[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

1 files changed, 3 insertions, 3 deletions

diff --git a/libcxx/include/mutex b/libcxx/include/mutex
index dca62202db1..8fc3c61221b 100644
--- a/libcxx/include/mutex
+++ b/libcxx/include/mutex
@@ -86,9 +86,9 @@ public:
     void unlock();
 };
 
-struct defer_lock_t {};
-struct try_to_lock_t {};
-struct adopt_lock_t {};
+struct defer_lock_t { explicit defer_lock_t() = default; };
+struct try_to_lock_t { explicit try_to_lock_t() = default; };
+struct adopt_lock_t { explicit adopt_lock_t() = default; };
 
 inline constexpr defer_lock_t  defer_lock{};
 inline constexpr try_to_lock_t try_to_lock{};