Fixed async lambda resolve bug.

Would be nice to have a newer c# language spec ...

ICSharpCode.NRefactory.CSharp/Resolver/ResolveVisitor.cs

@@ -2601,6 +2601,14 @@ namespace ICSharpCode.NRefactory.CSharp.Resolver
 					for (int i = 0; i < returnValues.Count; i++) {
 						returnValues[i] = resolveResultCache[returnExpressions[i]];
 					}
+
+					// async lambdas without return statements are resolved as Task return types.
+					if (returnExpressions.Count == 0 && isAsync) {
+						inferredReturnType = resolver.Compilation.FindType(KnownTypeCode.Task);
+						Log.WriteLine("Lambda return type was inferred to: " + inferredReturnType);
+						return;
+					} 
+
 					TypeInference ti = new TypeInference(resolver.Compilation, resolver.conversions);
 					bool tiSuccess;
 					inferredReturnType = ti.GetBestCommonType(returnValues, out tiSuccess);

ICSharpCode.NRefactory.Tests/CSharp/Resolver/LambdaTests.cs

@@ -635,5 +635,38 @@ class TestClass {
 			Assert.IsFalse(mrr.IsError);
 			Assert.AreEqual("System.Int32", mrr.Type.ReflectionName);
 		}
+
+		[Test]
+		public void AsyncLambdaWithAwait()
+		{
+			string program = @"
+using System;
+using System.Threading.Tasks;
+
+class A
+{
+	public Task OpenAsync ()
+	{
+		return null;
+	}
+}
+
+class C
+{
+	async void Foo ()
+	{
+			await $Test (async () => { await new A().OpenAsync (); })$;
+	}
+	
+	T Test<T> (Func<T> func)
+	{
+		return default (T);
+	}
+}
+";
+			var mrr = Resolve<CSharpInvocationResolveResult>(program);
+			Assert.IsFalse(mrr.IsError);
+			Assert.AreEqual("System.Threading.Tasks.Task", mrr.Type.ReflectionName);
+		}
 	}
 }