Update ILVerify to honor the "async" flag

src/coreclr/tools/ILVerification/ILImporter.Verify.cs

@@ -1875,7 +1875,22 @@ void ImportReturn()
 
             var declaredReturnType = _method.Signature.ReturnType;
 
-            if (declaredReturnType.IsVoid)
+            // For async methods, unwrap Task/ValueTask return types
+            TypeDesc expectedReturnType = declaredReturnType;
+            if (_method.IsAsync)
+            {
+                if (IsTaskOrValueTaskType(declaredReturnType, out TypeDesc unwrappedType))
+                {
+                    expectedReturnType = unwrappedType;
+                }
+                else
+                {
+                    // Async methods must return Task or ValueTask
+                    VerificationError(VerifierError.StackUnexpected);
+                }
+            }
+
+            if (expectedReturnType.IsVoid)
             {
                 Debug.Assert(_stackTop >= 0);
 
@@ -1891,11 +1906,49 @@ void ImportReturn()
                     Check(_stackTop == 1, VerifierError.ReturnEmpty);
 
                     var actualReturnType = Pop();
-                    CheckIsAssignable(actualReturnType, StackValue.CreateFromType(declaredReturnType));
+                    CheckIsAssignable(actualReturnType, StackValue.CreateFromType(expectedReturnType));
+
+                    Check((!expectedReturnType.IsByRef && !expectedReturnType.IsByRefLike) || actualReturnType.IsPermanentHome, VerifierError.ReturnPtrToStack);
+                }
+            }
+        }
+
+        bool IsTaskOrValueTaskType(TypeDesc type, out TypeDesc unwrappedType)
+        {
+            unwrappedType = null;
+
+            if (type is not MetadataType metadataType)
+                return false;
+
+            if (!metadataType.Namespace.SequenceEqual("System.Threading.Tasks"u8))
+                return false;
+
+            // Check for Task (non-generic)
+            if (metadataType.Name.SequenceEqual("Task"u8) && !metadataType.HasInstantiation)
+            {
+                unwrappedType = _typeSystemContext.GetWellKnownType(WellKnownType.Void);
+                return true;
+            }
 
-                    Check((!declaredReturnType.IsByRef && !declaredReturnType.IsByRefLike) || actualReturnType.IsPermanentHome, VerifierError.ReturnPtrToStack);
+            // Check for ValueTask (non-generic)
+            if (metadataType.Name.SequenceEqual("ValueTask"u8) && !metadataType.HasInstantiation)
+            {
+                unwrappedType = _typeSystemContext.GetWellKnownType(WellKnownType.Void);
+                return true;
+            }
+
+            // Check for Task<T> and ValueTask<T>
+            if (metadataType.HasInstantiation && metadataType.Instantiation.Length == 1)
+            {
+                if (metadataType.Name.SequenceEqual("Task`1"u8) || 
+                    metadataType.Name.SequenceEqual("ValueTask`1"u8))
+                {
+                    unwrappedType = metadataType.Instantiation[0];
+                    return true;
                 }
             }
+
+            return false;
         }
 
         void ImportFallthrough(BasicBlock next)

src/tests/ilverify/ILTests/RuntimeAsyncTests.il

@@ -0,0 +1,159 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+.assembly extern System.Runtime
+{
+}
+
+.assembly RuntimeAsyncTests
+{
+}
+
+.class public auto ansi beforefieldinit RuntimeAsyncTestsType
+        extends [System.Runtime]System.Object
+{
+    // Valid async method returning Task<int> with int on stack
+    .method public hidebysig instance class [System.Runtime]System.Threading.Tasks.Task`1<int32> AsyncTaskInt_Valid() cil managed async
+    {
+        ldc.i4.0
+        ret
+    }
+
+    // Valid async method returning ValueTask<int> with int on stack
+    .method public hidebysig instance valuetype [System.Runtime]System.Threading.Tasks.ValueTask`1<int32> AsyncValueTaskInt_Valid() cil managed async
+    {
+        ldc.i4.0
+        ret
+    }
+
+    // Valid async method returning ValueTask<bool> with bool (i4) on stack
+    .method public hidebysig instance valuetype [System.Runtime]System.Threading.Tasks.ValueTask`1<bool> AsyncValueTaskBool_Valid() cil managed async
+    {
+        ldc.i4.1
+        ret
+    }
+
+    // Valid async method returning ValueTask<object> with object on stack
+    .method public hidebysig instance valuetype [System.Runtime]System.Threading.Tasks.ValueTask`1<object> AsyncValueTaskObject_Valid() cil managed async
+    {
+        ldnull
+        ret
+    }
+
+    // Invalid: async method with wrong stack state - empty stack for ValueTask<int>
+    .method public hidebysig instance valuetype [System.Runtime]System.Threading.Tasks.ValueTask`1<int32> AsyncEmptyStack_Invalid_ReturnMissing() cil managed async
+    {
+        ret
+    }
+
+    // Invalid: async method with wrong type on stack
+    .method public hidebysig instance valuetype [System.Runtime]System.Threading.Tasks.ValueTask`1<int32> AsyncWrongType_Invalid_StackUnexpected() cil managed async
+    {
+        ldnull
+        ret
+    }
+
+    // Invalid: async method with extra items on stack
+    .method public hidebysig instance valuetype [System.Runtime]System.Threading.Tasks.ValueTask`1<int32> AsyncExtraStack_Invalid_ReturnEmpty() cil managed async
+    {
+        ldc.i4.0
+        ldc.i4.1
+        ret
+    }
+
+    // Invalid: async method with ValueTask return should have empty stack
+    .method public hidebysig instance valuetype [System.Runtime]System.Threading.Tasks.ValueTask AsyncReturnsInt_Invalid_ReturnVoid() cil managed async
+    {
+        ldc.i4.0
+        ret
+    }
+
+    // Invalid: method with async flag but returning integer type
+    .method public hidebysig instance int32 NonAsyncReturnType_Invalid_StackUnexpected() cil managed async
+    {
+        ldc.i4.0
+        ret
+    }
+
+    // Valid: async method returning Task with nothing on stack
+    .method public hidebysig instance class [System.Runtime]System.Threading.Tasks.Task AsyncTask_Valid() cil managed async
+    {
+        ret
+    }
+
+    // Valid: async method returning ValueTask with nothing on stack
+    .method public hidebysig instance valuetype [System.Runtime]System.Threading.Tasks.ValueTask AsyncValueTask_Valid() cil managed async
+    {
+        ret
+    }
+
+    // Invalid: async method returning from try block (not allowed)
+    .method public hidebysig instance valuetype [System.Runtime]System.Threading.Tasks.ValueTask`1<int32> AsyncReturnFromTry_Invalid_ReturnFromTry() cil managed async
+    {
+        .try
+        {
+            ldc.i4.0
+            ret
+        }
+        catch [System.Runtime]System.Object
+        {
+            pop
+            leave.s lbl_ret
+        }
+
+    lbl_ret:
+        ldc.i4.1
+        ret
+    }
+
+    // Invalid: async method returning from catch block (not allowed)
+    .method public hidebysig instance valuetype [System.Runtime]System.Threading.Tasks.ValueTask`1<int32> AsyncReturnFromCatch_Invalid_ReturnFromHandler() cil managed async
+    {
+        .try
+        {
+            leave.s lbl_ret
+        }
+        catch [System.Runtime]System.Object
+        {
+            pop
+            ldc.i4.0
+            ret
+        }
+
+    lbl_ret:
+        ldc.i4.1
+        ret
+    }
+
+    // Invalid: async method returning from filter (still not allowed)
+    .method public hidebysig instance valuetype [System.Runtime]System.Threading.Tasks.ValueTask`1<int32> AsyncReturnFromFilter_Invalid_ReturnFromFilter() cil managed async
+    {
+        .try
+        {
+            leave.s lbl_ret
+        }
+        filter
+        {
+            pop
+            ldc.i4.0
+            ret
+
+            endfilter
+        }
+        {
+            pop
+            leave.s lbl_ret
+        }
+
+    lbl_ret:
+        ldc.i4.1
+        ret
+    }
+
+    .method public hidebysig specialname rtspecialname instance void .ctor() cil managed
+    {
+        ldarg.0
+        call instance void [System.Runtime]System.Object::.ctor()
+        ret
+    }
+}

src/tests/ilverify/ILTests/RuntimeAsyncTests.ilproj

@@ -0,0 +1,3 @@
+<Project ToolsVersion="15.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <Import Project="ILTests.targets" />
+</Project>