Test convert nullable to non-nullable using custom awaitable (#39417)

src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs

@@ -123648,5 +123648,65 @@ void Method(Program x)
             Assert.Equal(CodeAnalysis.NullableAnnotation.None, model.GetTypeInfo(identifiers[3]).Nullability.Annotation);
             // Note: this discrepancy causes some issues with type simplification in the IDE layer
         }
+
+        [Fact, WorkItem(39417, "https://github.com/dotnet/roslyn/issues/39417")]
+        public void ConfigureAwait_DetectSettingNullableToNonNullableType()
+        {
+            var source =
+@"using System.Threading.Tasks;
+#nullable enable
+class Request
+{
+    public string? Name { get; }
+    public Task<string?> GetName() => Task.FromResult(Name);
+}
+class Handler
+{
+    public async Task Handle(Request request)
+    {
+        string a = request.Name;
+        string b = await request.GetName().ConfigureAwait(false);
+    }
+}
+";
+            var comp = CreateCompilation(source);
+            comp.VerifyDiagnostics(
+                Diagnostic(ErrorCode.WRN_ConvertingNullableToNonNullable, "request.Name").WithLocation(12, 20),
+                Diagnostic(ErrorCode.WRN_ConvertingNullableToNonNullable, "await request.GetName().ConfigureAwait(false)").WithLocation(13, 20)
+                );
+        }
+
+        [Fact, WorkItem(39417, "https://github.com/dotnet/roslyn/issues/39417")]
+        public void CustomAwaitable_DetectSettingNullableToNonNullableType()
+        {
+            var source =
+@"using System.Runtime.CompilerServices;
+using System.Threading.Tasks;
+#nullable enable
+static class TaskExtensions
+{
+    public static ConfiguredTaskAwaitable<TResult> NoSync<TResult>(this Task<TResult> task)
+    {
+        return task.ConfigureAwait(false);
+    }
+}
+class Request
+{
+    public string? Name { get; }
+    public Task<string?> GetName() => Task.FromResult(Name);
+}
+class Handler
+{
+    public async Task Handle(Request request)
+    {
+        string a = await request.GetName().NoSync();
+    }
+}
+";
+            var comp = CreateCompilation(source);
+            comp.VerifyDiagnostics(
+                Diagnostic(ErrorCode.WRN_ConvertingNullableToNonNullable, "await request.GetName().NoSync()").WithLocation(20, 20)
+                );
+        }
     }
 }