6 files changed, 73 insertions, 20 deletions
diff --git a/libcxx/include/__config b/libcxx/include/__config
index 51ac16cc572..1ed51699932 100644
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -1097,6 +1097,14 @@ _LIBCPP_FUNC_VIS extern "C" void __sanitizer_annotate_contiguous_container(
        _LIBCPP_HAS_NO_THREADS is defined.
 #endif
 
+// The Apple, glibc, and Bionic implementation of pthreads implements
+// pthread_mutex_destroy as nop for regular mutexes.
+// TODO(EricWF): Enable this optimization on Apple and Bionic platforms after
+// speaking to their respective stakeholders.
+#if defined(_LIBCPP_HAS_THREAD_API_PTHREAD) && defined(__GLIBC__)
+# define _LIBCPP_HAS_TRIVIAL_MUTEX_DESTRUCTION
+#endif
+
 // Systems that use capability-based security (FreeBSD with Capsicum,
 // Nuxi CloudABI) may only provide local filesystem access (using *at()).
 // Functions like open(), rename(), unlink() and stat() should not be
diff --git a/libcxx/include/__mutex_base b/libcxx/include/__mutex_base
index 0c34a3bf709..61abae979ac 100644
--- a/libcxx/include/__mutex_base
+++ b/libcxx/include/__mutex_base
@@ -36,28 +36,24 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 #  endif
 #endif  // _LIBCPP_THREAD_SAFETY_ANNOTATION
 
+
 class _LIBCPP_TYPE_VIS _LIBCPP_THREAD_SAFETY_ANNOTATION(capability("mutex")) mutex
 {
-#ifndef _LIBCPP_CXX03_LANG
     __libcpp_mutex_t __m_ = _LIBCPP_MUTEX_INITIALIZER;
-#else
-    __libcpp_mutex_t __m_;
-#endif
 
 public:
     _LIBCPP_INLINE_VISIBILITY
-#ifndef _LIBCPP_CXX03_LANG
-    constexpr mutex() = default;
+    _LIBCPP_CONSTEXPR mutex() = default;
+
+    mutex(const mutex&) = delete;
+    mutex& operator=(const mutex&) = delete;
+
+#if defined(_LIBCPP_HAS_TRIVIAL_MUTEX_DESTRUCTION)
+    ~mutex() = default;
 #else
-    mutex() _NOEXCEPT {__m_ = (__libcpp_mutex_t)_LIBCPP_MUTEX_INITIALIZER;}
-#endif
     ~mutex() _NOEXCEPT;
+#endif
 
-private:
-    mutex(const mutex&);// = delete;
-    mutex& operator=(const mutex&);// = delete;
-
-public:
     void lock() _LIBCPP_THREAD_SAFETY_ANNOTATION(acquire_capability());
     bool try_lock() _NOEXCEPT _LIBCPP_THREAD_SAFETY_ANNOTATION(try_acquire_capability(true));
     void unlock() _NOEXCEPT _LIBCPP_THREAD_SAFETY_ANNOTATION(release_capability());
diff --git a/libcxx/src/CMakeLists.txt b/libcxx/src/CMakeLists.txt
index 2a8ff2c2d89..56d7b9888e3 100644
--- a/libcxx/src/CMakeLists.txt
+++ b/libcxx/src/CMakeLists.txt
@@ -22,6 +22,7 @@ set(LIBCXX_SOURCES
   locale.cpp
   memory.cpp
   mutex.cpp
+  mutex_destructor.cpp
   new.cpp
   optional.cpp
   random.cpp
diff --git a/libcxx/src/mutex.cpp b/libcxx/src/mutex.cpp
index 33a8197dadf..0d69d7cacfa 100644
--- a/libcxx/src/mutex.cpp
+++ b/libcxx/src/mutex.cpp
@@ -25,10 +25,7 @@ const defer_lock_t  defer_lock = {};
 const try_to_lock_t try_to_lock = {};
 const adopt_lock_t  adopt_lock = {};
 
-mutex::~mutex() _NOEXCEPT
-{
-    __libcpp_mutex_destroy(&__m_);
-}
+// ~mutex is defined elsewhere
 
 void
 mutex::lock()
diff --git a/libcxx/src/mutex_destructor.cpp b/libcxx/src/mutex_destructor.cpp
new file mode 100644
index 00000000000..3a6b6b55eeb
--- /dev/null
+++ b/libcxx/src/mutex_destructor.cpp
@@ -0,0 +1,51 @@
+//===--------------------- mutex_destructor.cpp ---------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// Define ~mutex.
+//
+// On some platforms ~mutex has been made trivial and the definition is only
+// provided for ABI compatibility.
+//
+// In order to avoid ODR violations within libc++ itself, we need to ensure
+// that *nothing* sees the non-trivial mutex declaration. For this reason
+// we re-declare the entire class in this file instead of using
+// _LIBCPP_BUILDING_LIBRARY to change the definition in the headers.
+
+#include "__config"
+#include "__threading_support"
+
+#if !defined(_LIBCPP_HAS_NO_THREADS)
+#if _LIBCPP_ABI_VERSION == 1 || !defined(_LIBCPP_HAS_TRIVIAL_MUTEX_DESTRUCTION)
+#define NEEDS_MUTEX_DESTRUCTOR
+#endif
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#ifdef NEEDS_MUTEX_DESTRUCTOR
+class _LIBCPP_TYPE_VIS mutex
+{
+    __libcpp_mutex_t __m_ = _LIBCPP_MUTEX_INITIALIZER;
+
+public:
+    _LIBCPP_ALWAYS_INLINE _LIBCPP_INLINE_VISIBILITY
+    constexpr mutex() = default;
+    mutex(const mutex&) = delete;
+    mutex& operator=(const mutex&) = delete;
+    ~mutex() noexcept;
+};
+
+
+mutex::~mutex() _NOEXCEPT
+{
+    __libcpp_mutex_destroy(&__m_);
+}
+
+#endif // !_LIBCPP_HAS_NO_THREADS
+_LIBCPP_END_NAMESPACE_STD
+
diff --git a/libcxx/test/std/thread/thread.mutex/thread.mutex.requirements/thread.mutex.requirements.mutex/thread.mutex.class/default.pass.cpp b/libcxx/test/std/thread/thread.mutex/thread.mutex.requirements/thread.mutex.requirements.mutex/thread.mutex.class/default.pass.cpp
index e2fd416b6e4..f754e7b7031 100644
--- a/libcxx/test/std/thread/thread.mutex/thread.mutex.requirements/thread.mutex.requirements.mutex/thread.mutex.class/default.pass.cpp
+++ b/libcxx/test/std/thread/thread.mutex/thread.mutex.requirements/thread.mutex.requirements.mutex/thread.mutex.class/default.pass.cpp
@@ -21,8 +21,8 @@
 
 int main(int, char**)
 {
-    static_assert(std::is_nothrow_default_constructible<std::mutex>::value, "");
-    std::mutex m;
-
+  static_assert(std::is_nothrow_default_constructible<std::mutex>::value, "");
+  std::mutex m;
+  ((void)m);
   return 0;
 }