Merge pull request #20975 from rchande/fix20937

Show builder for async lambda

src/EditorFeatures/CSharpTest/Completion/CompletionProviders/SuggestionModeCompletionProviderTests.cs

@@ -761,6 +761,42 @@ class C {
             await VerifyNotBuilderAsync(markup);
         }
 
+        [WorkItem(20937, "https://github.com/dotnet/roslyn/issues/20937")]
+        [Fact, Trait(Traits.Feature, Traits.Features.Completion)]
+        public async Task AsyncLambda()
+        {
+            var markup = @"
+using System;
+using System.Threading.Tasks;
+class Program
+{
+    public void B(Func<int, int, Task<int>> f) { }
+
+    void A()
+    {
+        B(async($$";
+
+            await VerifyBuilderAsync(markup);
+        }
+
+        [WorkItem(20937, "https://github.com/dotnet/roslyn/issues/20937")]
+        [Fact, Trait(Traits.Feature, Traits.Features.Completion)]
+        public async Task AsyncLambdaAfterComma()
+        {
+            var markup = @"
+using System;
+using System.Threading.Tasks;
+class Program
+{
+    public void B(Func<int, int, Task<int>> f) { }
+
+    void A()
+    {
+        B(async(p1, $$";
+
+            await VerifyBuilderAsync(markup);
+        }
+
         private async Task VerifyNotBuilderAsync(string markup)
         {
             await VerifyWorkerAsync(markup, isBuilder: false);

src/Features/CSharp/Portable/Completion/SuggestionMode/CSharpSuggestionModeCompletionProvider.cs

@@ -168,6 +168,19 @@ private bool IsLambdaExpression(SemanticModel semanticModel, int position, Synta
                 return false;
             }
 
+            // async lambda: 
+            //    Foo(async($$
+            //    Foo(async(p1, $$
+            if (token.IsKind(SyntaxKind.OpenParenToken, SyntaxKind.CommaToken) && token.Parent.IsKind(SyntaxKind.ArgumentList)
+                && token.Parent.Parent is InvocationExpressionSyntax invocation
+                && invocation.Expression is IdentifierNameSyntax identifier)
+            {
+                if (identifier.Identifier.IsKindOrHasMatchingText(SyntaxKind.AsyncKeyword))
+                {
+                    return true;
+                }
+            }
+
             // If we're an argument to a function with multiple overloads, 
             // open the builder if any overload takes a delegate at our argument position
             var inferredTypeInfo = typeInferrer.GetTypeInferenceInfo(semanticModel, position, cancellationToken: cancellationToken);