Allow ?. expression as value of async Task method even if type is unconstrained

Fixes #74
Closes #799

src/Compilers/CSharp/Portable/Binder/Binder_Expressions.cs

@@ -6307,10 +6307,11 @@ private BoundConditionalAccess BindConditionalAccessExpression(ConditionalAccess
             // access cannot be a pointer
             if ((!accessType.IsReferenceType && !accessType.IsValueType) || accessType.IsPointerType())
             {
-                // Assume result type of the access is void when result value isn't used and cannot be made nullable.
-                // We are not doing this for types that can be made nullable to still allow expression evaluator to 
-                // to get the value.
-                bool resultIsNotUsed = false;
+                // Result type of the access is void when result value cannot be made nullable.
+                // For improved diagnostics we detect the cases where the value will be used and produce a
+                // more specific (though not technically correct) diagnostic here:
+                // "Error CS0023: Operator '?' cannot be applied to operand of type 'T'"
+                bool resultIsUsed = true;
                 CSharpSyntaxNode parent = node.Parent;
 
                 if (parent != null)
@@ -6318,37 +6319,35 @@ private BoundConditionalAccess BindConditionalAccessExpression(ConditionalAccess
                     switch (parent.Kind())
                     {
                         case SyntaxKind.ExpressionStatement:
-                            resultIsNotUsed = ((ExpressionStatementSyntax)parent).Expression == node;
+                            resultIsUsed = ((ExpressionStatementSyntax)parent).Expression != node;
                             break;
 
                         case SyntaxKind.SimpleLambdaExpression:
-                            resultIsNotUsed = (((SimpleLambdaExpressionSyntax)parent).Body == node) && ContainingMethodOrLambdaReturnsVoid();
+                            resultIsUsed = (((SimpleLambdaExpressionSyntax)parent).Body != node) || ContainingMethodOrLambdaRequiresValue();
                             break;
 
                         case SyntaxKind.ParenthesizedLambdaExpression:
-                            resultIsNotUsed = (((ParenthesizedLambdaExpressionSyntax)parent).Body == node) && ContainingMethodOrLambdaReturnsVoid();
+                            resultIsUsed = (((ParenthesizedLambdaExpressionSyntax)parent).Body != node) || ContainingMethodOrLambdaRequiresValue();
                             break;
 
                         case SyntaxKind.ArrowExpressionClause:
-                            resultIsNotUsed = (((ArrowExpressionClauseSyntax)parent).Expression == node) && ContainingMethodOrLambdaReturnsVoid();
+                            resultIsUsed = (((ArrowExpressionClauseSyntax)parent).Expression != node) || ContainingMethodOrLambdaRequiresValue();
                             break;
 
                         case SyntaxKind.ForStatement:
                             // Incrementors and Initializers doesn't have to produce a value
                             var loop = (ForStatementSyntax)parent;
-                            resultIsNotUsed = loop.Incrementors.Contains(node) || loop.Initializers.Contains(node);
+                            resultIsUsed = !loop.Incrementors.Contains(node) && !loop.Initializers.Contains(node);
                             break;
                     }
                 }
 
-                if (resultIsNotUsed)
-                {
-                    accessType = GetSpecialType(SpecialType.System_Void, diagnostics, node);
-                }
-                else
+                if (resultIsUsed)
                 {
                     return GenerateBadConditionalAccessNodeError(node, receiver, access, diagnostics);
                 }
+
+                accessType = GetSpecialType(SpecialType.System_Void, diagnostics, node);
             }
 
             // if access has value type, the type of the conditional access is nullable of that
@@ -6360,10 +6359,13 @@ private BoundConditionalAccess BindConditionalAccessExpression(ConditionalAccess
             return new BoundConditionalAccess(node, receiver, access, accessType);
         }
 
-        private bool ContainingMethodOrLambdaReturnsVoid()
+        private bool ContainingMethodOrLambdaRequiresValue()
         {
-            Symbol containingMember = ContainingMemberOrLambda;
-            return (object)containingMember != null && containingMember.Kind == SymbolKind.Method && ((MethodSymbol)containingMember).ReturnsVoid;
+            var containingMethod = ContainingMemberOrLambda as MethodSymbol;
+            return
+                (object)containingMethod == null ||
+                    !containingMethod.ReturnsVoid &&
+                    !containingMethod.IsTaskReturningAsync(this.Compilation);
         }
 
         private BoundConditionalAccess GenerateBadConditionalAccessNodeError(ConditionalAccessExpressionSyntax node, BoundExpression receiver, BoundExpression access, DiagnosticBag diagnostics)

src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenShortCircuitOperatorTests.cs

@@ -5542,5 +5542,57 @@ public T M()
 M
 ---");
         }
+
+        [WorkItem(74, "https://github.com/dotnet/roslyn/issues/74")]
+        [Fact]
+        public void ConditionalInAsyncTask()
+        {
+            var source = @"
+#pragma warning disable CS1998 // suppress 'no await in async' warning
+using System;
+using System.Threading.Tasks;
+
+class Foo<T>
+{
+    public T Method(int i)
+    {
+        Console.Write(i);
+        return default(T); // returns value of unconstrained type parameter type
+    }
+    public void M1(Foo<T> x) => x?.Method(4);
+    public async void M2(Foo<T> x) => x?.Method(5);
+    public async Task M3(Foo<T> x) => x?.Method(6);
+    public async Task M4() {
+        Foo<T> a = new Foo<T>();
+        Foo<T> b = null;
+
+        Action f1 = async () => a?.Method(1);
+        f1();
+        f1 = async () => b?.Method(0);
+        f1();
+
+        Func<Task> f2 = async () => a?.Method(2);
+        await f2();
+        Func<Task> f3 = async () => b?.Method(3);
+        await f3();
+
+        M1(a); M1(b);
+        M2(a); M2(b);
+        await M3(a);
+        await M3(b);
+    }
+}
+class Program
+{
+    public static void Main()
+    {
+        // this will complete synchronously as there are no truly async ops.
+        new Foo<int>().M4();
+    }
+}";
+            var compilation = CreateCompilationWithMscorlib45(
+                source, references: new[] { SystemRef_v4_0_30319_17929, SystemCoreRef_v4_0_30319_17929, CSharpRef }, options: TestOptions.DebugExe);
+            CompileAndVerify(compilation, expectedOutput: "12456");
+        }
     }
 }