Merge pull request #1910 from gafter/fix1830

Give an error when a type argument is inferred as void, and when a return statement has a void expression.

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

@@ -2908,15 +2908,8 @@ private BoundReturnStatement BindReturn(ReturnStatementSyntax syntax, Diagnostic
             }
             else
             {
-                // Check that the returned expression is not void. "return void" is not allowed anywhere outside
-                // expression-lambdas.
-                //
-                // Note: in the case of "return void" in non-async lambdas used in return-type-inference,
-                // simply return "void" as the inferred return type and give an error if that didn't work.
-                // We can't follow the same route in async methods, since empty-return-operands become a "Task"
-                // return type.
-
-                if ((object)arg?.Type != null && IsInAsyncMethod() && arg.Type.SpecialType == SpecialType.System_Void)
+                // Check that the returned expression is not void.
+                if ((object)arg?.Type != null && arg.Type.SpecialType == SpecialType.System_Void)
                 {
                     Error(diagnostics, ErrorCode.ERR_CantReturnVoid, expressionSyntax);
                 }

src/Compilers/CSharp/Portable/Symbols/ConstraintsHelper.cs

@@ -405,7 +405,7 @@ private static bool CheckConstraintsSingleType(TypeSymbol type, CheckConstraints
 
             var diagnosticsBuilder = ArrayBuilder<TypeParameterDiagnosticInfo>.GetInstance();
             ArrayBuilder<TypeParameterDiagnosticInfo> useSiteDiagnosticsBuilder = null;
-            var result = CheckTypeConstraints(type, conversions, currentCompilation, diagnosticsBuilder, ref useSiteDiagnosticsBuilder);
+            var result = !typeSyntax.HasErrors && CheckTypeConstraints(type, conversions, currentCompilation, diagnosticsBuilder, ref useSiteDiagnosticsBuilder);
 
             if (useSiteDiagnosticsBuilder != null)
             {
@@ -647,7 +647,7 @@ private static bool InterfacesAreDistinct(NamedTypeSymbol type, ConsList<Symbol>
                 return true;
             }
 
-            if (typeArgument.IsPointerType() || typeArgument.IsRestrictedType())
+            if (typeArgument.IsPointerType() || typeArgument.IsRestrictedType() || typeArgument.SpecialType == SpecialType.System_Void)
             {
                 // "The type '{0}' may not be used as a type argument"
                 diagnosticsBuilder.Add(new TypeParameterDiagnosticInfo(typeParameter, new CSDiagnosticInfo(ErrorCode.ERR_BadTypeArgument, typeArgument)));

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

@@ -1137,5 +1137,27 @@ void M()
                 //         var d = new System.Action(() => (new object()));
                 Diagnostic(ErrorCode.ERR_IllegalStatement, "(new object())").WithLocation(6, 41));
         }
+
+        [WorkItem(1830, "https://github.com/dotnet/roslyn/issues/1830")]
+        [Fact]
+        public void FuncOfVoid()
+        {
+            var comp = CreateCompilationWithMscorlib(@"
+using System;
+class Program
+{
+    void M1<T>(Func<T> f) {}
+    void Main(string[] args)
+    {
+        M1(() => { return System.Console.Beep(); });
+    }
+}
+");
+            comp.VerifyDiagnostics(
+                // (8,27): error CS4029: Cannot return an expression of type 'void'
+                //         M1(() => { return System.Console.Beep(); });
+                Diagnostic(ErrorCode.ERR_CantReturnVoid, "System.Console.Beep()").WithLocation(8, 27)
+                );
+        }
     }
 }