Implement LWG 2960: nonesuch is insufficiently useless

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@359526 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/experimental/utilities/meta/meta.detect/nonesuch.fail.cpp

@@ -0,0 +1,32 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11
+// <experimental/type_traits>
+//
+//  struct nonesuch;
+//	  nonesuch has no default constructor (C++17 §15.1) 
+//    or initializer-list constructor (C++17 §11.6.4), 
+//    and is not an aggregate (C++17 §11.6.1).
+
+
+#include <experimental/type_traits>
+#include <string>
+
+#include "test_macros.h"
+
+namespace ex = std::experimental;
+
+void doSomething (const ex::nonesuch &) {}
+
+int main(int, char**) {
+    ex::nonesuch *e0 = new ex::nonesuch; // expected-error {{no matching constructor for initialization of 'ex::nonesuch'}}
+    doSomething({}); // expected-error{{no matching function for call to 'doSomething'}}
+
+    return 0;
+}

test/std/experimental/utilities/meta/meta.detect/nonesuch.pass.cpp

@@ -0,0 +1,29 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11
+// <experimental/type_traits>
+//
+//  struct nonesuch
+
+#include <experimental/type_traits>
+#include <string>
+
+#include "test_macros.h"
+
+namespace ex = std::experimental;
+
+struct such {};
+void foo(const such &) {}
+void foo(const ex::nonesuch &) {}
+
+int main(int, char**) {
+    foo({});  // nonesuch is not an aggregate
+
+    return 0;
+}