Revert r372777: [libc++] Implement LWG 2510 and its follow-ups

This also reverts:
 - r372778: [libc++] Implement LWG 3158
 - r372782: [libc++] Try fixing tests that fail on GCC 5 and older
 - r372787: Purge mentions of GCC 4 from the test suite

Reason: the change breaks compilation of LLVM with libc++, for details see
http://lists.llvm.org/pipermail/libcxx-dev/2019-September/000599.html

llvm-svn: 372832

1 files changed, 3 insertions, 3 deletions

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