[libc++] Implement LWG 2510

Summary:
LWG2510 makes tag types like allocator_arg_t explicitly default
constructible instead of implicitly default constructible. It also
makes the constructors for std::pair and std::tuple conditionally
explicit based on the explicit-ness of the default constructibility
for the pair/tuple's elements.

Reviewers: mclow.lists, EricWF

Subscribers: christof, jkorous, dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D65161

llvm-svn: 372777

1 files changed, 26 insertions, 0 deletions

diff --git a/libcxx/test/std/language.support/support.dynamic/nothrow_t.fail.cpp b/libcxx/test/std/language.support/support.dynamic/nothrow_t.fail.cpp
new file mode 100644
index 00000000000..c1092c3516b
--- /dev/null
+++ b/libcxx/test/std/language.support/support.dynamic/nothrow_t.fail.cpp
@@ -0,0 +1,26 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03
+
+// struct nothrow_t {
+//   explicit nothrow_t() = default;
+// };
+// extern const nothrow_t nothrow;
+
+// This test checks for LWG 2510.
+
+#include <new>
+
+
+std::nothrow_t f() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
+
+int main(int, char**) {
+    return 0;
+}