                            parameters = m.Parameters;
                            break;
                        default:
                            throw ExceptionUtilities.Unreachable;
                    }

                    if (refKind == RefKind.None)
                    {
                        break;
                    }

                    return CheckInvocationEscape(
                        patternIndexer.Syntax,
                        patternIndexer.PatternSymbol,
                        patternIndexer.Receiver,
                        parameters,
                        ImmutableArray.Create<BoundExpression>(patternIndexer.Argument),
                        default,
                        default,
                        checkingReceiver,
                        escapeFrom,
                        escapeTo,
                        diagnostics,
                        isRefEscape: true);

                case BoundKind.FunctionPointerInvocation:
                    var functionPointerInvocation = (BoundFunctionPointerInvocation)expr;

                    FunctionPointerMethodSymbol signature = functionPointerInvocation.FunctionPointer.Signature;
                    if (signature.RefKind == RefKind.None)
                    {
                        break;
                    }

                    return CheckInvocationEscape(
                        functionPointerInvocation.Syntax,
                        signature,
                        functionPointerInvocation.InvokedExpression,
                        signature.Parameters,
                        functionPointerInvocation.Arguments,
                        functionPointerInvocation.ArgumentRefKindsOpt,
                        argsToParamsOpt: default,
                        checkingReceiver,
                        escapeFrom,
                        escapeTo,
                        diagnostics,
                        isRefEscape: true);

                case BoundKind.PropertyAccess:
                    var propertyAccess = (BoundPropertyAccess)expr;
                    var propertySymbol = propertyAccess.PropertySymbol;

                    if (propertySymbol.RefKind == RefKind.None)
                    {
                        break;
                    }

                    // not passing any arguments/parameters
                    return CheckInvocationEscape(
                        propertyAccess.Syntax,
                        propertySymbol,
                        propertyAccess.ReceiverOpt,
                        default,
                        default,
                        default,
                        default,
                        checkingReceiver,
                        escapeFrom,
                        escapeTo,
                        diagnostics,
                        isRefEscape: true);

                case BoundKind.AssignmentOperator:
                    var assignment = (BoundAssignmentOperator)expr;

                    // Only ref-assignments can be LValues
                    if (!assignment.IsRef)
                    {
                        break;
                    }

                    return CheckRefEscape(
                        node,
                        assignment.Left,
                        escapeFrom,
                        escapeTo,
                        checkingReceiver: false,
                        diagnostics);
            }

            // At this point we should have covered all the possible cases for anything that is not a strict RValue.
            Error(diagnostics, GetStandardRValueRefEscapeError(escapeTo), node);
            return false;
        }

        internal static uint GetBroadestValEscape(BoundTupleExpression expr, uint scopeOfTheContainingExpression)
        {
            uint broadest = scopeOfTheContainingExpression;
            foreach (var element in expr.Arguments)
            {
                uint valEscape;
                if (element is BoundTupleExpression te)
                {
                    valEscape = GetBroadestValEscape(te, scopeOfTheContainingExpression);
                }
                else
                {
                    valEscape = GetValEscape(element, scopeOfTheContainingExpression);
                }

                broadest = Math.Min(broadest, valEscape);
            }

            return broadest;
        }

        /// <summary>
        /// Computes the widest scope depth to which the given expression can escape by value.
        /// 
        /// NOTE: unless the type of expression is ref-like, the result is Binder.ExternalScope since ordinary values can always be returned from methods. 
        /// </summary>
        internal static uint GetValEscape(BoundExpression expr, uint scopeOfTheContainingExpression)
        {
            // cannot infer anything from errors
            if (expr.HasAnyErrors)
            {
                return Binder.ExternalScope;
            }

            // constants/literals cannot refer to local state
            if (expr.ConstantValue != null)
            {
                return Binder.ExternalScope;
            }

            // to have local-referring values an expression must have a ref-like type
            if (expr.Type?.IsRefLikeType != true)
            {
                return Binder.ExternalScope;
            }

            // cover case that can refer to local state
            // otherwise default to ExternalScope (ordinary values)
            switch (expr.Kind)
            {
                case BoundKind.DefaultLiteral:
                case BoundKind.DefaultExpression:
                case BoundKind.Parameter:
                case BoundKind.ThisReference:
                    // always returnable
                    return Binder.ExternalScope;

                case BoundKind.TupleLiteral:
                case BoundKind.ConvertedTupleLiteral:
                    var tupleLiteral = (BoundTupleExpression)expr;
                    return GetTupleValEscape(tupleLiteral.Arguments, scopeOfTheContainingExpression);

                case BoundKind.MakeRefOperator:
                case BoundKind.RefValueOperator:
                    // for compat reasons
                    // NB: it also means can`t assign stackalloc spans to a __refvalue
                    //     we are ok with that.
                    return Binder.ExternalScope;

                case BoundKind.DiscardExpression:
                    // same as uninitialized local
                    return Binder.ExternalScope;

                case BoundKind.DeconstructValuePlaceholder:
                    return ((BoundDeconstructValuePlaceholder)expr).ValEscape;

                case BoundKind.Local:
                    return ((BoundLocal)expr).LocalSymbol.ValEscapeScope;

                case BoundKind.StackAllocArrayCreation:
                case BoundKind.ConvertedStackAllocExpression:
                    return Binder.TopLevelScope;

                case BoundKind.ConditionalOperator:
                    var conditional = (BoundConditionalOperator)expr;

                    var consEscape = GetValEscape(conditional.Consequence, scopeOfTheContainingExpression);

                    if (conditional.IsRef)
                    {
                        // ref conditional defers to one operand. 
                        // the other one is the same or we will be reporting errors anyways.
                        return consEscape;
                    }

                    // val conditional gets narrowest of its operands
                    return Math.Max(consEscape,
                                    GetValEscape(conditional.Alternative, scopeOfTheContainingExpression));

                case BoundKind.NullCoalescingOperator:
                    var coalescingOp = (BoundNullCoalescingOperator)expr;

                    return Math.Max(GetValEscape(coalescingOp.LeftOperand, scopeOfTheContainingExpression),
                                    GetValEscape(coalescingOp.RightOperand, scopeOfTheContainingExpression));

                case BoundKind.FieldAccess:
                    var fieldAccess = (BoundFieldAccess)expr;
                    var fieldSymbol = fieldAccess.FieldSymbol;

                    if (fieldSymbol.IsStatic || !fieldSymbol.ContainingType.IsRefLikeType)
                    {
                        // Already an error state.
                        return Binder.ExternalScope;
                    }

                    // for ref-like fields defer to the receiver.
                    return GetValEscape(fieldAccess.ReceiverOpt, scopeOfTheContainingExpression);

                case BoundKind.Call:
                    var call = (BoundCall)expr;

                    return GetInvocationEscapeScope(
                        call.Method,
                        call.ReceiverOpt,
                        call.Method.Parameters,
                        call.Arguments,
                        call.ArgumentRefKindsOpt,
                        call.ArgsToParamsOpt,
                        scopeOfTheContainingExpression,
                        isRefEscape: false);

                case BoundKind.FunctionPointerInvocation:
                    var ptrInvocation = (BoundFunctionPointerInvocation)expr;
                    var ptrSymbol = ptrInvocation.FunctionPointer.Signature;

                    return GetInvocationEscapeScope(
                        ptrSymbol,
                        receiverOpt: null,
                        ptrSymbol.Parameters,
                        ptrInvocation.Arguments,
                        ptrInvocation.ArgumentRefKindsOpt,
                        argsToParamsOpt: default,
                        scopeOfTheContainingExpression,
                        isRefEscape: false);

                case BoundKind.IndexerAccess:
                    var indexerAccess = (BoundIndexerAccess)expr;
                    var indexerSymbol = indexerAccess.Indexer;

                    return GetInvocationEscapeScope(
                        indexerSymbol,
                        indexerAccess.ReceiverOpt,
                        indexerSymbol.Parameters,
                        indexerAccess.Arguments,
                        indexerAccess.ArgumentRefKindsOpt,
                        indexerAccess.ArgsToParamsOpt,
                        scopeOfTheContainingExpression,
                        isRefEscape: false);

                case BoundKind.IndexOrRangePatternIndexerAccess:
                    var patternIndexer = (BoundIndexOrRangePatternIndexerAccess)expr;
                    var parameters = patternIndexer.PatternSymbol switch
                    {
                        PropertySymbol p => p.Parameters,
                        MethodSymbol m => m.Parameters,
                        _ => throw ExceptionUtilities.UnexpectedValue(patternIndexer.PatternSymbol)
                    };

                    return GetInvocationEscapeScope(
                        patternIndexer.PatternSymbol,
                        patternIndexer.Receiver,
                        parameters,
                        default,
                        default,
                        default,
                        scopeOfTheContainingExpression,
                        isRefEscape: false);

                case BoundKind.PropertyAccess:
                    var propertyAccess = (BoundPropertyAccess)expr;

                    // not passing any arguments/parameters
                    return GetInvocationEscapeScope(
                        propertyAccess.PropertySymbol,
                        propertyAccess.ReceiverOpt,
                        default,
                        default,
                        default,
                        default,
                        scopeOfTheContainingExpression,
                        isRefEscape: false);

                case BoundKind.ObjectCreationExpression:
                    var objectCreation = (BoundObjectCreationExpression)expr;
                    var constructorSymbol = objectCreation.Constructor;

                    var escape = GetInvocationEscapeScope(
                        constructorSymbol,
                        null,
                        constructorSymbol.Parameters,
                        objectCreation.Arguments,
                        objectCreation.ArgumentRefKindsOpt,
                        objectCreation.ArgsToParamsOpt,
                        scopeOfTheContainingExpression,
                        isRefEscape: false);

                    var initializerOpt = objectCreation.InitializerExpressionOpt;
                    if (initializerOpt != null)
                    {
                        escape = Math.Max(escape, GetValEscape(initializerOpt, scopeOfTheContainingExpression));
                    }

                    return escape;

                case BoundKind.UnaryOperator:
                    return GetValEscape(((BoundUnaryOperator)expr).Operand, scopeOfTheContainingExpression);

                case BoundKind.Conversion:
                    var conversion = (BoundConversion)expr;
                    Debug.Assert(conversion.ConversionKind != ConversionKind.StackAllocToSpanType, "StackAllocToSpanType unexpected");

                    return GetValEscape(conversion.Operand, scopeOfTheContainingExpression);

                case BoundKind.AssignmentOperator:
                    return GetValEscape(((BoundAssignmentOperator)expr).Right, scopeOfTheContainingExpression);

                case BoundKind.IncrementOperator:
                    return GetValEscape(((BoundIncrementOperator)expr).Operand, scopeOfTheContainingExpression);

                case BoundKind.CompoundAssignmentOperator:
                    var compound = (BoundCompoundAssignmentOperator)expr;

                    return Math.Max(GetValEscape(compound.Left, scopeOfTheContainingExpression),
                                    GetValEscape(compound.Right, scopeOfTheContainingExpression));

                case BoundKind.BinaryOperator:
                    var binary = (BoundBinaryOperator)expr;

                    return Math.Max(GetValEscape(binary.Left, scopeOfTheContainingExpression),
                                    GetValEscape(binary.Right, scopeOfTheContainingExpression));

                case BoundKind.UserDefinedConditionalLogicalOperator:
                    var uo = (BoundUserDefinedConditionalLogicalOperator)expr;

                    return Math.Max(GetValEscape(uo.Left, scopeOfTheContainingExpression),
                                    GetValEscape(uo.Right, scopeOfTheContainingExpression));

                case BoundKind.QueryClause:
                    return GetValEscape(((BoundQueryClause)expr).Value, scopeOfTheContainingExpression);

                case BoundKind.RangeVariable:
                    return GetValEscape(((BoundRangeVariable)expr).Value, scopeOfTheContainingExpression);

                case BoundKind.ObjectInitializerExpression:
                    var initExpr = (BoundObjectInitializerExpression)expr;
                    return GetValEscapeOfObjectInitializer(initExpr, scopeOfTheContainingExpression);

                case BoundKind.CollectionInitializerExpression:
                    var colExpr = (BoundCollectionInitializerExpression)expr;
                    return GetValEscape(colExpr.Initializers, scopeOfTheContainingExpression);

                case BoundKind.CollectionElementInitializer:
                    var colElement = (BoundCollectionElementInitializer)expr;
                    return GetValEscape(colElement.Arguments, scopeOfTheContainingExpression);

                case BoundKind.ObjectInitializerMember:
                    // this node generally makes no sense outside of the context of containing initializer
                    // however binder uses it as a placeholder when binding assignments inside an object initializer
                    // just say it does not escape anywhere, so that we do not get false errors.
                    return scopeOfTheContainingExpression;

                case BoundKind.ImplicitReceiver:
                case BoundKind.ObjectOrCollectionValuePlaceholder:
                    // binder uses this as a placeholder when binding members inside an object initializer
                    // just say it does not escape anywhere, so that we do not get false errors.
                    return scopeOfTheContainingExpression;

                case BoundKind.DisposableValuePlaceholder:
                    // Disposable value placeholder is only ever used to lookup a pattern dispose method
                    // then immediately discarded. The actual expression will be generated during lowering 
                    return scopeOfTheContainingExpression;

                case BoundKind.AwaitableValuePlaceholder:
                    return ((BoundAwaitableValuePlaceholder)expr).ValEscape;

                case BoundKind.PointerElementAccess:
                case BoundKind.PointerIndirectionOperator:
                    // Unsafe code will always be allowed to escape.
                    return Binder.ExternalScope;

                case BoundKind.AsOperator:
                case BoundKind.AwaitExpression:
                case BoundKind.ConditionalAccess:
                case BoundKind.ArrayAccess:
                    // only possible in error cases (if possible at all)
                    return scopeOfTheContainingExpression;

                case BoundKind.ConvertedSwitchExpression:
                case BoundKind.UnconvertedSwitchExpression:
                    var switchExpr = (BoundSwitchExpression)expr;
                    return GetValEscape(switchExpr.SwitchArms.SelectAsArray(a => a.Value), scopeOfTheContainingExpression);

                default:
                    // in error situations some unexpected nodes could make here
                    // returning "scopeOfTheContainingExpression" seems safer than throwing.
                    // we will still assert to make sure that all nodes are accounted for. 
                    Debug.Assert(false, $"{expr.Kind} expression of {expr.Type} type");
                    return scopeOfTheContainingExpression;
            }
        }

        private static uint GetTupleValEscape(ImmutableArray<BoundExpression> elements, uint scopeOfTheContainingExpression)
        {
            uint narrowestScope = scopeOfTheContainingExpression;
            foreach (var element in elements)
            {
                narrowestScope = Math.Max(narrowestScope, GetValEscape(element, scopeOfTheContainingExpression));
            }

            return narrowestScope;
        }

        private static uint GetValEscapeOfObjectInitializer(BoundObjectInitializerExpression initExpr, uint scopeOfTheContainingExpression)
        {
            var result = Binder.ExternalScope;
            foreach (var expression in initExpr.Initializers)
            {
                if (expression.Kind == BoundKind.AssignmentOperator)
                {
                    var assignment = (BoundAssignmentOperator)expression;
                    result = Math.Max(result, GetValEscape(assignment.Right, scopeOfTheContainingExpression));

                    var left = (BoundObjectInitializerMember)assignment.Left;
                    result = Math.Max(result, GetValEscape(left.Arguments, scopeOfTheContainingExpression));
                }
                else
                {
                    result = Math.Max(result, GetValEscape(expression, scopeOfTheContainingExpression));
                }
            }

            return result;
        }

        private static uint GetValEscape(ImmutableArray<BoundExpression> expressions, uint scopeOfTheContainingExpression)
        {
            var result = Binder.ExternalScope;
            foreach (var expression in expressions)
            {
                result = Math.Max(result, GetValEscape(expression, scopeOfTheContainingExpression));
            }

            return result;
        }

        /// <summary>
        /// A counterpart to the GetValEscape, which validates if given escape demand can be met by the expression.
        /// The result indicates whether the escape is possible.
        /// Additionally, the method emits diagnostics (possibly more than one, recursively) that would help identify the cause for the failure.
        /// </summary>
        internal static bool CheckValEscape(SyntaxNode node, BoundExpression expr, uint escapeFrom, uint escapeTo, bool checkingReceiver, DiagnosticBag diagnostics)
        {
            Debug.Assert(!checkingReceiver || expr.Type.IsValueType || expr.Type.IsTypeParameter());

            if (escapeTo >= escapeFrom)
            {
                // escaping to same or narrower scope is ok.
                return true;
            }

            // cannot infer anything from errors
            if (expr.HasAnyErrors)
            {
                return true;
            }

            // constants/literals cannot refer to local state
            if (expr.ConstantValue != null)
            {
                return true;
            }

            // to have local-referring values an expression must have a ref-like type
            if (expr.Type?.IsRefLikeType != true)
            {
                return true;
            }

            switch (expr.Kind)
            {
                case BoundKind.DefaultLiteral:
                case BoundKind.DefaultExpression:
                case BoundKind.Parameter:
                case BoundKind.ThisReference:
                    // always returnable
                    return true;

                case BoundKind.TupleLiteral:
                case BoundKind.ConvertedTupleLiteral:
                    var tupleLiteral = (BoundTupleExpression)expr;
                    return CheckTupleValEscape(tupleLiteral.Arguments, escapeFrom, escapeTo, diagnostics);

                case BoundKind.MakeRefOperator:
                case BoundKind.RefValueOperator:
                    // for compat reasons
                    return true;

                case BoundKind.DiscardExpression:
                    // same as uninitialized local
                    return true;

                case BoundKind.DeconstructValuePlaceholder:
                    if (((BoundDeconstructValuePlaceholder)expr).ValEscape > escapeTo)
                    {
                        Error(diagnostics, ErrorCode.ERR_EscapeLocal, node, expr.Syntax);
                        return false;
                    }
                    return true;

                case BoundKind.AwaitableValuePlaceholder:
                    if (((BoundAwaitableValuePlaceholder)expr).ValEscape > escapeTo)
                    {
                        Error(diagnostics, ErrorCode.ERR_EscapeLocal, node, expr.Syntax);
                        return false;
                    }
                    return true;

                case BoundKind.Local:
                    var localSymbol = ((BoundLocal)expr).LocalSymbol;
                    if (localSymbol.ValEscapeScope > escapeTo)
                    {
                        Error(diagnostics, ErrorCode.ERR_EscapeLocal, node, localSymbol);
                        return false;
                    }
                    return true;

                case BoundKind.StackAllocArrayCreation:
                case BoundKind.ConvertedStackAllocExpression:
                    if (escapeTo < Binder.TopLevelScope)
                    {
                        Error(diagnostics, ErrorCode.ERR_EscapeStackAlloc, node, expr.Type);
                        return false;
                    }
                    return true;

                case BoundKind.UnconvertedConditionalOperator:
                    {
                        var conditional = (BoundUnconvertedConditionalOperator)expr;
                        return
                            CheckValEscape(conditional.Consequence.Syntax, conditional.Consequence, escapeFrom, escapeTo, checkingReceiver: false, diagnostics: diagnostics) &&
                            CheckValEscape(conditional.Alternative.Syntax, conditional.Alternative, escapeFrom, escapeTo, checkingReceiver: false, diagnostics: diagnostics);
                    }

                case BoundKind.ConditionalOperator:
                    {
                        var conditional = (BoundConditionalOperator)expr;

                        var consValid = CheckValEscape(conditional.Consequence.Syntax, conditional.Consequence, escapeFrom, escapeTo, checkingReceiver: false, diagnostics: diagnostics);

                        if (!consValid || conditional.IsRef)
                        {
                            // ref conditional defers to one operand. 
                            // the other one is the same or we will be reporting errors anyways.
                            return consValid;
                        }

                        return CheckValEscape(conditional.Alternative.Syntax, conditional.Alternative, escapeFrom, escapeTo, checkingReceiver: false, diagnostics: diagnostics);
                    }

                case BoundKind.NullCoalescingOperator:
                    var coalescingOp = (BoundNullCoalescingOperator)expr;
                    return CheckValEscape(coalescingOp.LeftOperand.Syntax, coalescingOp.LeftOperand, escapeFrom, escapeTo, checkingReceiver, diagnostics) &&
                            CheckValEscape(coalescingOp.RightOperand.Syntax, coalescingOp.RightOperand, escapeFrom, escapeTo, checkingReceiver, diagnostics);

                case BoundKind.FieldAccess:
                    var fieldAccess = (BoundFieldAccess)expr;
                    var fieldSymbol = fieldAccess.FieldSymbol;

                    if (fieldSymbol.IsStatic || !fieldSymbol.ContainingType.IsRefLikeType)
                    {
                        // Already an error state.
                        return true;
                    }

                    // for ref-like fields defer to the receiver.
                    return CheckValEscape(node, fieldAccess.ReceiverOpt, escapeFrom, escapeTo, true, diagnostics);

                case BoundKind.Call:
                    var call = (BoundCall)expr;
                    var methodSymbol = call.Method;

                    return CheckInvocationEscape(
                        call.Syntax,
                        methodSymbol,
                        call.ReceiverOpt,
                        methodSymbol.Parameters,
                        call.Arguments,
                        call.ArgumentRefKindsOpt,
                        call.ArgsToParamsOpt,
                        checkingReceiver,
                        escapeFrom,
                        escapeTo,
                        diagnostics,
                        isRefEscape: false);

                case BoundKind.FunctionPointerInvocation:
                    var ptrInvocation = (BoundFunctionPointerInvocation)expr;
                    var ptrSymbol = ptrInvocation.FunctionPointer.Signature;

                    return CheckInvocationEscape(
                        ptrInvocation.Syntax,
                        ptrSymbol,
                        receiverOpt: null,
                        ptrSymbol.Parameters,
                        ptrInvocation.Arguments,
                        ptrInvocation.ArgumentRefKindsOpt,
                        argsToParamsOpt: default,
                        checkingReceiver,
                        escapeFrom,
                        escapeTo,
                        diagnostics,
                        isRefEscape: false);

                case BoundKind.IndexerAccess:
                    var indexerAccess = (BoundIndexerAccess)expr;
                    var indexerSymbol = indexerAccess.Indexer;

                    return CheckInvocationEscape(
                        indexerAccess.Syntax,
                        indexerSymbol,
                        indexerAccess.ReceiverOpt,
                        indexerSymbol.Parameters,
                        indexerAccess.Arguments,
                        indexerAccess.ArgumentRefKindsOpt,
                        indexerAccess.ArgsToParamsOpt,
                        checkingReceiver,
                        escapeFrom,
                        escapeTo,
                        diagnostics,
                        isRefEscape: false);

                case BoundKind.IndexOrRangePatternIndexerAccess:
                    var patternIndexer = (BoundIndexOrRangePatternIndexerAccess)expr;
                    var patternSymbol = patternIndexer.PatternSymbol;
                    var parameters = patternSymbol switch
                    {
                        PropertySymbol p => p.Parameters,
                        MethodSymbol m => m.Parameters,
                        _ => throw ExceptionUtilities.Unreachable,
                    };

                    return CheckInvocationEscape(
                        patternIndexer.Syntax,
                        patternSymbol,
                        patternIndexer.Receiver,
                        parameters,
                        ImmutableArray.Create(patternIndexer.Argument),
                        default,
                        default,
                        checkingReceiver,
                        escapeFrom,
                        escapeTo,
                        diagnostics,
                        isRefEscape: false);

                case BoundKind.PropertyAccess:
                    var propertyAccess = (BoundPropertyAccess)expr;

                    // not passing any arguments/parameters
                    return CheckInvocationEscape(
                        propertyAccess.Syntax,
                        propertyAccess.PropertySymbol,
                        propertyAccess.ReceiverOpt,
                        default,
                        default,
                        default,
                        default,
                        checkingReceiver,
                        escapeFrom,
                        escapeTo,
                        diagnostics,
                        isRefEscape: false);

                case BoundKind.ObjectCreationExpression:
                    var objectCreation = (BoundObjectCreationExpression)expr;
                    var constructorSymbol = objectCreation.Constructor;

                    var escape = CheckInvocationEscape(
                        objectCreation.Syntax,
                        constructorSymbol,
                        null,
                        constructorSymbol.Parameters,
                        objectCreation.Arguments,
                        objectCreation.ArgumentRefKindsOpt,
                        objectCreation.ArgsToParamsOpt,
                        checkingReceiver,
                        escapeFrom,
                        escapeTo,
                        diagnostics,
                        isRefEscape: false);

                    var initializerExpr = objectCreation.InitializerExpressionOpt;
                    if (initializerExpr != null)
                    {
                        escape = escape &&
                            CheckValEscape(
                                initializerExpr.Syntax,
                                initializerExpr,
                                escapeFrom,
                                escapeTo,
                                checkingReceiver: false,
                                diagnostics: diagnostics);
                    }

                    return escape;

                case BoundKind.UnaryOperator:
                    var unary = (BoundUnaryOperator)expr;
                    return CheckValEscape(node, unary.Operand, escapeFrom, escapeTo, checkingReceiver: false, diagnostics: diagnostics);

                case BoundKind.Conversion:
                    var conversion = (BoundConversion)expr;
                    Debug.Assert(conversion.ConversionKind != ConversionKind.StackAllocToSpanType, "StackAllocToSpanType unexpected");
                    return CheckValEscape(node, conversion.Operand, escapeFrom, escapeTo, checkingReceiver: false, diagnostics: diagnostics);

                case BoundKind.AssignmentOperator:
                    var assignment = (BoundAssignmentOperator)expr;
                    return CheckValEscape(node, assignment.Left, escapeFrom, escapeTo, checkingReceiver: false, diagnostics: diagnostics);

                case BoundKind.IncrementOperator:
                    var increment = (BoundIncrementOperator)expr;
                    return CheckValEscape(node, increment.Operand, escapeFrom, escapeTo, checkingReceiver: false, diagnostics: diagnostics);

                case BoundKind.CompoundAssignmentOperator:
                    var compound = (BoundCompoundAssignmentOperator)expr;

                    return CheckValEscape(compound.Left.Syntax, compound.Left, escapeFrom, escapeTo, checkingReceiver: false, diagnostics: diagnostics) &&
                           CheckValEscape(compound.Right.Syntax, compound.Right, escapeFrom, escapeTo, checkingReceiver: false, diagnostics: diagnostics);

                case BoundKind.BinaryOperator:
                    var binary = (BoundBinaryOperator)expr;

                    return CheckValEscape(binary.Left.Syntax, binary.Left, escapeFrom, escapeTo, checkingReceiver: false, diagnostics: diagnostics) &&
                           CheckValEscape(binary.Right.Syntax, binary.Right, escapeFrom, escapeTo, checkingReceiver: false, diagnostics: diagnostics);

                case BoundKind.UserDefinedConditionalLogicalOperator:
                    var uo = (BoundUserDefinedConditionalLogicalOperator)expr;

                    return CheckValEscape(uo.Left.Syntax, uo.Left, escapeFrom, escapeTo, checkingReceiver: false, diagnostics: diagnostics) &&
                           CheckValEscape(uo.Right.Syntax, uo.Right, escapeFrom, escapeTo, checkingReceiver: false, diagnostics: diagnostics);

                case BoundKind.QueryClause:
                    var clauseValue = ((BoundQueryClause)expr).Value;
                    return CheckValEscape(clauseValue.Syntax, clauseValue, escapeFrom, escapeTo, checkingReceiver: false, diagnostics: diagnostics);

                case BoundKind.RangeVariable:
                    var variableValue = ((BoundRangeVariable)expr).Value;
                    return CheckValEscape(variableValue.Syntax, variableValue, escapeFrom, escapeTo, checkingReceiver: false, diagnostics: diagnostics);

                case BoundKind.ObjectInitializerExpression:
                    var initExpr = (BoundObjectInitializerExpression)expr;
                    return CheckValEscapeOfObjectInitializer(initExpr, escapeFrom, escapeTo, diagnostics);

                // this would be correct implementation for CollectionInitializerExpression 
                // however it is unclear if it is reachable since the initialized type must implement IEnumerable
                case BoundKind.CollectionInitializerExpression:
                    var colExpr = (BoundCollectionInitializerExpression)expr;
                    return CheckValEscape(colExpr.Initializers, escapeFrom, escapeTo, diagnostics);

                // this would be correct implementation for CollectionElementInitializer 
                // however it is unclear if it is reachable since the initialized type must implement IEnumerable
                case BoundKind.CollectionElementInitializer:
                    var colElement = (BoundCollectionElementInitializer)expr;
                    return CheckValEscape(colElement.Arguments, escapeFrom, escapeTo, diagnostics);

                case BoundKind.PointerElementAccess:
                    var accessedExpression = ((BoundPointerElementAccess)expr).Expression;
                    return CheckValEscape(accessedExpression.Syntax, accessedExpression, escapeFrom, escapeTo, checkingReceiver, diagnostics);

                case BoundKind.PointerIndirectionOperator:
                    var operandExpression = ((BoundPointerIndirectionOperator)expr).Operand;
                    return CheckValEscape(operandExpression.Syntax, operandExpression, escapeFrom, escapeTo, checkingReceiver, diagnostics);

                case BoundKind.AsOperator:
                case BoundKind.AwaitExpression:
                case BoundKind.ConditionalAccess:
                case BoundKind.ArrayAccess:
                    // only possible in error cases (if possible at all)
                    return false;

                case BoundKind.UnconvertedSwitchExpression:
                case BoundKind.ConvertedSwitchExpression:
                    foreach (var arm in ((BoundSwitchExpression)expr).SwitchArms)
                    {
                        var result = arm.Value;
                        if (!CheckValEscape(result.Syntax, result, escapeFrom, escapeTo, checkingReceiver: false, diagnostics: diagnostics))
                            return false;
                    }

                    return true;

                default:
                    // in error situations some unexpected nodes could make here
                    // returning "false" seems safer than throwing.
                    // we will still assert to make sure that all nodes are accounted for.
                    Debug.Assert(false, $"{expr.Kind} expression of {expr.Type} type");
                    diagnostics.Add(ErrorCode.ERR_InternalError, node.Location);
                    return false;

                    #region "cannot produce ref-like values"
                    //                case BoundKind.ThrowExpression:
                    //                case BoundKind.ArgListOperator:
                    //                case BoundKind.ArgList:
                    //                case BoundKind.RefTypeOperator:
                    //                case BoundKind.AddressOfOperator:
                    //                case BoundKind.TypeOfOperator:
                    //                case BoundKind.IsOperator:
                    //                case BoundKind.SizeOfOperator:
                    //                case BoundKind.DynamicMemberAccess:
                    //                case BoundKind.DynamicInvocation:
                    //                case BoundKind.NewT:
                    //                case BoundKind.DelegateCreationExpression:
                    //                case BoundKind.ArrayCreation:
                    //                case BoundKind.AnonymousObjectCreationExpression:
                    //                case BoundKind.NameOfOperator:
                    //                case BoundKind.InterpolatedString:
                    //                case BoundKind.StringInsert:
                    //                case BoundKind.DynamicIndexerAccess:
                    //                case BoundKind.Lambda:
                    //                case BoundKind.DynamicObjectCreationExpression:
                    //                case BoundKind.NoPiaObjectCreationExpression:
                    //                case BoundKind.BaseReference:
                    //                case BoundKind.Literal:
                    //                case BoundKind.IsPatternExpression:
                    //                case BoundKind.DeconstructionAssignmentOperator:
                    //                case BoundKind.EventAccess:

                    #endregion

                    #region "not expression that can produce a value"
                    //                case BoundKind.FieldEqualsValue:
                    //                case BoundKind.PropertyEqualsValue:
                    //                case BoundKind.ParameterEqualsValue:
                    //                case BoundKind.NamespaceExpression:
                    //                case BoundKind.TypeExpression:
                    //                case BoundKind.BadStatement:
                    //                case BoundKind.MethodDefIndex:
                    //                case BoundKind.SourceDocumentIndex:
                    //                case BoundKind.ArgList:
                    //                case BoundKind.ArgListOperator:
                    //                case BoundKind.Block:
                    //                case BoundKind.Scope:
                    //                case BoundKind.NoOpStatement:
                    //                case BoundKind.ReturnStatement:
                    //                case BoundKind.YieldReturnStatement:
                    //                case BoundKind.YieldBreakStatement:
                    //                case BoundKind.ThrowStatement:
                    //                case BoundKind.ExpressionStatement:
                    //                case BoundKind.SwitchStatement:
                    //                case BoundKind.SwitchSection:
                    //                case BoundKind.SwitchLabel:
                    //                case BoundKind.BreakStatement:
                    //                case BoundKind.LocalFunctionStatement:
                    //                case BoundKind.ContinueStatement:
                    //                case BoundKind.PatternSwitchStatement:
                    //                case BoundKind.PatternSwitchSection:
                    //                case BoundKind.PatternSwitchLabel:
                    //                case BoundKind.IfStatement:
                    //                case BoundKind.DoStatement:
                    //                case BoundKind.WhileStatement:
                    //                case BoundKind.ForStatement:
                    //                case BoundKind.ForEachStatement:
                    //                case BoundKind.ForEachDeconstructStep:
                    //                case BoundKind.UsingStatement:
                    //                case BoundKind.FixedStatement:
                    //                case BoundKind.LockStatement:
                    //                case BoundKind.TryStatement:
                    //                case BoundKind.CatchBlock:
                    //                case BoundKind.LabelStatement:
                    //                case BoundKind.GotoStatement:
                    //                case BoundKind.LabeledStatement:
                    //                case BoundKind.Label:
                    //                case BoundKind.StatementList:
                    //                case BoundKind.ConditionalGoto:
                    //                case BoundKind.LocalDeclaration:
                    //                case BoundKind.MultipleLocalDeclarations:
                    //                case BoundKind.ArrayInitialization:
                    //                case BoundKind.AnonymousPropertyDeclaration:
                    //                case BoundKind.MethodGroup:
                    //                case BoundKind.PropertyGroup:
                    //                case BoundKind.EventAssignmentOperator:
                    //                case BoundKind.Attribute:
                    //                case BoundKind.FixedLocalCollectionInitializer:
                    //                case BoundKind.DynamicObjectInitializerMember:
                    //                case BoundKind.DynamicCollectionElementInitializer:
                    //                case BoundKind.ImplicitReceiver:
                    //                case BoundKind.FieldInitializer:
                    //                case BoundKind.GlobalStatementInitializer:
                    //                case BoundKind.TypeOrInstanceInitializers:
                    //                case BoundKind.DeclarationPattern:
                    //                case BoundKind.ConstantPattern:
                    //                case BoundKind.WildcardPattern:

                    #endregion

                    #region "not found as an operand in no-error unlowered bound tree"
                    //                case BoundKind.MaximumMethodDefIndex:
                    //                case BoundKind.InstrumentationPayloadRoot:
                    //                case BoundKind.ModuleVersionId:
                    //                case BoundKind.ModuleVersionIdString:
                    //                case BoundKind.Dup:
                    //                case BoundKind.TypeOrValueExpression:
                    //                case BoundKind.BadExpression:
                    //                case BoundKind.ArrayLength:
                    //                case BoundKind.MethodInfo:
                    //                case BoundKind.FieldInfo:
                    //                case BoundKind.SequencePoint:
                    //                case BoundKind.SequencePointExpression:
                    //                case BoundKind.SequencePointWithSpan:
                    //                case BoundKind.StateMachineScope:
                    //                case BoundKind.ConditionalReceiver:
                    //                case BoundKind.ComplexConditionalReceiver:
                    //                case BoundKind.PreviousSubmissionReference:
                    //                case BoundKind.HostObjectMemberReference:
                    //                case BoundKind.UnboundLambda:
                    //                case BoundKind.LoweredConditionalAccess:
                    //                case BoundKind.Sequence:
                    //                case BoundKind.HoistedFieldAccess:
                    //                case BoundKind.OutVariablePendingInference:
                    //                case BoundKind.DeconstructionVariablePendingInference:
                    //                case BoundKind.OutDeconstructVarPendingInference:
                    //                case BoundKind.PseudoVariable:

                    #endregion
            }
        }

        private static bool CheckTupleValEscape(ImmutableArray<BoundExpression> elements, uint escapeFrom, uint escapeTo, DiagnosticBag diagnostics)
        {
            foreach (var element in elements)
            {
                if (!CheckValEscape(element.Syntax, element, escapeFrom, escapeTo, checkingReceiver: false, diagnostics: diagnostics))
                {
                    return false;
                }
            }

            return true;
        }

        private static bool CheckValEscapeOfObjectInitializer(BoundObjectInitializerExpression initExpr, uint escapeFrom, uint escapeTo, DiagnosticBag diagnostics)
        {
            foreach (var expression in initExpr.Initializers)
            {
                if (expression.Kind == BoundKind.AssignmentOperator)
                {
                    var assignment = (BoundAssignmentOperator)expression;
                    if (!CheckValEscape(expression.Syntax, assignment.Right, escapeFrom, escapeTo, checkingReceiver: false, diagnostics: diagnostics))
                    {
                        return false;
                    }

                    var left = (BoundObjectInitializerMember)assignment.Left;
                    if (!CheckValEscape(left.Arguments, escapeFrom, escapeTo, diagnostics: diagnostics))
                    {
                        return false;
                    }
                }
                else
                {
                    if (!CheckValEscape(expression.Syntax, expression, escapeFrom, escapeTo, checkingReceiver: false, diagnostics: diagnostics))
                    {
                        return false;
                    }
                }
            }

            return true;
        }

        private static bool CheckValEscape(ImmutableArray<BoundExpression> expressions, uint escapeFrom, uint escapeTo, DiagnosticBag diagnostics)
        {
            foreach (var expression in expressions)
            {
                if (!CheckValEscape(expression.Syntax, expression, escapeFrom, escapeTo, checkingReceiver: false, diagnostics: diagnostics))
                {
                    return false;
                }
            }

            return true;
        }

        internal enum AddressKind
        {
            // reference may be written to
            Writeable,

            // reference itself will not be written to, but may be used for call, callvirt.
            // for all purposes it is the same as Writeable, except when fetching an address of an array element
            // where it results in a ".readonly" prefix to deal with array covariance.
            Constrained,

            // reference itself will not be written to, nor it will be used to modify fields.
            ReadOnly,

            // same as ReadOnly, but we are not supposed to get a reference to a clone
            // regardless of compat settings.
            ReadOnlyStrict,
        }

        internal static bool IsAnyReadOnly(AddressKind addressKind) => addressKind >= AddressKind.ReadOnly;

        /// <summary>
        /// Checks if expression directly or indirectly represents a value with its own home. In
        /// such cases it is possible to get a reference without loading into a temporary.
        /// </summary>
        internal static bool HasHome(
            BoundExpression expression,
            AddressKind addressKind,
            MethodSymbol method,
            bool peVerifyCompatEnabled,
            HashSet<LocalSymbol> stackLocalsOpt)
        {
            Debug.Assert(method is object);

            switch (expression.Kind)
            {
                case BoundKind.ArrayAccess:
                    if (addressKind == AddressKind.ReadOnly && !expression.Type.IsValueType && peVerifyCompatEnabled)
                    {
                        // due to array covariance getting a reference may throw ArrayTypeMismatch when element is not a struct, 
                        // passing "readonly." prefix would prevent that, but it is unverifiable, so will make a copy in compat case
                        return false;
                    }

                    return true;

                case BoundKind.PointerIndirectionOperator:
                case BoundKind.RefValueOperator:
                    return true;

                case BoundKind.ThisReference:
                    var type = expression.Type;
                    if (type.IsReferenceType)
                    {
                        Debug.Assert(IsAnyReadOnly(addressKind), "`this` is readonly in classes");
                        return true;
                    }

                    if (!IsAnyReadOnly(addressKind) && method.IsEffectivelyReadOnly)
                    {
                        return false;
                    }

                    return true;

                case BoundKind.ThrowExpression:
                    // vacuously this is true, we can take address of throw without temps
                    return true;

                case BoundKind.Parameter:
                    return IsAnyReadOnly(addressKind) ||
                        ((BoundParameter)expression).ParameterSymbol.RefKind != RefKind.In;

                case BoundKind.Local:
                    // locals have home unless they are byval stack locals or ref-readonly
                    // locals in a mutating call
                    var local = ((BoundLocal)expression).LocalSymbol;
                    return !((CodeGenerator.IsStackLocal(local, stackLocalsOpt) && local.RefKind == RefKind.None) ||
                        (!IsAnyReadOnly(addressKind) && local.RefKind == RefKind.RefReadOnly));

                case BoundKind.Call:
                    var methodRefKind = ((BoundCall)expression).Method.RefKind;
                    return methodRefKind == RefKind.Ref ||
                           (IsAnyReadOnly(addressKind) && methodRefKind == RefKind.RefReadOnly);

                case BoundKind.Dup:
                    //NB: Dup represents locals that do not need IL slot
                    var dupRefKind = ((BoundDup)expression).RefKind;
                    return dupRefKind == RefKind.Ref ||
                        (IsAnyReadOnly(addressKind) && dupRefKind == RefKind.RefReadOnly);

                case BoundKind.FieldAccess:
                    return HasHome((BoundFieldAccess)expression, addressKind, method, peVerifyCompatEnabled, stackLocalsOpt);

                case BoundKind.Sequence:
                    return HasHome(((BoundSequence)expression).Value, addressKind, method, peVerifyCompatEnabled, stackLocalsOpt);

                case BoundKind.AssignmentOperator:
                    var assignment = (BoundAssignmentOperator)expression;
                    if (!assignment.IsRef)
                    {
                        return false;
                    }
                    var lhsRefKind = assignment.Left.GetRefKind();
                    return lhsRefKind == RefKind.Ref ||
                        (IsAnyReadOnly(addressKind) && lhsRefKind == RefKind.RefReadOnly);

                case BoundKind.ComplexConditionalReceiver:
                    Debug.Assert(HasHome(
                        ((BoundComplexConditionalReceiver)expression).ValueTypeReceiver,
                        addressKind,
                        method,
                        peVerifyCompatEnabled,
                        stackLocalsOpt));
                    Debug.Assert(HasHome(
                        ((BoundComplexConditionalReceiver)expression).ReferenceTypeReceiver,
                        addressKind,
                        method,
                        peVerifyCompatEnabled,
                        stackLocalsOpt));
                    goto case BoundKind.ConditionalReceiver;

                case BoundKind.ConditionalReceiver:
                    //ConditionalReceiver is a noop from Emit point of view. - it represents something that has already been pushed. 
                    //We should never need a temp for it. 
                    return true;

                case BoundKind.ConditionalOperator:
                    var conditional = (BoundConditionalOperator)expression;

                    // only ref conditional may be referenced as a variable
                    if (!conditional.IsRef)
                    {
                        return false;
                    }

                    // branch that has no home will need a temporary
                    // if both have no home, just say whole expression has no home 
                    // so we could just use one temp for the whole thing
                    return HasHome(conditional.Consequence, addressKind, method, peVerifyCompatEnabled, stackLocalsOpt)
                        && HasHome(conditional.Alternative, addressKind, method, peVerifyCompatEnabled, stackLocalsOpt);

                default:
                    return false;
            }
        }

        /// <summary>
        /// Special HasHome for fields. 
        /// Fields have readable homes when they are not constants.
        /// Fields have writeable homes unless they are readonly and used outside of the constructor.
        /// </summary>
        private static bool HasHome(
            BoundFieldAccess fieldAccess,
            AddressKind addressKind,
            MethodSymbol method,
            bool peVerifyCompatEnabled,
            HashSet<LocalSymbol> stackLocalsOpt)
        {
            Debug.Assert(method is object);

            FieldSymbol field = fieldAccess.FieldSymbol;

            // const fields are literal values with no homes. (ex: decimal.Zero)
            if (field.IsConst)
            {
                return false;
            }

            // in readonly situations where ref to a copy is not allowed, consider fields as addressable
            if (addressKind == AddressKind.ReadOnlyStrict)
            {
                return true;
            }

            // ReadOnly references can always be taken unless we are in peverify compat mode
            if (addressKind == AddressKind.ReadOnly && !peVerifyCompatEnabled)
            {
                return true;
            }

            // Some field accesses must be values; values do not have homes.
            if (fieldAccess.IsByValue)
            {
                return false;
            }

            if (!field.IsReadOnly)
            {
                // in a case if we have a writeable struct field with a receiver that only has a readable home we would need to pass it via a temp.
                // it would be advantageous to make a temp for the field, not for the outer struct, since the field is smaller and we can get to is by fetching references.
                // NOTE: this would not be profitable if we have to satisfy verifier, since for verifiability 
                //       we would not be able to dig for the inner field using references and the outer struct will have to be copied to a temp anyways.
                if (!peVerifyCompatEnabled)
                {
                    Debug.Assert(!IsAnyReadOnly(addressKind));

                    var receiver = fieldAccess.ReceiverOpt;
                    if (receiver?.Type.IsValueType == true)
                    {
                        // Check receiver:
                        // has writeable home -> return true - the whole chain has writeable home (also a more common case)
                        // has readable home -> return false - we need to copy the field
                        // otherwise         -> return true  - the copy will be made at higher level so the leaf field can have writeable home

                        return HasHome(receiver, addressKind, method, peVerifyCompatEnabled, stackLocalsOpt)
                            || !HasHome(receiver, AddressKind.ReadOnly, method, peVerifyCompatEnabled, stackLocalsOpt);
                    }
                }

                return true;
            }

            // while readonly fields have home it is not valid to refer to it when not constructing.
            if (!TypeSymbol.Equals(field.ContainingType, method.ContainingType, TypeCompareKind.AllIgnoreOptions))
            {
                return false;
            }

            if (field.IsStatic)
            {
                return method.MethodKind == MethodKind.StaticConstructor;
            }
            else
            {
                return (method.MethodKind == MethodKind.Constructor || method.IsInitOnly) &&
                    fieldAccess.ReceiverOpt.Kind == BoundKind.ThisReference;
            }
        }
    }
}