Correct StructLayout size emission for data-less struct unions

docs/release-notes/.FSharp.Compiler.Service/11.0.100.md

@@ -1,5 +1,6 @@
 ### Fixed
 
+* Fix incorrect `StructLayout(Size = 1)` emission for data-less struct unions where the compiler-generated tag field makes the actual runtime size larger. ([PR #19759](https://github.com/dotnet/fsharp/pull/19759))
 * Honor `--nowarn` and `--warnaserror` for warnings emitted during command-line option parsing ([Issue #19576](https://github.com/dotnet/fsharp/issues/19576), [PR #19776](https://github.com/dotnet/fsharp/pull/19776))
 * Fix `[<return: X>]` prefix attributes being silently dropped on class members, and fix false-positive `AllowMultiple=false` errors when `[<X>]` and `[<return: X>]` are applied to the same binding. ([Issue #17904](https://github.com/dotnet/fsharp/issues/17904), [Issue #19020](https://github.com/dotnet/fsharp/issues/19020), [PR #19738](https://github.com/dotnet/fsharp/pull/19738))
 * Fix attributes on return type of unparenthesized tuple methods being silently dropped from IL. ([Issue #462](https://github.com/dotnet/fsharp/issues/462), [PR #19714](https://github.com/dotnet/fsharp/pull/19714))

src/Compiler/CodeGen/IlxGen.fs

@@ -12031,18 +12031,10 @@ and GenTypeDef cenv mgbuf lazyInitInfo eenv m (tycon: Tycon) : ILTypeRef option
                         }
 
                     let layout =
-                        // Structs with no instance fields get size 1, pack 0
+                        // Multi-case struct unions carry a hidden tag field; single-case struct unions
+                        // are handled by the CLR's minimum-1-byte guarantee. No explicit size needed.
                         if isStructTy g thisTy then
-                            if
-                                (tycon.AllFieldsArray.Length = 0
-                                 || tycon.AllFieldsArray |> Array.exists (fun f -> not f.IsStatic))
-                                && (alternatives
-                                    |> Array.collect (fun a -> a.FieldDefs)
-                                    |> Array.exists (fun fd -> not fd.ILField.IsStatic))
-                            then
-                                ILTypeDefLayout.Sequential { Size = None; Pack = None }
-                            else
-                                ILTypeDefLayout.Sequential { Size = Some 1; Pack = Some 0us }
+                            ILTypeDefLayout.Sequential { Size = None; Pack = None }
                         else
                             ILTypeDefLayout.Auto
 

tests/FSharp.Compiler.ComponentTests/Conformance/BasicGrammarElements/CustomAttributes/Basic/Basic.fs

@@ -397,7 +397,7 @@ module CustomAttributes_Basic =
         ]
 
     [<Fact>]
-    let ``StructLayoutAttribute has size=1 for struct DUs with no instance fields`` () =
+    let ``StructLayoutAttribute doesn't have size=1 for multi-case struct DUs with no instance fields`` () =
         Fsx """
         [<Struct>] type Option<'T> = None | Some
         """
@@ -413,8 +413,6 @@ module CustomAttributes_Basic =
                    [runtime]System.IComparable,
                    [runtime]System.Collections.IStructuralComparable
   {
-    .pack 0
-    .size 1
     .custom instance void [FSharp.Core]Microsoft.FSharp.Core.StructAttribute::.ctor() = ( 01 00 00 00 ) 
     .custom instance void [runtime]System.Diagnostics.DebuggerDisplayAttribute::.ctor(string) = ( 01 00 15 7B 5F 5F 44 65 62 75 67 44 69 73 70 6C   
                                                                                                   61 79 28 29 2C 6E 71 7D 00 00 )                   
@@ -426,4 +424,28 @@ module CustomAttributes_Basic =
       .field public static literal int32 Some = int32(0x00000001)
     } 
         """
-        ]
+        ]
+
+    [<Fact>]
+    let ``StructLayoutAttribute doesn't have size=1 for single-case struct DU`` () =
+        Fsx """
+        [<Struct>] type X = | Y
+        """
+        |> compile
+        |> shouldSucceed
+        |> verifyIL [
+        """
+        .class sequential autochar serializable sealed nested public beforefieldinit X
+        extends [runtime]System.ValueType
+        implements class [runtime]System.IEquatable`1<valuetype Test/X>,
+                   [runtime]System.Collections.IStructuralEquatable,
+                   class [runtime]System.IComparable`1<valuetype Test/X>,
+                   [runtime]System.IComparable,
+                   [runtime]System.Collections.IStructuralComparable
+  {
+    .custom instance void [FSharp.Core]Microsoft.FSharp.Core.StructAttribute::.ctor() = ( 01 00 00 00 ) 
+    .custom instance void [runtime]System.Diagnostics.DebuggerDisplayAttribute::.ctor(string) = ( 01 00 15 7B 5F 5F 44 65 62 75 67 44 69 73 70 6C   
+                                                                                                  61 79 28 29 2C 6E 71 7D 00 00 )                   
+    .custom instance void [FSharp.Core]Microsoft.FSharp.Core.CompilationMappingAttribute::.ctor(valuetype [FSharp.Core]Microsoft.FSharp.Core.SourceConstructFlags) = ( 01 00 01 00 00 00 00 00 ) 
+        """
+        ]