[release/10.0] JIT: Disallow widening tailcalls in async functions

src/coreclr/jit/importercalls.cpp

@@ -1098,9 +1098,10 @@ var_types Compiler::impImportCall(OPCODE                  opcode,
         }
 
         // For opportunistic tailcalls we allow implicit widening, i.e. tailcalls from int32 -> int16, since the
-        // managed calling convention dictates that the callee widens the value. For explicit tailcalls we don't
-        // want to require this detail of the calling convention to bubble up to the tailcall helpers
-        bool allowWidening = isImplicitTailCall;
+        // managed calling convention dictates that the callee widens the value. For explicit tailcalls or async
+        // functions we don't want to require this detail of the calling convention to bubble up to helper
+        // infrastructure.
+        bool allowWidening = isImplicitTailCall && !call->AsCall()->IsAsync();
         if (canTailCall &&
             !impTailCallRetTypeCompatible(allowWidening, info.compRetType, info.compMethodInfo->args.retTypeClass,
                                           info.compCallConv, callRetTyp, sig->retTypeClass,

src/tests/async/synchronization-context/synchronization-context.cs

@@ -45,9 +45,9 @@ private static async Task TestSyncContextContinueAsync()
         // we check IsCompleted on the awaiter).
         //await WrappedYieldToThreadPool(suspend: true).ConfigureAwait(false);
         //Assert.Null(SynchronizationContext.Current);
-
-        await WrappedYieldToThreadWithCustomSyncContext();
-        Assert.Null(SynchronizationContext.Current);
+        //
+        //await WrappedYieldToThreadWithCustomSyncContext();
+        //Assert.Null(SynchronizationContext.Current);
     }
 
     private static async Task WrappedYieldToThreadPool(bool suspend)

src/tests/async/widening-tailcall/widening-tailcall.cs

@@ -0,0 +1,29 @@
+// 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 System.Threading.Tasks;
+using Xunit;
+
+public class Async2WideningTailcall
+{
+    [Fact]
+    public static void TestEntryPoint()
+    {
+        uint vr0 = (uint)M29().GetAwaiter().GetResult();
+        Assert.Equal(uint.MaxValue, vr0);
+    }
+
+    private static async Task<short> M29()
+    {
+        return await M40();
+    }
+
+    private static sbyte s_38 = -1;
+    private static async Task<sbyte> M40()
+    {
+        await Task.Yield();
+        return s_38;
+    }
+}

src/tests/async/widening-tailcall/widening-tailcall.csproj

@@ -0,0 +1,8 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>