JIT: use type argument as the exact type for Activator.CreateInstance<T>

src/coreclr/jit/importercalls.cpp

@@ -3898,6 +3898,12 @@ GenTree* Compiler::impIntrinsic(CORINFO_CLASS_HANDLE    clsHnd,
                 break;
             }
 
+            case NI_System_Activator_CreateInstance_T:
+            {
+                isSpecial = true;
+                break;
+            }
+
             case NI_System_Span_get_Item:
             case NI_System_ReadOnlySpan_get_Item:
             {
@@ -9989,6 +9995,40 @@ CORINFO_CLASS_HANDLE Compiler::impGetSpecialIntrinsicExactReturnType(GenTreeCall
             break;
         }
 
+        case NI_System_Activator_CreateInstance_T:
+        {
+            // Expect one method generic parameter; figure out which it is.
+            CORINFO_SIG_INFO sig;
+            info.compCompHnd->getMethodSig(methodHnd, &sig);
+            assert(sig.sigInst.methInstCount == 1);
+            assert(sig.sigInst.classInstCount == 0);
+
+            CORINFO_CLASS_HANDLE typeHnd = sig.sigInst.methInst[0];
+            assert(typeHnd != nullptr);
+
+            CallArg* instParam = call->gtArgs.FindWellKnownArg(WellKnownArg::InstParam);
+            if (instParam != nullptr)
+            {
+                assert(instParam->GetNext() == nullptr);
+                CORINFO_METHOD_HANDLE hMethod = gtGetHelperArgMethodHandle(instParam->GetNode());
+                if (hMethod != NO_METHOD_HANDLE)
+                {
+                    result = getMethodInstantiationArgument(hMethod, 0);
+                }
+            }
+
+            if (result != NO_CLASS_HANDLE)
+            {
+                JITDUMP("Special intrinsic: return type is %s\n",
+                        result != nullptr ? eeGetClassName(result) : "unknown");
+            }
+            else
+            {
+                JITDUMP("Special intrinsic: return type undetermined or inexact, so deferring opt\n");
+            }
+            break;
+        }
+
         default:
         {
             JITDUMP("This special intrinsic not handled, sorry...\n");
@@ -10726,6 +10766,15 @@ NamedIntrinsic Compiler::lookupNamedIntrinsic(CORINFO_METHOD_HANDLE method)
                         {
                             result = NI_System_Activator_AllocatorOf;
                         }
+                        else if (strcmp(methodName, "CreateInstance") == 0)
+                        {
+                            CORINFO_SIG_INFO sig;
+                            eeGetMethodSig(method, &sig);
+                            if ((sig.sigInst.methInstCount == 1) && (sig.sigInst.classInstCount == 0))
+                            {
+                                result = NI_System_Activator_CreateInstance_T;
+                            }
+                        }
                         else if (strcmp(methodName, "DefaultConstructorOf") == 0)
                         {
                             result = NI_System_Activator_DefaultConstructorOf;

src/coreclr/jit/namedintrinsiclist.h

@@ -100,6 +100,7 @@ enum NamedIntrinsic : unsigned short
     NI_System_Type_op_Inequality,
     NI_System_Type_GetTypeFromHandle,
     NI_System_Type_GetGenericTypeDefinition,
+    NI_System_Activator_CreateInstance_T,
     NI_System_Array_Clone,
     NI_System_Array_GetLength,
     NI_System_Array_GetLowerBound,