Fix compareTypesForEquality

src/coreclr/tools/Common/Compiler/TypeExtensions.cs

@@ -182,43 +182,68 @@ public static bool IsArrayTypeWithoutGenericInterfaces(this TypeDesc type)
 
         public static bool? CompareTypesForEquality(TypeDesc type1, TypeDesc type2)
         {
-            bool? result = null;
-
             // If neither type is a canonical subtype, type handle comparison suffices
             if (!type1.IsCanonicalSubtype(CanonicalFormKind.Any) && !type2.IsCanonicalSubtype(CanonicalFormKind.Any))
             {
-                result = type1 == type2;
+                return type1 == type2;
             }
+
             // If either or both types are canonical subtypes, we can sometimes prove inequality.
-            else
+            if (AreGuaranteedToRepresentDifferentTypes(type1, type2))
+            {
+                return false;
+            }
+
+            return null;
+
+            static bool AreGuaranteedToRepresentDifferentTypes(TypeDesc type1, TypeDesc type2)
             {
-                // If either is a value type then the types cannot
-                // be equal unless the type defs are the same.
-                if (type1.IsValueType || type2.IsValueType)
+                if (type1.IsCanonicalDefinitionType(CanonicalFormKind.Any) || type2.IsCanonicalDefinitionType(CanonicalFormKind.Any))
                 {
-                    if (!type1.IsCanonicalDefinitionType(CanonicalFormKind.Universal) && !type2.IsCanonicalDefinitionType(CanonicalFormKind.Universal))
-                    {
-                        if (!type1.HasSameTypeDefinition(type2))
-                        {
-                            result = false;
-                        }
-                    }
+                    // Universal canonical definition can match any type. We can't prove inequality.
+                    if (type1.IsCanonicalDefinitionType(CanonicalFormKind.Universal) || type2.IsCanonicalDefinitionType(CanonicalFormKind.Universal))
+                        return false;
+
+                    return type1.IsGCPointer != type2.IsGCPointer;
                 }
-                // If we have two ref types that are not __Canon, then the
-                // types cannot be equal unless the type defs are the same.
-                else
+
+                TypeFlags category = type1.Category;
+                if (category != type2.Category)
+                    return true;
+
+                switch (category)
                 {
-                    if (!type1.IsCanonicalDefinitionType(CanonicalFormKind.Any) && !type2.IsCanonicalDefinitionType(CanonicalFormKind.Any))
-                    {
-                        if (!type1.HasSameTypeDefinition(type2))
+                    case TypeFlags.Array:
+                        if (((ArrayType)type1).Rank != ((ArrayType)type2).Rank)
+                            return true;
+                        return AreGuaranteedToRepresentDifferentTypes(((ArrayType)type1).ElementType, ((ArrayType)type2).ElementType);
+                    case TypeFlags.SzArray:
+                    case TypeFlags.ByRef:
+                    case TypeFlags.Pointer:
+                        return AreGuaranteedToRepresentDifferentTypes(((ParameterizedType)type1).ParameterType, ((ParameterizedType)type2).ParameterType);
+
+                    default:
+                        if (type1.IsDefType || type2.IsDefType)
                         {
-                            result = false;
+                            if (!type1.HasSameTypeDefinition(type2))
+                                return true;
+
+                            Instantiation inst1 = type1.Instantiation;
+                            if (inst1.Length != 0)
+                            {
+                                var inst2 = type2.Instantiation;
+                                Debug.Assert(inst1.Length == inst2.Length);
+                                for (int i = 0; i < inst1.Length; i++)
+                                {
+                                    if (AreGuaranteedToRepresentDifferentTypes(inst1[i], inst2[i]))
+                                        return true;
+                                }
+                            }
                         }
-                    }
+                        break;
                 }
+                return false;
             }
-
-            return result;
         }
 
         public static TypeDesc MergeTypesToCommonParent(TypeDesc ta, TypeDesc tb)

src/coreclr/vm/jitinterface.cpp

@@ -4448,6 +4448,61 @@ TypeCompareState CEEInfo::compareTypesForCast(
     return result;
 }
 
+// Returns true if hnd1 and hnd2 are guaranteed to represent different types. Returns false if hnd1 and hnd2 may represent the same type.
+static bool AreGuaranteedToRepresentDifferentTypes(TypeHandle hnd1, TypeHandle hnd2)
+{
+    STANDARD_VM_CONTRACT;
+
+    CorElementType et1 = hnd1.GetSignatureCorElementType();
+    CorElementType et2 = hnd2.GetSignatureCorElementType();
+
+    TypeHandle canonHnd = TypeHandle(g_pCanonMethodTableClass);
+    if ((hnd1 == canonHnd) || (hnd2 == canonHnd))
+    {
+        return CorTypeInfo::IsObjRef(et1) != CorTypeInfo::IsObjRef(et2);
+    }
+
+    if (et1 != et2)
+    {
+        return true;
+    }
+
+    switch (et1)
+    {
+    case ELEMENT_TYPE_ARRAY:
+        if (hnd1.GetRank() != hnd2.GetRank())
+            return true;
+        return AreGuaranteedToRepresentDifferentTypes(hnd1.GetTypeParam(), hnd2.GetTypeParam());
+    case ELEMENT_TYPE_SZARRAY:
+    case ELEMENT_TYPE_BYREF:
+    case ELEMENT_TYPE_PTR:
+        return AreGuaranteedToRepresentDifferentTypes(hnd1.GetTypeParam(), hnd2.GetTypeParam());
+
+    default:
+        if (!hnd1.IsTypeDesc())
+        {
+            if (!hnd1.AsMethodTable()->HasSameTypeDefAs(hnd2.AsMethodTable()))
+                return true;
+
+            if (hnd1.AsMethodTable()->HasInstantiation())
+            {
+                Instantiation inst1 = hnd1.AsMethodTable()->GetInstantiation();
+                Instantiation inst2 = hnd2.AsMethodTable()->GetInstantiation();
+                _ASSERTE(inst1.GetNumArgs() == inst2.GetNumArgs());
+
+                for (DWORD i = 0; i < inst1.GetNumArgs(); i++)
+                {
+                    if (AreGuaranteedToRepresentDifferentTypes(inst1[i], inst2[i]))
+                        return true;
+                }
+            }
+        }
+        break;
+    }
+
+    return false;
+}
+
 /*********************************************************************/
 // See if types represented by cls1 and cls2 compare equal, not
 // equal, or the comparison needs to be resolved at runtime.
@@ -4473,30 +4528,12 @@ TypeCompareState CEEInfo::compareTypesForEquality(
     {
         result = (hnd1 == hnd2 ? TypeCompareState::Must : TypeCompareState::MustNot);
     }
-    // If either or both types are canonical subtypes, we can sometimes prove inequality.
     else
     {
-        // If either is a value type then the types cannot
-        // be equal unless the type defs are the same.
-        if (hnd1.IsValueType() || hnd2.IsValueType())
+        // If either or both types are canonical subtypes, we can sometimes prove inequality.
+        if (AreGuaranteedToRepresentDifferentTypes(hnd1, hnd2))
         {
-            if (!hnd1.GetMethodTable()->HasSameTypeDefAs(hnd2.GetMethodTable()))
-            {
-                result = TypeCompareState::MustNot;
-            }
-        }
-        // If we have two ref types that are not __Canon, then the
-        // types cannot be equal unless the type defs are the same.
-        else
-        {
-            TypeHandle canonHnd = TypeHandle(g_pCanonMethodTableClass);
-            if ((hnd1 != canonHnd) && (hnd2 != canonHnd))
-            {
-                if (!hnd1.GetMethodTable()->HasSameTypeDefAs(hnd2.GetMethodTable()))
-                {
-                    result = TypeCompareState::MustNot;
-                }
-            }
+            result = TypeCompareState::MustNot;
         }
     }
 

src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/CollectionsMarshal.cs

@@ -34,8 +34,7 @@ public static Span<T> AsSpan<T>(List<T>? list)
                     ThrowHelper.ThrowInvalidOperationException_ConcurrentOperationsNotSupported();
                 }
 
-                // Commented out to workaround https://github.com/dotnet/runtime/issues/96876
-                // Debug.Assert(typeof(T[]) == list._items.GetType(), "Implementation depends on List<T> always using a T[] and not U[] where U : T.");
+                Debug.Assert(typeof(T[]) == list._items.GetType(), "Implementation depends on List<T> always using a T[] and not U[] where U : T.");
                 span = new Span<T>(ref MemoryMarshal.GetArrayDataReference(items), size);
             }
 

src/tests/Regressions/coreclr/GitHub_96876/test96876.cs

@@ -0,0 +1,26 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+using System;
+using System.Runtime.CompilerServices;
+using Xunit;
+
+public class Test96876
+{
+    [Fact]
+    public static void TestEntryPoint()
+    {
+        Assert.True(foo<string>(new string[1]));
+        Assert.False(foo<object>(new string[1]));
+
+        Assert.True(foo2<string>());
+        Assert.False(foo2<object>());
+    }
+
+    // Validate that the type equality involving shared array types is handled correctly
+    // in shared generic code.
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    static bool foo<T>(string[] list) => typeof(T[]) == list.GetType();
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    static bool foo2<T>() => typeof(T[]) == typeof(string[]);
+}

src/tests/Regressions/coreclr/GitHub_96876/test96876.csproj

@@ -0,0 +1,11 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <CLRTestPriority>1</CLRTestPriority>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="test96876.cs" />
+  </ItemGroup>
+  <ItemGroup>
+    <ProjectReference Include="$(TestSourceDir)Common/CoreCLRTestLibrary/CoreCLRTestLibrary.csproj" />
+  </ItemGroup>
+</Project>