diff --git a/libcxx/include/atomic b/libcxx/include/atomic
diff --git a/libcxx/include/barrier b/libcxx/include/barrier
@@ -0,0 +1,322 @@
+// -*- C++ -*-
+//===--------------------------- barrier ----------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP_BARRIER
+#define _LIBCPP_BARRIER
+
+/*
+    barrier synopsis
+
+namespace std
+{
+
+  template<class CompletionFunction = see below>
+  class barrier
+  {
+  public:
+    using arrival_token = see below;
+
+    constexpr explicit barrier(ptrdiff_t phase_count,
+                               CompletionFunction f = CompletionFunction());
+    ~barrier();
+
+    barrier(const barrier&) = delete;
+    barrier& operator=(const barrier&) = delete;
+
+    [[nodiscard]] arrival_token arrive(ptrdiff_t update = 1);
+    void wait(arrival_token&& arrival) const;
+
+    void arrive_and_wait();
+    void arrive_and_drop();
+
+  private:
+    CompletionFunction completion; // exposition only
+  };
+
+}
+
+*/
+
+#include <__config>
+#include <atomic>
+#ifndef _LIBCPP_HAS_NO_TREE_BARRIER
+# include <memory>
+#endif
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+#ifdef _LIBCPP_HAS_NO_THREADS
+# error <barrier> is not supported on this single threaded system
+#endif
+
+#if _LIBCPP_STD_VER < 14
+# error <barrier> requires C++14 or later
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+struct __empty_completion
+{
+    inline _LIBCPP_INLINE_VISIBILITY
+    void operator()() noexcept
+    {
+    }
+};
+
+#ifndef _LIBCPP_HAS_NO_TREE_BARRIER
+
+/*
+
+The default implementation of __barrier_base is a classic tree barrier.
+
+It looks different from literature pseudocode for two main reasons:
+ 1. Threads that call into std::barrier functions do not provide indices,
+    so a numbering step is added before the actual barrier algorithm,
+    appearing as an N+1 round to the N rounds of the tree barrier.
+ 2. A great deal of attention has been paid to avoid cache line thrashing
+    by flattening the tree structure into cache-line sized arrays, that
+    are indexed in an efficient way.
+
+*/
+
+using __barrier_phase_t = uint8_t;
+
+class __barrier_algorithm_base;
+
+_LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI
+__barrier_algorithm_base* __construct_barrier_algorithm_base(ptrdiff_t& __expected);
+
+_LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI
+bool __arrive_barrier_algorithm_base(__barrier_algorithm_base* __barrier,
+                                     __barrier_phase_t __old_phase);
+
+_LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI
+void __destroy_barrier_algorithm_base(__barrier_algorithm_base* __barrier);
+
+template<class _CompletionF>
+class __barrier_base {
+
+    ptrdiff_t                                               __expected;
+    unique_ptr<__barrier_algorithm_base,
+               decltype(&__destroy_barrier_algorithm_base)> __base;
+    __atomic_base<ptrdiff_t>                                __expected_adjustment;
+    _CompletionF                                            __completion;
+    __atomic_base<__barrier_phase_t>                        __phase;
+
+public:
+    using arrival_token = __barrier_phase_t;
+
+    static constexpr ptrdiff_t max() noexcept {
+        return numeric_limits<ptrdiff_t>::max();
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    __barrier_base(ptrdiff_t __expected, _CompletionF __completion = _CompletionF())
+            : __expected(__expected), __base(__construct_barrier_algorithm_base(this->__expected),
+                                             &__destroy_barrier_algorithm_base),
+              __expected_adjustment(0), __completion(move(__completion)), __phase(0)
+    {
+    }
+    [[nodiscard]] _LIBCPP_INLINE_VISIBILITY
+    arrival_token arrive(ptrdiff_t update)
+    {
+        auto const __old_phase = __phase.load(memory_order_relaxed);
+        for(; update; --update)
+            if(__arrive_barrier_algorithm_base(__base.get(), __old_phase)) {
+                __completion();
+                __expected += __expected_adjustment.load(memory_order_relaxed);
+                __expected_adjustment.store(0, memory_order_relaxed);
+                __phase.store(__old_phase + 2, memory_order_release);
+                __phase.notify_all();
+            }
+        return __old_phase;
+    }
+    _LIBCPP_INLINE_VISIBILITY
+    void wait(arrival_token&& __old_phase) const
+    {
+        auto const __test_fn = [=]() -> bool {
+            return __phase.load(memory_order_acquire) != __old_phase;
+        };
+        __libcpp_thread_poll_with_backoff(__test_fn, __libcpp_timed_backoff_policy());
+    }
+    _LIBCPP_INLINE_VISIBILITY
+    void arrive_and_drop()
+    {
+        __expected_adjustment.fetch_sub(1, memory_order_relaxed);
+        (void)arrive(1);
+    }
+};
+
+#else
+
+/*
+
+The alternative implementation of __barrier_base is a central barrier.
+
+Two versions of this algorithm are provided:
+ 1. A fairly straightforward implementation of the litterature for the
+    general case where the completion function is not empty.
+ 2. An optimized implementation that exploits 2's complement arithmetic
+    and well-defined overflow in atomic arithmetic, to handle the phase
+    roll-over for free.
+
+*/
+
+template<class _CompletionF>
+class __barrier_base {
+
+    __atomic_base<ptrdiff_t> __expected;
+    __atomic_base<ptrdiff_t> __arrived;
+    _CompletionF             __completion;
+    __atomic_base<bool>      __phase;
+public:
+    using arrival_token = bool;
+
+    static constexpr ptrdiff_t max() noexcept {
+        return numeric_limits<ptrdiff_t>::max();
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    __barrier_base(ptrdiff_t __expected, _CompletionF __completion = _CompletionF())
+        : __expected(__expected), __arrived(__expected), __completion(move(__completion)), __phase(false)
+    {
+    }
+    [[nodiscard]] _LIBCPP_INLINE_VISIBILITY
+    arrival_token arrive(ptrdiff_t update)
+    {
+        auto const __old_phase = __phase.load(memory_order_relaxed);
+        auto const __result = __arrived.fetch_sub(update, memory_order_acq_rel) - update;
+        auto const new_expected = __expected.load(memory_order_relaxed);
+        if(0 == __result) {
+            __completion();
+            __arrived.store(new_expected, memory_order_relaxed);
+            __phase.store(!__old_phase, memory_order_release);
+            __phase.notify_all();
+        }
+        return __old_phase;
+    }
+    _LIBCPP_INLINE_VISIBILITY
+    void wait(arrival_token&& __old_phase) const
+    {
+        __phase.wait(__old_phase, memory_order_acquire);
+    }
+    _LIBCPP_INLINE_VISIBILITY
+    void arrive_and_drop()
+    {
+        __expected.fetch_sub(1, memory_order_relaxed);
+        (void)arrive(1);
+    }
+};
+
+template<>
+class __barrier_base<__empty_completion> {
+
+    static constexpr uint64_t __expected_unit = 1ull;
+    static constexpr uint64_t __arrived_unit = 1ull << 32;
+    static constexpr uint64_t __expected_mask = __arrived_unit - 1;
+    static constexpr uint64_t __phase_bit = 1ull << 63;
+    static constexpr uint64_t __arrived_mask = (__phase_bit - 1) & ~__expected_mask;
+
+    __atomic_base<uint64_t>   __phase_arrived_expected;
+
+    static _LIBCPP_INLINE_VISIBILITY
+    constexpr uint64_t __init(ptrdiff_t __count) _NOEXCEPT
+    {
+        return ((uint64_t(1u << 31) - __count) << 32)
+              | (uint64_t(1u << 31) - __count);
+    }
+
+public:
+    using arrival_token = uint64_t;
+
+    static constexpr ptrdiff_t max() noexcept {
+        return ptrdiff_t(1u << 31) - 1;
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    explicit inline __barrier_base(ptrdiff_t __count, __empty_completion = __empty_completion())
+        : __phase_arrived_expected(__init(__count))
+    {
+    }
+    [[nodiscard]] inline _LIBCPP_INLINE_VISIBILITY
+    arrival_token arrive(ptrdiff_t update)
+    {
+        auto const __inc = __arrived_unit * update;
+        auto const __old = __phase_arrived_expected.fetch_add(__inc, memory_order_acq_rel);
+        if((__old ^ (__old + __inc)) & __phase_bit) {
+            __phase_arrived_expected.fetch_add((__old & __expected_mask) << 32, memory_order_relaxed);
+            __phase_arrived_expected.notify_all();
+        }
+        return __old & __phase_bit;
+    }
+    inline _LIBCPP_INLINE_VISIBILITY
+    void wait(arrival_token&& __phase) const
+    {
+        auto const __test_fn = [=]() -> bool {
+            uint64_t const __current = __phase_arrived_expected.load(memory_order_acquire);
+            return ((__current & __phase_bit) != __phase);
+        };
+        __libcpp_thread_poll_with_backoff(__test_fn, __libcpp_timed_backoff_policy());
+    }
+    inline _LIBCPP_INLINE_VISIBILITY
+    void arrive_and_drop()
+    {
+        __phase_arrived_expected.fetch_add(__expected_unit, memory_order_relaxed);
+        (void)arrive(1);
+    }
+};
+
+#endif //_LIBCPP_HAS_NO_TREE_BARRIER
+
+template<class _CompletionF = __empty_completion>
+class barrier {
+
+    __barrier_base<_CompletionF> __b;
+public:
+    using arrival_token = typename __barrier_base<_CompletionF>::arrival_token;
+
+    static constexpr ptrdiff_t max() noexcept {
+        return __barrier_base<_CompletionF>::max();
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    barrier(ptrdiff_t __count, _CompletionF __completion = _CompletionF())
+        : __b(__count, std::move(__completion)) {
+    }
+
+    barrier(barrier const&) = delete;
+    barrier& operator=(barrier const&) = delete;
+
+    [[nodiscard]] _LIBCPP_INLINE_VISIBILITY
+    arrival_token arrive(ptrdiff_t update = 1)
+    {
+        return __b.arrive(update);
+    }
+    _LIBCPP_INLINE_VISIBILITY
+    void wait(arrival_token&& __phase) const
+    {
+        __b.wait(std::move(__phase));
+    }
+	_LIBCPP_INLINE_VISIBILITY
+    void arrive_and_wait()
+    {
+        wait(arrive());
+	}
+    _LIBCPP_INLINE_VISIBILITY
+    void arrive_and_drop()
+    {
+        __b.arrive_and_drop();
+    }
+};
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif //_LIBCPP_BARRIER
diff --git a/libcxx/include/latch b/libcxx/include/latch
@@ -0,0 +1,104 @@
+// -*- C++ -*-
+//===--------------------------- latch -----------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP_LATCH
+#define _LIBCPP_LATCH
+
+/*
+    latch synopsis
+
+namespace std
+{
+
+  class latch
+  {
+  public:
+    constexpr explicit latch(ptrdiff_t __expected);
+    ~latch();
+
+    latch(const latch&) = delete;
+    latch& operator=(const latch&) = delete;
+
+    void count_down(ptrdiff_t __update = 1);
+    bool try_wait() const noexcept;
+    void wait() const;
+    void arrive_and_wait(ptrdiff_t __update = 1);
+
+  private:
+    ptrdiff_t __counter; // exposition only
+  };
+
+}
+
+*/
+
+#include <__config>
+#include <atomic>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+#ifdef _LIBCPP_HAS_NO_THREADS
+# error <latch> is not supported on this single threaded system
+#endif
+
+#if _LIBCPP_STD_VER < 14
+# error <latch> requires C++14 or later
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+class latch
+{
+    __atomic_base<ptrdiff_t> __a;
+
+public:
+    static constexpr ptrdiff_t max() noexcept {
+        return numeric_limits<ptrdiff_t>::max();
+    }
+
+    inline _LIBCPP_INLINE_VISIBILITY
+    constexpr explicit latch(ptrdiff_t __expected) : __a(__expected) { }
+
+    ~latch() = default;
+    latch(const latch&) = delete;
+    latch& operator=(const latch&) = delete;
+
+    inline _LIBCPP_INLINE_VISIBILITY
+    void count_down(ptrdiff_t __update = 1)
+    {
+        auto const __old = __a.fetch_sub(__update, memory_order_release);
+        if(__old == __update)
+            __a.notify_all();
+    }
+    inline _LIBCPP_INLINE_VISIBILITY
+    bool try_wait() const noexcept
+    {
+        return 0 == __a.load(memory_order_acquire);
+    }
+    inline _LIBCPP_INLINE_VISIBILITY
+    void wait() const
+    {
+        auto const __test_fn = [=]() -> bool {
+            return try_wait();
+        };
+        __cxx_atomic_wait(&__a.__a_, __test_fn);
+    }
+    inline _LIBCPP_INLINE_VISIBILITY
+    void arrive_and_wait(ptrdiff_t __update = 1)
+    {
+        count_down(__update);
+        wait();
+    }
+};
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif //_LIBCPP_LATCH
diff --git a/libcxx/include/module.modulemap b/libcxx/include/module.modulemap
@@ -231,6 +231,10 @@ module std [system] {
     header "atomic"
     export *
   }
+  module barrier {
+    header "barrier"
+    export *
+  }
   module bit {
     header "bit"
     export *
@@ -334,6 +338,10 @@ module std [system] {
     header "iterator"
     export *
   }
+  module latch {
+    header "latch"
+    export *
+  }
   module limits {
     header "limits"
     export *
@@ -400,6 +408,10 @@ module std [system] {
     header "scoped_allocator"
     export *
   }
+  module semaphore {
+    header "semaphore"
+    export *
+  }
   module set {
     header "set"
     export initializer_list

diff --git a/libcxx/include/semaphore b/libcxx/include/semaphore
@@ -0,0 +1,233 @@
+// -*- C++ -*-
+//===--------------------------- semaphore --------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP_SEMAPHORE
+#define _LIBCPP_SEMAPHORE
+
+/*
+    semaphore synopsis
+
+namespace std {
+
+template<ptrdiff_t least_max_value = implementation-defined>
+class counting_semaphore
+{
+public:
+static constexpr ptrdiff_t max() noexcept;
+
+constexpr explicit counting_semaphore(ptrdiff_t desired);
+~counting_semaphore();
+
+counting_semaphore(const counting_semaphore&) = delete;
+counting_semaphore& operator=(const counting_semaphore&) = delete;
+
+void release(ptrdiff_t update = 1);
+void acquire();
+bool try_acquire() noexcept;
+template<class Rep, class Period>
+    bool try_acquire_for(const chrono::duration<Rep, Period>& rel_time);
+template<class Clock, class Duration>
+    bool try_acquire_until(const chrono::time_point<Clock, Duration>& abs_time);
+
+private:
+ptrdiff_t counter; // exposition only
+};
+
+using binary_semaphore = counting_semaphore<1>;
+
+}
+
+*/
+
+#include <__config>
+#include <__threading_support>
+#include <atomic>
+#include <cassert>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+#ifdef _LIBCPP_HAS_NO_THREADS
+# error <semaphore> is not supported on this single threaded system
+#endif
+
+#if _LIBCPP_STD_VER < 14
+# error <semaphore> is requires C++14 or later
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+/*
+
+__atomic_semaphore_base is the general-case implementation, to be used for
+user-requested least-max values that exceed the OS implementation support
+(incl. when the OS has no support of its own) and for binary semaphores.
+
+It is a typical Dijsktra semaphore algorithm over atomics, wait and notify
+functions. It avoids contention against users' own use of those facilities.
+
+*/
+
+class __atomic_semaphore_base
+{
+    __atomic_base<ptrdiff_t> __a;
+
+public:
+    _LIBCPP_INLINE_VISIBILITY
+    __atomic_semaphore_base(ptrdiff_t __count) : __a(__count)
+    {
+    }
+    _LIBCPP_INLINE_VISIBILITY
+    void release(ptrdiff_t __update = 1)
+    {
+        if(0 < __a.fetch_add(__update, memory_order_release))
+            ;
+        else if(__update > 1)
+            __a.notify_all();
+        else
+            __a.notify_one();
+    }
+    _LIBCPP_INLINE_VISIBILITY
+    void acquire()
+    {
+        auto const __test_fn = [=]() -> bool {
+            auto __old = __a.load(memory_order_relaxed);
+            return (__old != 0) && __a.compare_exchange_strong(__old, __old - 1, memory_order_acquire, memory_order_relaxed);
+        };
+        __cxx_atomic_wait(&__a.__a_, __test_fn);
+    }
+    template <class Rep, class Period>
+    _LIBCPP_INLINE_VISIBILITY
+    bool try_acquire_for(chrono::duration<Rep, Period> const& __rel_time)
+    {
+        auto const __test_fn = [=]() -> bool {
+            auto __old = __a.load(memory_order_acquire);
+            while(1) {
+                if (__old == 0)
+                    return false;
+                if(__a.compare_exchange_strong(__old, __old - 1, memory_order_acquire, memory_order_relaxed))
+                    return true;
+            }
+        };
+        return __libcpp_thread_poll_with_backoff(__test_fn, __libcpp_timed_backoff_policy(), __rel_time);
+    }
+};
+
+#ifndef _LIBCPP_NO_NATIVE_SEMAPHORES
+
+/*
+
+__platform_semaphore_base a simple wrapper for the OS semaphore type. That
+is, every call is routed to the OS in the most direct manner possible.
+
+*/
+
+class __platform_semaphore_base
+{
+    __libcpp_semaphore_t __semaphore;
+
+public:
+    _LIBCPP_INLINE_VISIBILITY
+    __platform_semaphore_base(ptrdiff_t __count) :
+        __semaphore()
+    {
+        __libcpp_semaphore_init(&__semaphore, __count);
+    }
+    _LIBCPP_INLINE_VISIBILITY
+    ~__platform_semaphore_base() {
+        __libcpp_semaphore_destroy(&__semaphore);
+    }
+    _LIBCPP_INLINE_VISIBILITY
+    void release(ptrdiff_t __update)
+    {
+        for(; __update; --__update)
+            __libcpp_semaphore_post(&__semaphore);
+    }
+    _LIBCPP_INLINE_VISIBILITY
+    void acquire()
+    {
+        __libcpp_semaphore_wait(&__semaphore);
+    }
+    _LIBCPP_INLINE_VISIBILITY
+    bool try_acquire_for(chrono::nanoseconds __rel_time)
+    {
+        return __libcpp_semaphore_wait_timed(&__semaphore, __rel_time);
+    }
+};
+
+template<ptrdiff_t __least_max_value>
+using __semaphore_base =
+    typename conditional<(__least_max_value > 1 && __least_max_value <= _LIBCPP_SEMAPHORE_MAX),
+                         __platform_semaphore_base,
+                         __atomic_semaphore_base>::type;
+
+#else
+
+template<ptrdiff_t __least_max_value>
+using __semaphore_base =
+    __atomic_semaphore_base;
+
+#endif //_LIBCPP_NO_NATIVE_SEMAPHORES
+
+template<ptrdiff_t __least_max_value = _LIBCPP_SEMAPHORE_MAX>
+class counting_semaphore
+{
+    __semaphore_base<__least_max_value> __semaphore;
+
+public:
+    static constexpr ptrdiff_t max() noexcept {
+        return __least_max_value;
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    counting_semaphore(ptrdiff_t __count = 0) : __semaphore(__count) { }
+    ~counting_semaphore() = default;
+
+    counting_semaphore(const counting_semaphore&) = delete;
+    counting_semaphore& operator=(const counting_semaphore&) = delete;
+
+    _LIBCPP_INLINE_VISIBILITY
+    void release(ptrdiff_t __update = 1)
+    {
+        __semaphore.release(__update);
+    }
+    _LIBCPP_INLINE_VISIBILITY
+    void acquire()
+    {
+        __semaphore.acquire();
+    }
+    template<class Rep, class Period>
+    _LIBCPP_INLINE_VISIBILITY
+    bool try_acquire_for(chrono::duration<Rep, Period> const& __rel_time)
+    {
+        return __semaphore.try_acquire_for(chrono::duration_cast<chrono::nanoseconds>(__rel_time));
+    }
+    _LIBCPP_INLINE_VISIBILITY
+    bool try_acquire()
+    {
+        return try_acquire_for(chrono::nanoseconds::zero());
+    }
+    template <class Clock, class Duration>
+    _LIBCPP_INLINE_VISIBILITY
+    bool try_acquire_until(chrono::time_point<Clock, Duration> const& __abs_time)
+    {
+        auto const current = Clock::now();
+        if(current >= __abs_time)
+            return try_acquire();
+        else
+            return try_acquire_for(__abs_time - current);
+    }
+};
+
+using binary_semaphore = counting_semaphore<1>;
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif //_LIBCPP_SEMAPHORE
diff --git a/libcxx/lib/abi/CHANGELOG.TXT b/libcxx/lib/abi/CHANGELOG.TXT
@@ -12,6 +12,34 @@ Afterwards the ABI list should be updated to include the new changes.
 
 New entries should be added directly below the "Version" header.
 
+------------
+Version 10.0
+------------
+
+* TODO - [libc++] Implementation of C++20's P1135R6 for libcxx
+
+  libc++ now implements P1135R6 (The C++20 Synchronization Library), which
+  adds a few symbols to the dylib. This is backwards-compatible since we're
+  just adding new symbols.
+
+  x86_64-unknown-linux-gnu
+  ------------------------
+  TODO
+
+  x86_64-apple-apple-darwin
+  -------------------------
+  Symbol added: __ZNSt3__120__libcpp_atomic_waitEPVKNS_17__cxx_atomic_implIxNS_22__cxx_atomic_base_implIxEEEEx
+  Symbol added: __ZNSt3__120__libcpp_atomic_waitEPVKvx
+  Symbol added: __ZNSt3__123__cxx_atomic_notify_allEPVKNS_17__cxx_atomic_implIxNS_22__cxx_atomic_base_implIxEEEE
+  Symbol added: __ZNSt3__123__cxx_atomic_notify_allEPVKv
+  Symbol added: __ZNSt3__123__cxx_atomic_notify_oneEPVKNS_17__cxx_atomic_implIxNS_22__cxx_atomic_base_implIxEEEE
+  Symbol added: __ZNSt3__123__cxx_atomic_notify_oneEPVKv
+  Symbol added: __ZNSt3__123__libcpp_atomic_monitorEPVKNS_17__cxx_atomic_implIxNS_22__cxx_atomic_base_implIxEEEE
+  Symbol added: __ZNSt3__123__libcpp_atomic_monitorEPVKv
+  Symbol added: __ZNSt3__131__arrive_barrier_algorithm_baseEPNS_24__barrier_algorithm_baseEh
+  Symbol added: __ZNSt3__132__destroy_barrier_algorithm_baseEPNS_24__barrier_algorithm_baseE
+  Symbol added: __ZNSt3__134__construct_barrier_algorithm_baseERl
+
 -----------
 Version 9.0
 -----------

diff --git a/libcxx/lib/abi/x86_64-apple-darwin.v1.abilist b/libcxx/lib/abi/x86_64-apple-darwin.v1.abilist
@@ -2420,3 +2420,14 @@
 {'type': 'OBJECT', 'is_defined': True, 'name': '__ZTINSt3__14__fs10filesystem16filesystem_errorE', 'size': 0}
 {'type': 'OBJECT', 'is_defined': True, 'name': '__ZTSNSt3__14__fs10filesystem16filesystem_errorE', 'size': 0}
 {'type': 'OBJECT', 'is_defined': True, 'name': '__ZTVNSt3__14__fs10filesystem16filesystem_errorE', 'size': 0}
+{'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__120__libcpp_atomic_waitEPVKNS_17__cxx_atomic_implIxNS_22__cxx_atomic_base_implIxEEEEx'}
+{'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__120__libcpp_atomic_waitEPVKvx'}
+{'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__123__cxx_atomic_notify_allEPVKNS_17__cxx_atomic_implIxNS_22__cxx_atomic_base_implIxEEEE'}
+{'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__123__cxx_atomic_notify_allEPVKv'}
+{'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__123__cxx_atomic_notify_oneEPVKNS_17__cxx_atomic_implIxNS_22__cxx_atomic_base_implIxEEEE'}
+{'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__123__cxx_atomic_notify_oneEPVKv'}
+{'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__123__libcpp_atomic_monitorEPVKNS_17__cxx_atomic_implIxNS_22__cxx_atomic_base_implIxEEEE'}
+{'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__123__libcpp_atomic_monitorEPVKv'}
+{'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__131__arrive_barrier_algorithm_baseEPNS_24__barrier_algorithm_baseEh'}
+{'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__132__destroy_barrier_algorithm_baseEPNS_24__barrier_algorithm_baseE'}
+{'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__134__construct_barrier_algorithm_baseERl'}
diff --git a/libcxx/lib/abi/x86_64-apple-darwin.v2.abilist b/libcxx/lib/abi/x86_64-apple-darwin.v2.abilist
@@ -2376,3 +2376,14 @@
 {'is_defined': True, 'name': '___dynamic_cast', 'type': 'I'}
 {'is_defined': False, 'name': '___gxx_personality_v0', 'type': 'U'}
 {'is_defined': True, 'name': '___gxx_personality_v0', 'type': 'I'}
+{'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__220__libcpp_atomic_waitEPVKNS_17__cxx_atomic_implIxNS_22__cxx_atomic_base_implIxEEEEx'}
+{'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__220__libcpp_atomic_waitEPVKvx'}
+{'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__223__cxx_atomic_notify_allEPVKNS_17__cxx_atomic_implIxNS_22__cxx_atomic_base_implIxEEEE'}
+{'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__223__cxx_atomic_notify_allEPVKv'}
+{'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__223__cxx_atomic_notify_oneEPVKNS_17__cxx_atomic_implIxNS_22__cxx_atomic_base_implIxEEEE'}
+{'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__223__cxx_atomic_notify_oneEPVKv'}
+{'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__223__libcpp_atomic_monitorEPVKNS_17__cxx_atomic_implIxNS_22__cxx_atomic_base_implIxEEEE'}
+{'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__223__libcpp_atomic_monitorEPVKv'}
+{'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__231__arrive_barrier_algorithm_baseEPNS_24__barrier_algorithm_baseEh'}
+{'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__232__destroy_barrier_algorithm_baseEPNS_24__barrier_algorithm_baseE'}
+{'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__234__construct_barrier_algorithm_baseERl'}
diff --git a/libcxx/src/CMakeLists.txt b/libcxx/src/CMakeLists.txt
@@ -4,6 +4,8 @@ set(LIBCXX_LIB_CMAKEFILES_DIR "${CMAKE_CURRENT_BINARY_DIR}${CMAKE_FILES_DIRECTOR
 set(LIBCXX_SOURCES
   algorithm.cpp
   any.cpp
+  atomic.cpp
+  barrier.cpp
   bind.cpp
   charconv.cpp
   chrono.cpp

diff --git a/libcxx/src/atomic.cpp b/libcxx/src/atomic.cpp
@@ -0,0 +1,189 @@
+//===------------------------- atomic.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
+//
+//===----------------------------------------------------------------------===//
+
+#include <__config>
+#ifndef _LIBCPP_HAS_NO_THREADS
+
+#include <climits>
+#include <atomic>
+#include <functional>
+
+#include <iostream>
+
+#ifdef __linux__
+
+#include <unistd.h>
+#include <linux/futex.h>
+#include <sys/syscall.h>
+
+#else // <- Add other operating systems here
+
+// Baseline needs no new headers
+
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#ifdef __linux__
+
+static void __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr,
+                                              __cxx_contention_t __val)
+{
+    static constexpr timespec __timeout = { 2, 0 };
+    syscall(SYS_futex, __ptr, FUTEX_WAIT_PRIVATE, __val, &__timeout, 0, 0);
+}
+
+static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile* __ptr,
+                                              bool __notify_one)
+{
+    syscall(SYS_futex, __ptr, FUTEX_WAKE_PRIVATE, __notify_one ? 1 : INT_MAX, 0, 0, 0);
+}
+
+#elif defined(__APPLE__) && defined(_LIBCPP_USE_ULOCK)
+
+extern "C" int __ulock_wait(uint32_t operation, void *addr, uint64_t value,
+		uint32_t timeout); /* timeout is specified in microseconds */
+extern "C" int __ulock_wake(uint32_t operation, void *addr, uint64_t wake_value);
+
+#define UL_COMPARE_AND_WAIT				1
+#define ULF_WAKE_ALL					0x00000100
+
+static void __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr,
+                                              __cxx_contention_t __val)
+{
+    __ulock_wait(UL_COMPARE_AND_WAIT,
+                 const_cast<__cxx_atomic_contention_t*>(__ptr), __val, 0);
+}
+
+static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile* __ptr,
+                                              bool __notify_one)
+{
+    __ulock_wake(UL_COMPARE_AND_WAIT | (__notify_one ? 0 : ULF_WAKE_ALL),
+                 const_cast<__cxx_atomic_contention_t*>(__ptr), 0);
+}
+
+#else // <- Add other operating systems here
+
+// Baseline is just a timed backoff
+
+static void __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr,
+                                              __cxx_contention_t __val)
+{
+    __libcpp_thread_poll_with_backoff([=]() -> bool {
+        return !__cxx_nonatomic_compare_equal(__cxx_atomic_load(__ptr, memory_order_relaxed), __val);
+    }, __libcpp_timed_backoff_policy());
+}
+
+static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile*, bool) { }
+
+#endif // __linux__
+
+static constexpr size_t __libcpp_contention_table_size = (1 << 8);  /* < there's no magic in this number */
+
+struct alignas(64) /*  aim to avoid false sharing */ __libcpp_contention_table_entry
+{
+    __cxx_atomic_contention_t __contention_state;
+    __cxx_atomic_contention_t __platform_state;
+    inline constexpr __libcpp_contention_table_entry() :
+        __contention_state(0), __platform_state(0) { }
+};
+
+static __libcpp_contention_table_entry __libcpp_contention_table[ __libcpp_contention_table_size ];
+
+static hash<void const volatile*> __libcpp_contention_hasher;
+
+static __libcpp_contention_table_entry* __libcpp_contention_state(void const volatile * p)
+{
+    return &__libcpp_contention_table[__libcpp_contention_hasher(p) & (__libcpp_contention_table_size - 1)];
+}
+
+/* Given an atomic to track contention and an atomic to actually wait on, which may be
+   the same atomic, we try to detect contention to avoid spuriously calling the platform. */
+
+static void __libcpp_contention_notify(__cxx_atomic_contention_t volatile* __contention_state,
+                                       __cxx_atomic_contention_t const volatile* __platform_state,
+                                       bool __notify_one)
+{
+    if(0 != __cxx_atomic_load(__contention_state, memory_order_seq_cst))
+        // We only call 'wake' if we consumed a contention bit here.
+        __libcpp_platform_wake_by_address(__platform_state, __notify_one);
+}
+static __cxx_contention_t __libcpp_contention_monitor_for_wait(__cxx_atomic_contention_t volatile* __contention_state,
+                                                               __cxx_atomic_contention_t const volatile* __platform_state)
+{
+    // We will monitor this value.
+    return __cxx_atomic_load(__platform_state, memory_order_acquire);
+}
+static void __libcpp_contention_wait(__cxx_atomic_contention_t volatile* __contention_state,
+                                     __cxx_atomic_contention_t const volatile* __platform_state,
+                                     __cxx_contention_t __old_value)
+{
+    __cxx_atomic_fetch_add(__contention_state, __cxx_contention_t(1), memory_order_seq_cst);
+    // We sleep as long as the monitored value hasn't changed.
+    __libcpp_platform_wait_on_address(__platform_state, __old_value);
+    __cxx_atomic_fetch_sub(__contention_state, __cxx_contention_t(1), memory_order_release);
+}
+
+/* When the incoming atomic is the wrong size for the platform wait size, need to
+   launder the value sequence through an atomic from our table. */
+
+static void __libcpp_atomic_notify(void const volatile* __location)
+{
+    auto const __entry = __libcpp_contention_state(__location);
+    // The value sequence laundering happens on the next line below.
+    __cxx_atomic_fetch_add(&__entry->__platform_state, __cxx_contention_t(1), memory_order_release);
+    __libcpp_contention_notify(&__entry->__contention_state,
+                               &__entry->__platform_state,
+                               false /* when laundering, we can't handle notify_one */);
+}
+_LIBCPP_EXPORTED_FROM_ABI
+void __cxx_atomic_notify_one(void const volatile* __location)
+    { __libcpp_atomic_notify(__location); }
+_LIBCPP_EXPORTED_FROM_ABI
+void __cxx_atomic_notify_all(void const volatile* __location)
+    { __libcpp_atomic_notify(__location); }
+_LIBCPP_EXPORTED_FROM_ABI
+__cxx_contention_t __libcpp_atomic_monitor(void const volatile* __location)
+{
+    auto const __entry = __libcpp_contention_state(__location);
+    return __libcpp_contention_monitor_for_wait(&__entry->__contention_state, &__entry->__platform_state);
+}
+_LIBCPP_EXPORTED_FROM_ABI
+void __libcpp_atomic_wait(void const volatile* __location, __cxx_contention_t __old_value)
+{
+    auto const __entry = __libcpp_contention_state(__location);
+    __libcpp_contention_wait(&__entry->__contention_state, &__entry->__platform_state, __old_value);
+}
+
+/* When the incoming atomic happens to be the platform wait size, we still need to use the
+   table for the contention detection, but we can use the atomic directly for the wait. */
+
+_LIBCPP_EXPORTED_FROM_ABI
+void __cxx_atomic_notify_one(__cxx_atomic_contention_t const volatile* __location)
+{
+    __libcpp_contention_notify(&__libcpp_contention_state(__location)->__contention_state, __location, true);
+}
+_LIBCPP_EXPORTED_FROM_ABI
+void __cxx_atomic_notify_all(__cxx_atomic_contention_t const volatile* __location)
+{
+    __libcpp_contention_notify(&__libcpp_contention_state(__location)->__contention_state, __location, false);
+}
+_LIBCPP_EXPORTED_FROM_ABI
+__cxx_contention_t __libcpp_atomic_monitor(__cxx_atomic_contention_t const volatile* __location)
+{
+    return __libcpp_contention_monitor_for_wait(&__libcpp_contention_state(__location)->__contention_state, __location);
+}
+_LIBCPP_EXPORTED_FROM_ABI
+void __libcpp_atomic_wait(__cxx_atomic_contention_t const volatile* __location, __cxx_contention_t __old_value)
+{
+    __libcpp_contention_wait(&__libcpp_contention_state(__location)->__contention_state, __location, __old_value);
+}
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif //_LIBCPP_HAS_NO_THREADS
diff --git a/libcxx/src/barrier.cpp b/libcxx/src/barrier.cpp
@@ -0,0 +1,95 @@
+//===------------------------- barrier.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
+//
+//===----------------------------------------------------------------------===//
+
+#include <__config>
+
+#ifndef _LIBCPP_HAS_NO_THREADS
+
+#include <barrier>
+#include <thread>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if !defined(_LIBCPP_HAS_NO_TREE_BARRIER) && (_LIBCPP_STD_VER > 11)
+
+class __barrier_algorithm_base {
+public:
+    struct alignas(64) /* naturally-align the heap state */ __state_t
+    {
+        struct {
+            __atomic_base<__barrier_phase_t> __phase = ATOMIC_VAR_INIT(0);
+        } __tickets[64];
+    };
+
+    ptrdiff_t&              __expected;
+    unique_ptr<__state_t[]> __state;
+
+    _LIBCPP_HIDDEN
+    __barrier_algorithm_base(ptrdiff_t& __expected)
+        : __expected(__expected), __state(new __barrier_algorithm_base::__state_t[(__expected + 1) >> 1])
+    {
+    }
+    _LIBCPP_HIDDEN
+    bool __arrive(__barrier_phase_t __old_phase)
+    {
+        __barrier_phase_t const __half_step = __old_phase + 1,
+                                __full_step = __old_phase + 2;
+        size_t __current_expected = __expected,
+            __current = hash<thread::id>()(this_thread::get_id()) % ((__expected + 1) >> 1);
+        for(int __round = 0;; ++__round) {
+            if(__current_expected <= 1)
+                return true;
+            size_t const __end_node = ((__current_expected + 1) >> 1),
+                         __last_node = __end_node - 1;
+            for(;;++__current) {
+                if(__current == __end_node)
+                __current = 0;
+                __barrier_phase_t expect = __old_phase;
+                if(__current == __last_node && (__current_expected & 1))
+                {
+                    if(__state[__current].__tickets[__round].__phase.compare_exchange_strong(expect, __full_step, memory_order_acq_rel))
+                        break;    // I'm 1 in 1, go to next __round
+                }
+                else if(__state[__current].__tickets[__round].__phase.compare_exchange_strong(expect, __half_step, memory_order_acq_rel))
+                {
+                    return false; // I'm 1 in 2, done with arrival
+                }
+                else if(expect == __half_step)
+                {
+                    if(__state[__current].__tickets[__round].__phase.compare_exchange_strong(expect, __full_step, memory_order_acq_rel))
+                        break;    // I'm 2 in 2, go to next __round
+                }
+            }
+            __current_expected = __last_node + 1;
+            __current >>= 1;
+        }
+    }
+};
+
+_LIBCPP_EXPORTED_FROM_ABI
+__barrier_algorithm_base * __construct_barrier_algorithm_base(ptrdiff_t& __expected)
+{
+    return new __barrier_algorithm_base(__expected);
+}
+_LIBCPP_EXPORTED_FROM_ABI
+bool __arrive_barrier_algorithm_base(__barrier_algorithm_base* __barrier,
+                                     __barrier_phase_t __old_phase)
+{
+    return __barrier->__arrive(__old_phase);
+}
+_LIBCPP_EXPORTED_FROM_ABI
+void __destroy_barrier_algorithm_base(__barrier_algorithm_base* __barrier)
+{
+    delete __barrier;
+}
+
+#endif //!defined(_LIBCPP_HAS_NO_TREE_BARRIER) && (_LIBCPP_STD_VER >= 11)
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif //_LIBCPP_HAS_NO_THREADS
diff --git a/libcxx/src/include/apple_availability.h b/libcxx/src/include/apple_availability.h
@@ -46,6 +46,24 @@
 #endif
 #endif // __ENVIRONMENT_.*_VERSION_MIN_REQUIRED__
 
+#if defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__)
+#if __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ >= 101500
+#define _LIBCPP_USE_ULOCK
+#endif
+#elif defined(__ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__)
+#if __ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__ >= 130000
+#define _LIBCPP_USE_ULOCK
+#endif
+#elif defined(__ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__)
+#if __ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__ >= 130000
+#define _LIBCPP_USE_ULOCK
+#endif
+#elif defined(__ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__)
+#if __ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__ >= 60000
+#define _LIBCPP_USE_ULOCK
+#endif
+#endif // __ENVIRONMENT_.*_VERSION_MIN_REQUIRED__
+
 #endif // __APPLE__
 
 #endif // _LIBCPP_SRC_INCLUDE_APPLE_AVAILABILITY_H
diff --git a/libcxx/test/libcxx/double_include.sh.cpp b/libcxx/test/libcxx/double_include.sh.cpp
@@ -25,6 +25,9 @@
 #include <array>
 #ifndef _LIBCPP_HAS_NO_THREADS
 #include <atomic>
+#include <latch>
+#include <barrier>
+#include <semaphore>
 #endif
 #include <bit>
 #include <bitset>

diff --git a/...t/std/atomics/atomics.types.operations/atomics.types.operations.wait/atomic_wait.pass.cpp b/...t/std/atomics/atomics.types.operations/atomics.types.operations.wait/atomic_wait.pass.cpp
@@ -0,0 +1,65 @@
+//===----------------------------------------------------------------------===//
+//
+// 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: libcpp-has-no-threads
+// XFAIL: c++98, c++03
+
+// This test requires the dylib support introduced in D68480
+// XFAIL: with_system_cxx_lib=macosx10.15
+// XFAIL: with_system_cxx_lib=macosx10.14
+// XFAIL: with_system_cxx_lib=macosx10.13
+// XFAIL: with_system_cxx_lib=macosx10.12
+// XFAIL: with_system_cxx_lib=macosx10.11
+// XFAIL: with_system_cxx_lib=macosx10.10
+// XFAIL: with_system_cxx_lib=macosx10.9
+
+// <atomic>
+
+#include <atomic>
+#include <type_traits>
+#include <cassert>
+#include <thread>
+
+#include "test_macros.h"
+#include "../atomics.types.operations.req/atomic_helpers.h"
+
+template <class T>
+struct TestFn {
+  void operator()() const {
+    typedef std::atomic<T> A;
+
+    A t;
+    std::atomic_init(&t, T(1));
+    assert(std::atomic_load(&t) == T(1));
+    std::atomic_wait(&t, T(0));
+    std::thread t_([&](){
+      std::atomic_store(&t, T(3));
+      std::atomic_notify_one(&t);
+    });
+    std::atomic_wait(&t, T(1));
+    t_.join();
+
+    volatile A vt;
+    std::atomic_init(&vt, T(2));
+    assert(std::atomic_load(&vt) == T(2));
+    std::atomic_wait(&vt, T(1));
+    std::thread t2_([&](){
+      std::atomic_store(&vt, T(4));
+      std::atomic_notify_one(&vt);
+    });
+    std::atomic_wait(&vt, T(2));
+    t2_.join();
+  }
+};
+
+int main(int, char**)
+{
+    TestEachAtomicType<TestFn>()();
+
+  return 0;
+}
diff --git a/libcxx/test/std/atomics/types.pass.cpp b/libcxx/test/std/atomics/types.pass.cpp
@@ -0,0 +1,125 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <atomic>
+
+// Test nested types
+
+// template <class T>
+// class atomic
+// {
+// public:
+//     typedef T                                        value_type;
+// };
+
+#include <atomic>
+#include <type_traits>
+
+#include <thread>
+#include <chrono>
+#if TEST_STD_VER >= 20
+# include <memory>
+#endif
+
+#include "test_macros.h"
+
+template <class A>
+void
+test_atomic()
+{
+    A a;
+#if TEST_STD_VER >= 17
+    static_assert((std::is_same<typename A::value_type, decltype(a.load())>::value), "");
+#endif
+}
+
+template <class T>
+void
+test()
+{
+    using A = std::atomic<T>;
+#if TEST_STD_VER >= 17
+    static_assert((std::is_same<typename A::value_type, T>::value), "");
+#endif
+    test_atomic<A>();
+}
+
+struct TriviallyCopyable {
+    int i_;
+};
+
+int main(int, char**)
+{
+    test<bool>               ();
+    test<char>               ();
+    test<signed char>        ();
+    test<unsigned char>      ();
+    test<short>              ();
+    test<unsigned short>     ();
+    test<int>                ();
+    test<unsigned int>       ();
+    test<long>               ();
+    test<unsigned long>      ();
+    test<long long>          ();
+    test<unsigned long long> ();
+    test<char16_t>           ();
+    test<char32_t>           ();
+    test<wchar_t>            ();
+
+    test<int_least8_t>   ();
+    test<uint_least8_t>  ();
+    test<int_least16_t>  ();
+    test<uint_least16_t> ();
+    test<int_least32_t>  ();
+    test<uint_least32_t> ();
+    test<int_least64_t>  ();
+    test<uint_least64_t> ();
+
+    test<int_fast8_t>   ();
+    test<uint_fast8_t>  ();
+    test<int_fast16_t>  ();
+    test<uint_fast16_t> ();
+    test<int_fast32_t>  ();
+    test<uint_fast32_t> ();
+    test<int_fast64_t>  ();
+    test<uint_fast64_t> ();
+
+    test< int8_t>  ();
+    test<uint8_t>  ();
+    test< int16_t> ();
+    test<uint16_t> ();
+    test< int32_t> ();
+    test<uint32_t> ();
+    test< int64_t> ();
+    test<uint64_t> ();
+
+    test<intptr_t>  ();
+    test<uintptr_t> ();
+    test<size_t>    ();
+    test<ptrdiff_t> ();
+    test<intmax_t>  ();
+    test<uintmax_t> ();
+
+    test<uintmax_t> ();
+    test<uintmax_t> ();
+
+    test<TriviallyCopyable>();
+    test<std::thread::id>();
+    test<std::chrono::nanoseconds>();
+    test<float>();
+
+#if TEST_STD_VER >= 20
+    test_atomic<std::atomic_signed_lock_free>();
+    test_atomic<std::atomic_unsigned_lock_free>();
+/*
+    test<std::shared_ptr<int>>();
+*/
+#endif
+
+    return 0;
+}
diff --git a/libcxx/test/std/thread/thread.barrier/arrive.pass.cpp b/libcxx/test/std/thread/thread.barrier/arrive.pass.cpp
@@ -0,0 +1,42 @@
+//===----------------------------------------------------------------------===//
+//
+// 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: libcpp-has-no-threads
+// UNSUPPORTED: c++98, c++03, c++11
+
+// This test requires the dylib support introduced in D68480
+// XFAIL: with_system_cxx_lib=macosx10.15
+// XFAIL: with_system_cxx_lib=macosx10.14
+// XFAIL: with_system_cxx_lib=macosx10.13
+// XFAIL: with_system_cxx_lib=macosx10.12
+// XFAIL: with_system_cxx_lib=macosx10.11
+// XFAIL: with_system_cxx_lib=macosx10.10
+// XFAIL: with_system_cxx_lib=macosx10.9
+
+// <barrier>
+
+#include <barrier>
+#include <thread>
+
+#include "test_macros.h"
+
+int main(int, char**)
+{
+  std::barrier<> b(2);
+
+  auto tok = b.arrive();
+  std::thread t([&](){
+    (void)b.arrive();
+  });
+  b.wait(std::move(tok));
+  t.join();
+
+  auto tok2 = b.arrive(2);
+  b.wait(std::move(tok2));
+  return 0;
+}
diff --git a/libcxx/test/std/thread/thread.barrier/arrive_and_drop.pass.cpp b/libcxx/test/std/thread/thread.barrier/arrive_and_drop.pass.cpp
@@ -0,0 +1,41 @@
+//===----------------------------------------------------------------------===//
+//
+// 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: libcpp-has-no-threads
+// UNSUPPORTED: c++98, c++03, c++11
+
+// This test requires the dylib support introduced in D68480
+// XFAIL: with_system_cxx_lib=macosx10.15
+// XFAIL: with_system_cxx_lib=macosx10.14
+// XFAIL: with_system_cxx_lib=macosx10.13
+// XFAIL: with_system_cxx_lib=macosx10.12
+// XFAIL: with_system_cxx_lib=macosx10.11
+// XFAIL: with_system_cxx_lib=macosx10.10
+// XFAIL: with_system_cxx_lib=macosx10.9
+
+// <barrier>
+
+#include <barrier>
+#include <thread>
+#include <cassert>
+
+#include "test_macros.h"
+
+int main(int, char**)
+{
+  std::barrier<> b(2);
+
+  std::thread t([&](){
+    b.arrive_and_drop();
+  });
+
+  b.arrive_and_wait();
+  b.arrive_and_wait();
+  t.join();
+  return 0;
+}
diff --git a/libcxx/test/std/thread/thread.barrier/arrive_and_wait.pass.cpp b/libcxx/test/std/thread/thread.barrier/arrive_and_wait.pass.cpp
@@ -0,0 +1,41 @@
+//===----------------------------------------------------------------------===//
+//
+// 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: libcpp-has-no-threads
+// UNSUPPORTED: c++98, c++03, c++11
+
+// This test requires the dylib support introduced in D68480
+// XFAIL: with_system_cxx_lib=macosx10.15
+// XFAIL: with_system_cxx_lib=macosx10.14
+// XFAIL: with_system_cxx_lib=macosx10.13
+// XFAIL: with_system_cxx_lib=macosx10.12
+// XFAIL: with_system_cxx_lib=macosx10.11
+// XFAIL: with_system_cxx_lib=macosx10.10
+// XFAIL: with_system_cxx_lib=macosx10.9
+
+// <barrier>
+
+#include <barrier>
+#include <thread>
+
+#include "test_macros.h"
+
+int main(int, char**)
+{
+  std::barrier<> b(2);
+
+  std::thread t([&](){
+    for(int i = 0; i < 10; ++i)
+      b.arrive_and_wait();
+  });
+  for(int i = 0; i < 10; ++i)
+    b.arrive_and_wait();
+  t.join();
+
+  return 0;
+}
diff --git a/libcxx/test/std/thread/thread.barrier/completion.pass.cpp b/libcxx/test/std/thread/thread.barrier/completion.pass.cpp
@@ -0,0 +1,46 @@
+//===----------------------------------------------------------------------===//
+//
+// 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: libcpp-has-no-threads
+// UNSUPPORTED: c++98, c++03, c++11
+
+// This test requires the dylib support introduced in D68480
+// XFAIL: with_system_cxx_lib=macosx10.15
+// XFAIL: with_system_cxx_lib=macosx10.14
+// XFAIL: with_system_cxx_lib=macosx10.13
+// XFAIL: with_system_cxx_lib=macosx10.12
+// XFAIL: with_system_cxx_lib=macosx10.11
+// XFAIL: with_system_cxx_lib=macosx10.10
+// XFAIL: with_system_cxx_lib=macosx10.9
+
+// <barrier>
+
+#include <barrier>
+#include <thread>
+#include <cassert>
+
+#include "test_macros.h"
+
+int main(int, char**)
+{
+  int x = 0;
+  auto comp = [&]() { x += 1; };
+  std::barrier<decltype(comp)> b(2, comp);
+
+  std::thread t([&](){
+      for(int i = 0; i < 10; ++i)
+        b.arrive_and_wait();
+  });
+
+  for(int i = 0; i < 10; ++i)
+    b.arrive_and_wait();
+
+  assert(x == 10);
+  t.join();
+  return 0;
+}
diff --git a/libcxx/test/std/thread/thread.barrier/max.pass.cpp b/libcxx/test/std/thread/thread.barrier/max.pass.cpp
@@ -0,0 +1,26 @@
+//===----------------------------------------------------------------------===//
+//
+// 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: libcpp-has-no-threads
+// UNSUPPORTED: c++98, c++03, c++11
+
+// <barrier>
+
+#include <barrier>
+#include <thread>
+#include <cassert>
+
+#include "test_macros.h"
+
+int main(int, char**)
+{
+  static_assert(std::barrier<>::max() > 0, "");
+  auto l = [](){};
+  static_assert(std::barrier<decltype(l)>::max() > 0, "");
+  return 0;
+}
diff --git a/libcxx/test/std/thread/thread.barrier/version.pass.cpp b/libcxx/test/std/thread/thread.barrier/version.pass.cpp
@@ -0,0 +1,25 @@
+//===----------------------------------------------------------------------===//
+//
+// 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: libcpp-has-no-threads
+// UNSUPPORTED: c++98, c++03, c++11
+
+// <barrier>
+
+#include <barrier>
+
+#include "test_macros.h"
+
+#ifndef _LIBCPP_VERSION
+#error _LIBCPP_VERSION not defined
+#endif
+
+int main(int, char**)
+{
+  return 0;
+}
diff --git a/libcxx/test/std/thread/thread.latch/arrive_and_wait.pass.cpp b/libcxx/test/std/thread/thread.latch/arrive_and_wait.pass.cpp
@@ -0,0 +1,39 @@
+//===----------------------------------------------------------------------===//
+//
+// 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: libcpp-has-no-threads
+// UNSUPPORTED: c++98, c++03, c++11
+
+// This test requires the dylib support introduced in D68480
+// XFAIL: with_system_cxx_lib=macosx10.15
+// XFAIL: with_system_cxx_lib=macosx10.14
+// XFAIL: with_system_cxx_lib=macosx10.13
+// XFAIL: with_system_cxx_lib=macosx10.12
+// XFAIL: with_system_cxx_lib=macosx10.11
+// XFAIL: with_system_cxx_lib=macosx10.10
+// XFAIL: with_system_cxx_lib=macosx10.9
+
+// <latch>
+
+#include <latch>
+#include <thread>
+
+#include "test_macros.h"
+
+int main(int, char**)
+{
+  std::latch l(2);
+
+  std::thread t([&](){
+      l.arrive_and_wait();
+  });
+  l.arrive_and_wait();
+  t.join();
+
+  return 0;
+}
diff --git a/libcxx/test/std/thread/thread.latch/count_down.pass.cpp b/libcxx/test/std/thread/thread.latch/count_down.pass.cpp
@@ -0,0 +1,40 @@
+//===----------------------------------------------------------------------===//
+//
+// 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: libcpp-has-no-threads
+// UNSUPPORTED: c++98, c++03, c++11
+
+// This test requires the dylib support introduced in D68480
+// XFAIL: with_system_cxx_lib=macosx10.15
+// XFAIL: with_system_cxx_lib=macosx10.14
+// XFAIL: with_system_cxx_lib=macosx10.13
+// XFAIL: with_system_cxx_lib=macosx10.12
+// XFAIL: with_system_cxx_lib=macosx10.11
+// XFAIL: with_system_cxx_lib=macosx10.10
+// XFAIL: with_system_cxx_lib=macosx10.9
+
+// <latch>
+
+#include <latch>
+#include <thread>
+
+#include "test_macros.h"
+
+int main(int, char**)
+{
+  std::latch l(2);
+
+  l.count_down();
+  std::thread t([&](){
+    l.count_down();
+  });
+  l.wait();
+  t.join();
+
+  return 0;
+}
diff --git a/libcxx/test/std/thread/thread.latch/max.pass.cpp b/libcxx/test/std/thread/thread.latch/max.pass.cpp
@@ -0,0 +1,23 @@
+//===----------------------------------------------------------------------===//
+//
+// 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: libcpp-has-no-threads
+// UNSUPPORTED: c++98, c++03, c++11
+
+// <latch>
+
+#include <latch>
+#include <cassert>
+
+#include "test_macros.h"
+
+int main(int, char**)
+{
+  static_assert(std::latch::max() > 0, "");
+  return 0;
+}
diff --git a/libcxx/test/std/thread/thread.latch/try_wait.pass.cpp b/libcxx/test/std/thread/thread.latch/try_wait.pass.cpp
@@ -0,0 +1,37 @@
+//===----------------------------------------------------------------------===//
+//
+// 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: libcpp-has-no-threads
+// UNSUPPORTED: c++98, c++03, c++11
+
+// This test requires the dylib support introduced in D68480
+// XFAIL: with_system_cxx_lib=macosx10.15
+// XFAIL: with_system_cxx_lib=macosx10.14
+// XFAIL: with_system_cxx_lib=macosx10.13
+// XFAIL: with_system_cxx_lib=macosx10.12
+// XFAIL: with_system_cxx_lib=macosx10.11
+// XFAIL: with_system_cxx_lib=macosx10.10
+// XFAIL: with_system_cxx_lib=macosx10.9
+
+// <latch>
+
+#include <latch>
+#include <cassert>
+
+#include "test_macros.h"
+
+int main(int, char**)
+{
+  std::latch l(1);
+
+  l.count_down();
+  bool const b = l.try_wait();
+  assert(b);
+
+  return 0;
+}
diff --git a/libcxx/test/std/thread/thread.latch/version.pass.cpp b/libcxx/test/std/thread/thread.latch/version.pass.cpp
@@ -0,0 +1,25 @@
+//===----------------------------------------------------------------------===//
+//
+// 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: libcpp-has-no-threads
+// UNSUPPORTED: c++98, c++03, c++11
+
+// <latch>
+
+#include <latch>
+
+#include "test_macros.h"
+
+#ifndef _LIBCPP_VERSION
+#error _LIBCPP_VERSION not defined
+#endif
+
+int main(int, char**)
+{
+  return 0;
+}
diff --git a/libcxx/test/std/thread/thread.semaphore/acquire.pass.cpp b/libcxx/test/std/thread/thread.semaphore/acquire.pass.cpp
@@ -0,0 +1,40 @@
+//===----------------------------------------------------------------------===//
+//
+// 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: libcpp-has-no-threads
+// UNSUPPORTED: c++98, c++03, c++11
+
+// This test requires the dylib support introduced in D68480
+// XFAIL: with_system_cxx_lib=macosx10.15
+// XFAIL: with_system_cxx_lib=macosx10.14
+// XFAIL: with_system_cxx_lib=macosx10.13
+// XFAIL: with_system_cxx_lib=macosx10.12
+// XFAIL: with_system_cxx_lib=macosx10.11
+// XFAIL: with_system_cxx_lib=macosx10.10
+// XFAIL: with_system_cxx_lib=macosx10.9
+
+// <semaphore>
+
+#include <semaphore>
+#include <thread>
+
+#include "test_macros.h"
+
+int main(int, char**)
+{
+  std::counting_semaphore<> s(2);
+
+  std::thread t([&](){
+    s.acquire();
+  });
+  t.join();
+
+  s.acquire();
+
+  return 0;
+}
diff --git a/libcxx/test/std/thread/thread.semaphore/binary.pass.cpp b/libcxx/test/std/thread/thread.semaphore/binary.pass.cpp
@@ -0,0 +1,47 @@
+//===----------------------------------------------------------------------===//
+//
+// 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: libcpp-has-no-threads
+// UNSUPPORTED: c++98, c++03, c++11
+
+// This test requires the dylib support introduced in D68480
+// XFAIL: with_system_cxx_lib=macosx10.15
+// XFAIL: with_system_cxx_lib=macosx10.14
+// XFAIL: with_system_cxx_lib=macosx10.13
+// XFAIL: with_system_cxx_lib=macosx10.12
+// XFAIL: with_system_cxx_lib=macosx10.11
+// XFAIL: with_system_cxx_lib=macosx10.10
+// XFAIL: with_system_cxx_lib=macosx10.9
+
+// <semaphore>
+
+#include <semaphore>
+#include <chrono>
+#include <thread>
+
+#include "test_macros.h"
+
+int main(int, char**)
+{
+  std::binary_semaphore s(1);
+
+  auto l = [&](){
+    for(int i = 0; i < 1024; ++i) {
+        s.acquire();
+        std::this_thread::sleep_for(std::chrono::microseconds(1));
+        s.release();
+    }
+  };
+
+  std::thread t(l);
+  l();
+
+  t.join();
+
+  return 0;
+}
diff --git a/libcxx/test/std/thread/thread.semaphore/max.pass.cpp b/libcxx/test/std/thread/thread.semaphore/max.pass.cpp
@@ -0,0 +1,28 @@
+//===----------------------------------------------------------------------===//
+//
+// 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: libcpp-has-no-threads
+// UNSUPPORTED: c++98, c++03, c++11
+
+// <semaphore>
+
+#include <semaphore>
+#include <thread>
+
+#include "test_macros.h"
+
+int main(int, char**)
+{
+  static_assert(std::counting_semaphore<>::max() > 0, "");
+  static_assert(std::counting_semaphore<1>::max() >= 1, "");
+  static_assert(std::counting_semaphore<std::numeric_limits<int>::max()>::max() >= 1, "");
+  static_assert(std::counting_semaphore<std::numeric_limits<unsigned>::max()>::max() >= 1, "");
+  static_assert(std::counting_semaphore<std::numeric_limits<ptrdiff_t>::max()>::max() >= 1, "");
+  static_assert(std::counting_semaphore<1>::max() == std::binary_semaphore::max(), "");
+  return 0;
+}
diff --git a/libcxx/test/std/thread/thread.semaphore/release.pass.cpp b/libcxx/test/std/thread/thread.semaphore/release.pass.cpp
@@ -0,0 +1,43 @@
+//===----------------------------------------------------------------------===//
+//
+// 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: libcpp-has-no-threads
+// UNSUPPORTED: c++98, c++03, c++11
+
+// This test requires the dylib support introduced in D68480
+// XFAIL: with_system_cxx_lib=macosx10.15
+// XFAIL: with_system_cxx_lib=macosx10.14
+// XFAIL: with_system_cxx_lib=macosx10.13
+// XFAIL: with_system_cxx_lib=macosx10.12
+// XFAIL: with_system_cxx_lib=macosx10.11
+// XFAIL: with_system_cxx_lib=macosx10.10
+// XFAIL: with_system_cxx_lib=macosx10.9
+
+// <semaphore>
+
+#include <semaphore>
+#include <thread>
+
+#include "test_macros.h"
+
+int main(int, char**)
+{
+  std::counting_semaphore<> s(0);
+
+  s.release();
+  s.acquire();
+
+  std::thread t([&](){
+    s.acquire();
+  });
+  s.release(2);
+  t.join();
+  s.acquire();
+
+  return 0;
+}
diff --git a/libcxx/test/std/thread/thread.semaphore/timed.pass.cpp b/libcxx/test/std/thread/thread.semaphore/timed.pass.cpp
@@ -0,0 +1,53 @@
+//===----------------------------------------------------------------------===//
+//
+// 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: libcpp-has-no-threads
+// UNSUPPORTED: c++98, c++03, c++11
+
+// This test requires the dylib support introduced in D68480
+// XFAIL: with_system_cxx_lib=macosx10.15
+// XFAIL: with_system_cxx_lib=macosx10.14
+// XFAIL: with_system_cxx_lib=macosx10.13
+// XFAIL: with_system_cxx_lib=macosx10.12
+// XFAIL: with_system_cxx_lib=macosx10.11
+// XFAIL: with_system_cxx_lib=macosx10.10
+// XFAIL: with_system_cxx_lib=macosx10.9
+
+// <semaphore>
+
+#include <semaphore>
+#include <thread>
+#include <chrono>
+
+#include "test_macros.h"
+
+int main(int, char**)
+{
+  auto const start = std::chrono::steady_clock::now();
+
+  std::counting_semaphore<> s(0);
+
+  assert(!s.try_acquire_until(start + std::chrono::milliseconds(250)));
+  assert(!s.try_acquire_for(std::chrono::milliseconds(250)));
+
+  std::thread t([&](){
+    std::this_thread::sleep_for(std::chrono::milliseconds(250));
+    s.release();
+    std::this_thread::sleep_for(std::chrono::milliseconds(250));
+    s.release();
+  });
+
+  assert(s.try_acquire_until(start + std::chrono::seconds(2)));
+  assert(s.try_acquire_for(std::chrono::seconds(2)));
+  t.join();
+
+  auto const end = std::chrono::steady_clock::now();
+  assert(end - start < std::chrono::seconds(10));
+
+  return 0;
+}
diff --git a/libcxx/test/std/thread/thread.semaphore/try_acquire.pass.cpp b/libcxx/test/std/thread/thread.semaphore/try_acquire.pass.cpp
@@ -0,0 +1,43 @@
+//===----------------------------------------------------------------------===//
+//
+// 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: libcpp-has-no-threads
+// UNSUPPORTED: c++98, c++03, c++11
+
+// This test requires the dylib support introduced in D68480
+// XFAIL: with_system_cxx_lib=macosx10.15
+// XFAIL: with_system_cxx_lib=macosx10.14
+// XFAIL: with_system_cxx_lib=macosx10.13
+// XFAIL: with_system_cxx_lib=macosx10.12
+// XFAIL: with_system_cxx_lib=macosx10.11
+// XFAIL: with_system_cxx_lib=macosx10.10
+// XFAIL: with_system_cxx_lib=macosx10.9
+
+// <semaphore>
+
+#include <semaphore>
+#include <thread>
+
+#include "test_macros.h"
+
+int main(int, char**)
+{
+  std::counting_semaphore<> s(1);
+
+  assert(s.try_acquire());
+  s.release();
+  assert(s.try_acquire());
+  s.release(2);
+  std::thread t([&](){
+    assert(s.try_acquire());
+  });
+  t.join();
+  assert(s.try_acquire());
+
+  return 0;
+}
diff --git a/libcxx/test/std/thread/thread.semaphore/version.pass.cpp b/libcxx/test/std/thread/thread.semaphore/version.pass.cpp
@@ -0,0 +1,25 @@
+//===----------------------------------------------------------------------===//
+//
+// 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: libcpp-has-no-threads
+// UNSUPPORTED: c++98, c++03, c++11
+
+// <semaphore>
+
+#include <semaphore>
+
+#include "test_macros.h"
+
+#ifndef _LIBCPP_VERSION
+#error _LIBCPP_VERSION not defined
+#endif
+
+int main(int, char**)
+{
+  return 0;
+}
diff --git a/libcxx/www/cxx2a_status.html b/libcxx/www/cxx2a_status.html
@@ -169,7 +169,7 @@ <h3>Paper Status</h3>
 	<tr><td><a href="https://wg21.link/P1004">P1004</a></td><td>LWG</td><td>Making std::vector constexpr</td><td>Cologne</td><td></td><td></td></tr>
 	<tr><td><a href="https://wg21.link/P1035">P1035</a></td><td>LWG</td><td>Input Range Adaptors</td><td>Cologne</td><td></td><td></td></tr>
 	<tr><td><a href="https://wg21.link/P1065">P1065</a></td><td>LWG</td><td>Constexpr INVOKE</td><td>Cologne</td><td></td><td></td></tr>
-	<tr><td><a href="https://wg21.link/P1135">P1135</a></td><td>LWG</td><td>The C++20 Synchronization Library</td><td>Cologne</td><td></td><td></td></tr>
+	<tr><td><a href="https://wg21.link/P1135">P1135</a></td><td>LWG</td><td>The C++20 Synchronization Library</td><td>Cologne</td><td>Complete</td><td></td></tr>
 	<tr><td><a href="https://wg21.link/P1207">P1207</a></td><td>LWG</td><td>Movability of Single-pass Iterators</td><td>Cologne</td><td></td><td></td></tr>
 	<tr><td><a href="https://wg21.link/P1208">P1208</a></td><td>LWG</td><td>Adopt source_location for C++20</td><td>Cologne</td><td></td><td></td></tr>
 	<tr><td><a href="https://wg21.link/P1355">P1355</a></td><td>LWG</td><td>Exposing a narrow contract for ceil2</td><td>Cologne</td><td>Complete</td><td>9.0</td></tr>