[libc++] Take 2: 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.

This was previously committed as r372777 and reverted in r372832 due to
the commit breaking LLVM's build in C++14 mode. This issue has now been
addressed.

Reviewers: mclow.lists

Subscribers: christof, jkorous, dexonsmith, libcxx-commits

Tags: #libc

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

llvm-svn: 372983

2 files changed, 31 insertions, 2 deletions

diff --git a/libcxx/test/std/utilities/memory/allocator.tag/allocator_arg.fail.cpp b/libcxx/test/std/utilities/memory/allocator.tag/allocator_arg.fail.cpp
new file mode 100644
index 00000000000..fb991d425f1
--- /dev/null
+++ b/libcxx/test/std/utilities/memory/allocator.tag/allocator_arg.fail.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
+
+// Before GCC 6, aggregate initialization kicks in.
+// See https://stackoverflow.com/q/41799015/627587.
+// UNSUPPORTED: gcc-5
+
+// <memory>
+
+// struct allocator_arg_t { explicit allocator_arg_t() = default; };
+// const allocator_arg_t allocator_arg = allocator_arg_t();
+
+// This test checks for LWG 2510.
+
+#include <memory>
+
+
+std::allocator_arg_t f() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
+
+int main(int, char**) {
+    return 0;
+}
diff --git a/libcxx/test/std/utilities/memory/allocator.tag/allocator_arg.pass.cpp b/libcxx/test/std/utilities/memory/allocator.tag/allocator_arg.pass.cpp
index 0253243319a..e22bec1e514 100644
--- a/libcxx/test/std/utilities/memory/allocator.tag/allocator_arg.pass.cpp
+++ b/libcxx/test/std/utilities/memory/allocator.tag/allocator_arg.pass.cpp
@@ -8,7 +8,7 @@
 
 // <memory>
 
-// struct allocator_arg_t { };
+// struct allocator_arg_t { explicit allocator_arg_t() = default; };
 // const allocator_arg_t allocator_arg = allocator_arg_t();
 
 #include <memory>
@@ -21,5 +21,5 @@ int main(int, char**)
 {
     test(std::allocator_arg);
 
-  return 0;
+    return 0;
 }