diff --git a/libcxx/.clang-format b/libcxx/.clang-format
index 39ae1322ffa8a..c37ab817bca90 100644
--- a/libcxx/.clang-format
+++ b/libcxx/.clang-format
@@ -24,6 +24,7 @@ AttributeMacros: [
                   '_LIBCPP_CONSTEXPR_SINCE_CXX23',
                   '_LIBCPP_CONSTEXPR',
                   '_LIBCPP_CONSTINIT',
+                  '_LIBCPP_DEPRECATED_ATOMIC_SYNC',
                   '_LIBCPP_DEPRECATED_IN_CXX11',
                   '_LIBCPP_DEPRECATED_IN_CXX14',
                   '_LIBCPP_DEPRECATED_IN_CXX17',
diff --git a/libcxx/docs/ReleaseNotes/19.rst b/libcxx/docs/ReleaseNotes/19.rst
index cac42f9c3c3f7..1d9efa08550dc 100644
--- a/libcxx/docs/ReleaseNotes/19.rst
+++ b/libcxx/docs/ReleaseNotes/19.rst
@@ -55,6 +55,10 @@ Improvements and New Features
 Deprecations and Removals
 -------------------------
 
+- The C++20 synchronization library (``<barrier>``, ``<latch>``, ``atomic::wait``, etc.) has been deprecated
+  in language modes prior to C++20. If you are using these features prior to C++20, please update to ``-std=c++20``.
+  In LLVM 20, the C++20 synchronization library will be removed entirely in language modes prior to C++20.
+
 - TODO: The ``LIBCXX_ENABLE_ASSERTIONS`` CMake variable that was used to enable the safe mode has been deprecated and setting
   it triggers an error; use the ``LIBCXX_HARDENING_MODE`` CMake variable with the value ``extensive`` instead. Similarly,
   the ``_LIBCPP_ENABLE_ASSERTIONS`` macro has been deprecated (setting it to ``1`` still enables the extensive mode in
diff --git a/libcxx/include/__atomic/atomic.h b/libcxx/include/__atomic/atomic.h
index 3dfb6937d0325..bd3f659c22df0 100644
--- a/libcxx/include/__atomic/atomic.h
+++ b/libcxx/include/__atomic/atomic.h
@@ -462,22 +462,26 @@ atomic_wait_explicit(const atomic<_Tp>* __o, typename atomic<_Tp>::value_type __
 // atomic_notify_one
 
 template <class _Tp>
-_LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void atomic_notify_one(volatile atomic<_Tp>* __o) _NOEXCEPT {
+_LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void
+atomic_notify_one(volatile atomic<_Tp>* __o) _NOEXCEPT {
   __o->notify_one();
 }
 template <class _Tp>
-_LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void atomic_notify_one(atomic<_Tp>* __o) _NOEXCEPT {
+_LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void
+atomic_notify_one(atomic<_Tp>* __o) _NOEXCEPT {
   __o->notify_one();
 }
 
 // atomic_notify_all
 
 template <class _Tp>
-_LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void atomic_notify_all(volatile atomic<_Tp>* __o) _NOEXCEPT {
+_LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void
+atomic_notify_all(volatile atomic<_Tp>* __o) _NOEXCEPT {
   __o->notify_all();
 }
 template <class _Tp>
-_LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void atomic_notify_all(atomic<_Tp>* __o) _NOEXCEPT {
+_LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void
+atomic_notify_all(atomic<_Tp>* __o) _NOEXCEPT {
   __o->notify_all();
 }
 
diff --git a/libcxx/include/__atomic/atomic_flag.h b/libcxx/include/__atomic/atomic_flag.h
index 084366237c16e..3ec3366ecaaf9 100644
--- a/libcxx/include/__atomic/atomic_flag.h
+++ b/libcxx/include/__atomic/atomic_flag.h
@@ -49,22 +49,26 @@ struct atomic_flag {
     __cxx_atomic_store(&__a_, _LIBCPP_ATOMIC_FLAG_TYPE(false), __m);
   }
 
-  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void wait(bool __v, memory_order __m = memory_order_seq_cst) const
-      volatile _NOEXCEPT {
+  _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void
+  wait(bool __v, memory_order __m = memory_order_seq_cst) const volatile _NOEXCEPT {
     std::__atomic_wait(*this, _LIBCPP_ATOMIC_FLAG_TYPE(__v), __m);
   }
-  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void
+  _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void
   wait(bool __v, memory_order __m = memory_order_seq_cst) const _NOEXCEPT {
     std::__atomic_wait(*this, _LIBCPP_ATOMIC_FLAG_TYPE(__v), __m);
   }
-  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void notify_one() volatile _NOEXCEPT {
+  _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void notify_one() volatile _NOEXCEPT {
+    std::__atomic_notify_one(*this);
+  }
+  _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void notify_one() _NOEXCEPT {
     std::__atomic_notify_one(*this);
   }
-  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void notify_one() _NOEXCEPT { std::__atomic_notify_one(*this); }
   _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void notify_all() volatile _NOEXCEPT {
     std::__atomic_notify_all(*this);
   }
-  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void notify_all() _NOEXCEPT { std::__atomic_notify_all(*this); }
+  _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void notify_all() _NOEXCEPT {
+    std::__atomic_notify_all(*this);
+  }
 
 #if _LIBCPP_STD_VER >= 20
   _LIBCPP_HIDE_FROM_ABI constexpr atomic_flag() _NOEXCEPT : __a_(false) {}
@@ -141,41 +145,43 @@ inline _LIBCPP_HIDE_FROM_ABI void atomic_flag_clear_explicit(atomic_flag* __o, m
   __o->clear(__m);
 }
 
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void
+inline _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void
 atomic_flag_wait(const volatile atomic_flag* __o, bool __v) _NOEXCEPT {
   __o->wait(__v);
 }
 
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void
+inline _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void
 atomic_flag_wait(const atomic_flag* __o, bool __v) _NOEXCEPT {
   __o->wait(__v);
 }
 
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void
+inline _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void
 atomic_flag_wait_explicit(const volatile atomic_flag* __o, bool __v, memory_order __m) _NOEXCEPT {
   __o->wait(__v, __m);
 }
 
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void
+inline _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void
 atomic_flag_wait_explicit(const atomic_flag* __o, bool __v, memory_order __m) _NOEXCEPT {
   __o->wait(__v, __m);
 }
 
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void
+inline _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void
 atomic_flag_notify_one(volatile atomic_flag* __o) _NOEXCEPT {
   __o->notify_one();
 }
 
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void atomic_flag_notify_one(atomic_flag* __o) _NOEXCEPT {
+inline _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void
+atomic_flag_notify_one(atomic_flag* __o) _NOEXCEPT {
   __o->notify_one();
 }
 
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void
+inline _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void
 atomic_flag_notify_all(volatile atomic_flag* __o) _NOEXCEPT {
   __o->notify_all();
 }
 
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void atomic_flag_notify_all(atomic_flag* __o) _NOEXCEPT {
+inline _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void
+atomic_flag_notify_all(atomic_flag* __o) _NOEXCEPT {
   __o->notify_all();
 }
 
diff --git a/libcxx/include/__config b/libcxx/include/__config
index 132cace3d5b2c..879f8f3321c3c 100644
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -954,6 +954,14 @@ typedef __char32_t char32_t;
 #    define _LIBCPP_DEPRECATED_(m)
 #  endif
 
+#  if _LIBCPP_STD_VER < 20
+#    define _LIBCPP_DEPRECATED_ATOMIC_SYNC                                                                             \
+      _LIBCPP_DEPRECATED_("The C++20 synchronization library has been deprecated prior to C++20. Please update to "    \
+                          "using -std=c++20 if you need to use these facilities.")
+#  else
+#    define _LIBCPP_DEPRECATED_ATOMIC_SYNC /* nothing */
+#  endif
+
 #  if !defined(_LIBCPP_CXX03_LANG)
 #    define _LIBCPP_DEPRECATED_IN_CXX11 _LIBCPP_DEPRECATED
 #  else
diff --git a/libcxx/include/barrier b/libcxx/include/barrier
index c5fd84b91925b..d776078267625 100644
--- a/libcxx/include/barrier
+++ b/libcxx/include/barrier
@@ -257,7 +257,7 @@ public:
 #  endif // !_LIBCPP_HAS_NO_TREE_BARRIER
 
 template <class _CompletionF = __empty_completion>
-class barrier {
+class _LIBCPP_DEPRECATED_ATOMIC_SYNC barrier {
   __barrier_base<_CompletionF> __b_;
 
 public:
diff --git a/libcxx/include/latch b/libcxx/include/latch
index 3cc7258381143..1937617f7dcc6 100644
--- a/libcxx/include/latch
+++ b/libcxx/include/latch
@@ -66,7 +66,7 @@ _LIBCPP_PUSH_MACROS
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-class latch {
+class _LIBCPP_DEPRECATED_ATOMIC_SYNC latch {
   __atomic_base<ptrdiff_t> __a_;
 
 public:
diff --git a/libcxx/include/semaphore b/libcxx/include/semaphore
index 1375ec3f7c04b..cb2f42c106ca8 100644
--- a/libcxx/include/semaphore
+++ b/libcxx/include/semaphore
@@ -127,7 +127,7 @@ private:
 };
 
 template <ptrdiff_t __least_max_value = _LIBCPP_SEMAPHORE_MAX>
-class counting_semaphore {
+class _LIBCPP_DEPRECATED_ATOMIC_SYNC counting_semaphore {
   __atomic_semaphore_base __semaphore_;
 
 public:
@@ -172,7 +172,9 @@ public:
   }
 };
 
-using binary_semaphore = counting_semaphore<1>;
+_LIBCPP_SUPPRESS_DEPRECATED_PUSH
+using binary_semaphore _LIBCPP_DEPRECATED_ATOMIC_SYNC = counting_semaphore<1>;
+_LIBCPP_SUPPRESS_DEPRECATED_POP
 
 _LIBCPP_END_NAMESPACE_STD
 
diff --git a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.wait/atomic_notify_all.pass.cpp b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.wait/atomic_notify_all.pass.cpp
index 2b9f34b731f87..0ec530c922e70 100644
--- a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.wait/atomic_notify_all.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.wait/atomic_notify_all.pass.cpp
@@ -7,9 +7,12 @@
 //===----------------------------------------------------------------------===//
 //
 // UNSUPPORTED: no-threads
-// XFAIL: c++03
+// UNSUPPORTED: c++03
 // XFAIL: !has-1024-bit-atomics
 
+// Until we drop support for the synchronization library in C++11/14/17
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
+
 // XFAIL: availability-synchronization_library-missing
 
 // <atomic>
diff --git a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.wait/atomic_notify_one.pass.cpp b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.wait/atomic_notify_one.pass.cpp
index dfa781c566009..c21b67d479ae2 100644
--- a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.wait/atomic_notify_one.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.wait/atomic_notify_one.pass.cpp
@@ -7,9 +7,12 @@
 //===----------------------------------------------------------------------===//
 //
 // UNSUPPORTED: no-threads
-// XFAIL: c++03
+// UNSUPPORTED: c++03
 // XFAIL: !has-1024-bit-atomics
 
+// Until we drop support for the synchronization library in C++11/14/17
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
+
 // XFAIL: availability-synchronization_library-missing
 
 // <atomic>
diff --git a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.wait/atomic_wait.pass.cpp b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.wait/atomic_wait.pass.cpp
index 38142b336e72c..af99113f13499 100644
--- a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.wait/atomic_wait.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.wait/atomic_wait.pass.cpp
@@ -7,9 +7,12 @@
 //===----------------------------------------------------------------------===//
 //
 // UNSUPPORTED: no-threads
-// XFAIL: c++03
+// UNSUPPORTED: c++03
 // XFAIL: !has-1024-bit-atomics
 
+// Until we drop support for the synchronization library in C++11/14/17
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
+
 // XFAIL: availability-synchronization_library-missing
 
 // <atomic>
diff --git a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.wait/atomic_wait_explicit.pass.cpp b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.wait/atomic_wait_explicit.pass.cpp
index 2db95a0b67a7f..bb8c64593b54b 100644
--- a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.wait/atomic_wait_explicit.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.wait/atomic_wait_explicit.pass.cpp
@@ -7,9 +7,12 @@
 //===----------------------------------------------------------------------===//
 //
 // UNSUPPORTED: no-threads
-// XFAIL: c++03
+// UNSUPPORTED: c++03
 // XFAIL: !has-1024-bit-atomics
 
+// Until we drop support for the synchronization library in C++11/14/17
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
+
 // XFAIL: availability-synchronization_library-missing
 
 // <atomic>
diff --git a/libcxx/test/std/thread/thread.barrier/arrive.pass.cpp b/libcxx/test/std/thread/thread.barrier/arrive.pass.cpp
index 18cdc6d654ac2..d9d9c1dba6bbb 100644
--- a/libcxx/test/std/thread/thread.barrier/arrive.pass.cpp
+++ b/libcxx/test/std/thread/thread.barrier/arrive.pass.cpp
@@ -9,6 +9,9 @@
 // UNSUPPORTED: no-threads
 // UNSUPPORTED: c++03, c++11
 
+// Until we drop support for the synchronization library in C++11/14/17
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
+
 // XFAIL: availability-synchronization_library-missing
 
 // <barrier>
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
index 3fc48261de1b1..aff7b26e16f70 100644
--- a/libcxx/test/std/thread/thread.barrier/arrive_and_drop.pass.cpp
+++ b/libcxx/test/std/thread/thread.barrier/arrive_and_drop.pass.cpp
@@ -9,6 +9,9 @@
 // UNSUPPORTED: no-threads
 // UNSUPPORTED: c++03, c++11
 
+// Until we drop support for the synchronization library in C++11/14/17
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
+
 // XFAIL: availability-synchronization_library-missing
 
 // <barrier>
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
index 2aee8624ae3d5..8c45ba9278f28 100644
--- a/libcxx/test/std/thread/thread.barrier/arrive_and_wait.pass.cpp
+++ b/libcxx/test/std/thread/thread.barrier/arrive_and_wait.pass.cpp
@@ -9,6 +9,9 @@
 // UNSUPPORTED: no-threads
 // UNSUPPORTED: c++03, c++11
 
+// Until we drop support for the synchronization library in C++11/14/17
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
+
 // XFAIL: availability-synchronization_library-missing
 
 // <barrier>
diff --git a/libcxx/test/std/thread/thread.barrier/completion.pass.cpp b/libcxx/test/std/thread/thread.barrier/completion.pass.cpp
index 7354dbe6ffe8a..633a0c8bf2366 100644
--- a/libcxx/test/std/thread/thread.barrier/completion.pass.cpp
+++ b/libcxx/test/std/thread/thread.barrier/completion.pass.cpp
@@ -9,6 +9,9 @@
 // UNSUPPORTED: no-threads
 // UNSUPPORTED: c++03, c++11
 
+// Until we drop support for the synchronization library in C++11/14/17
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
+
 // XFAIL: availability-synchronization_library-missing
 
 // <barrier>
diff --git a/libcxx/test/std/thread/thread.barrier/ctor.compile.pass.cpp b/libcxx/test/std/thread/thread.barrier/ctor.compile.pass.cpp
index d47127a18613b..fe7068d2a574c 100644
--- a/libcxx/test/std/thread/thread.barrier/ctor.compile.pass.cpp
+++ b/libcxx/test/std/thread/thread.barrier/ctor.compile.pass.cpp
@@ -9,6 +9,9 @@
 // UNSUPPORTED: no-threads
 // UNSUPPORTED: c++03, c++11
 
+// Until we drop support for the synchronization library in C++11/14/17
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
+
 // <barrier>
 
 // explicit barrier(ptrdiff_t __count, _CompletionF __completion = _CompletionF());
diff --git a/libcxx/test/std/thread/thread.barrier/max.pass.cpp b/libcxx/test/std/thread/thread.barrier/max.pass.cpp
index ec03c5c87a09c..b09a02e1bdef4 100644
--- a/libcxx/test/std/thread/thread.barrier/max.pass.cpp
+++ b/libcxx/test/std/thread/thread.barrier/max.pass.cpp
@@ -9,6 +9,9 @@
 // UNSUPPORTED: no-threads
 // UNSUPPORTED: c++03, c++11
 
+// Until we drop support for the synchronization library in C++11/14/17
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
+
 // <barrier>
 
 #include <barrier>
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
index ddc06d2038cc8..8ca4f37b73b95 100644
--- a/libcxx/test/std/thread/thread.latch/arrive_and_wait.pass.cpp
+++ b/libcxx/test/std/thread/thread.latch/arrive_and_wait.pass.cpp
@@ -9,6 +9,9 @@
 // UNSUPPORTED: no-threads
 // UNSUPPORTED: c++03, c++11
 
+// Until we drop support for the synchronization library in C++11/14/17
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
+
 // XFAIL: availability-synchronization_library-missing
 
 // <latch>
diff --git a/libcxx/test/std/thread/thread.latch/count_down.pass.cpp b/libcxx/test/std/thread/thread.latch/count_down.pass.cpp
index 1503c09509a6c..eb524abd24b98 100644
--- a/libcxx/test/std/thread/thread.latch/count_down.pass.cpp
+++ b/libcxx/test/std/thread/thread.latch/count_down.pass.cpp
@@ -9,6 +9,9 @@
 // UNSUPPORTED: no-threads
 // UNSUPPORTED: c++03, c++11
 
+// Until we drop support for the synchronization library in C++11/14/17
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
+
 // XFAIL: availability-synchronization_library-missing
 
 // <latch>
diff --git a/libcxx/test/std/thread/thread.latch/ctor.pass.cpp b/libcxx/test/std/thread/thread.latch/ctor.pass.cpp
index 1983f6409cb5a..bca4561bd2f74 100644
--- a/libcxx/test/std/thread/thread.latch/ctor.pass.cpp
+++ b/libcxx/test/std/thread/thread.latch/ctor.pass.cpp
@@ -9,6 +9,9 @@
 // UNSUPPORTED: no-threads
 // UNSUPPORTED: c++03, c++11
 
+// Until we drop support for the synchronization library in C++11/14/17
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
+
 // <latch>
 
 // inline constexpr explicit latch(ptrdiff_t __expected);
diff --git a/libcxx/test/std/thread/thread.latch/max.pass.cpp b/libcxx/test/std/thread/thread.latch/max.pass.cpp
index 8b9176c8cac57..bcf353ed9712e 100644
--- a/libcxx/test/std/thread/thread.latch/max.pass.cpp
+++ b/libcxx/test/std/thread/thread.latch/max.pass.cpp
@@ -9,6 +9,9 @@
 // UNSUPPORTED: no-threads
 // UNSUPPORTED: c++03, c++11
 
+// Until we drop support for the synchronization library in C++11/14/17
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
+
 // <latch>
 
 #include <latch>
diff --git a/libcxx/test/std/thread/thread.latch/try_wait.pass.cpp b/libcxx/test/std/thread/thread.latch/try_wait.pass.cpp
index 70ef2cdf71254..8f354463a8697 100644
--- a/libcxx/test/std/thread/thread.latch/try_wait.pass.cpp
+++ b/libcxx/test/std/thread/thread.latch/try_wait.pass.cpp
@@ -9,6 +9,9 @@
 // UNSUPPORTED: no-threads
 // UNSUPPORTED: c++03, c++11
 
+// Until we drop support for the synchronization library in C++11/14/17
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
+
 // XFAIL: availability-synchronization_library-missing
 
 // <latch>
diff --git a/libcxx/test/std/thread/thread.semaphore/acquire.pass.cpp b/libcxx/test/std/thread/thread.semaphore/acquire.pass.cpp
index 3f6e3107e8bce..22eed736c6b75 100644
--- a/libcxx/test/std/thread/thread.semaphore/acquire.pass.cpp
+++ b/libcxx/test/std/thread/thread.semaphore/acquire.pass.cpp
@@ -9,6 +9,9 @@
 // UNSUPPORTED: no-threads
 // UNSUPPORTED: c++03, c++11
 
+// Until we drop support for the synchronization library in C++11/14/17
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
+
 // XFAIL: availability-synchronization_library-missing
 
 // <semaphore>
diff --git a/libcxx/test/std/thread/thread.semaphore/binary.pass.cpp b/libcxx/test/std/thread/thread.semaphore/binary.pass.cpp
index 111a650b5ea39..c01c78506587c 100644
--- a/libcxx/test/std/thread/thread.semaphore/binary.pass.cpp
+++ b/libcxx/test/std/thread/thread.semaphore/binary.pass.cpp
@@ -9,6 +9,9 @@
 // UNSUPPORTED: no-threads
 // UNSUPPORTED: c++03, c++11
 
+// Until we drop support for the synchronization library in C++11/14/17
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
+
 // XFAIL: availability-synchronization_library-missing
 
 // <semaphore>
diff --git a/libcxx/test/std/thread/thread.semaphore/ctor.compile.pass.cpp b/libcxx/test/std/thread/thread.semaphore/ctor.compile.pass.cpp
index 28ccc0124d489..dcc298ce11ce8 100644
--- a/libcxx/test/std/thread/thread.semaphore/ctor.compile.pass.cpp
+++ b/libcxx/test/std/thread/thread.semaphore/ctor.compile.pass.cpp
@@ -9,6 +9,9 @@
 // UNSUPPORTED: no-threads
 // UNSUPPORTED: c++03, c++11
 
+// Until we drop support for the synchronization library in C++11/14/17
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
+
 // <semaphore>
 
 // constexpr explicit counting_semaphore(ptrdiff_t desired);
diff --git a/libcxx/test/std/thread/thread.semaphore/max.pass.cpp b/libcxx/test/std/thread/thread.semaphore/max.pass.cpp
index ca7ad0c92e60e..6f3ed5e345e0b 100644
--- a/libcxx/test/std/thread/thread.semaphore/max.pass.cpp
+++ b/libcxx/test/std/thread/thread.semaphore/max.pass.cpp
@@ -9,6 +9,9 @@
 // UNSUPPORTED: no-threads
 // UNSUPPORTED: c++03, c++11
 
+// Until we drop support for the synchronization library in C++11/14/17
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
+
 // <semaphore>
 
 #include <semaphore>
diff --git a/libcxx/test/std/thread/thread.semaphore/release.pass.cpp b/libcxx/test/std/thread/thread.semaphore/release.pass.cpp
index bf3dd7f7d814f..3c4d179e50433 100644
--- a/libcxx/test/std/thread/thread.semaphore/release.pass.cpp
+++ b/libcxx/test/std/thread/thread.semaphore/release.pass.cpp
@@ -9,6 +9,9 @@
 // UNSUPPORTED: no-threads
 // UNSUPPORTED: c++03, c++11
 
+// Until we drop support for the synchronization library in C++11/14/17
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
+
 // XFAIL: availability-synchronization_library-missing
 
 // <semaphore>
diff --git a/libcxx/test/std/thread/thread.semaphore/timed.pass.cpp b/libcxx/test/std/thread/thread.semaphore/timed.pass.cpp
index 9fa01fc035904..77f15ece221d4 100644
--- a/libcxx/test/std/thread/thread.semaphore/timed.pass.cpp
+++ b/libcxx/test/std/thread/thread.semaphore/timed.pass.cpp
@@ -9,6 +9,9 @@
 // UNSUPPORTED: no-threads
 // UNSUPPORTED: c++03, c++11
 
+// Until we drop support for the synchronization library in C++11/14/17
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
+
 // XFAIL: availability-synchronization_library-missing
 
 // <semaphore>
diff --git a/libcxx/test/std/thread/thread.semaphore/try_acquire.pass.cpp b/libcxx/test/std/thread/thread.semaphore/try_acquire.pass.cpp
index 0d0f7792592fb..ec159daf87a3f 100644
--- a/libcxx/test/std/thread/thread.semaphore/try_acquire.pass.cpp
+++ b/libcxx/test/std/thread/thread.semaphore/try_acquire.pass.cpp
@@ -9,6 +9,9 @@
 // UNSUPPORTED: no-threads
 // UNSUPPORTED: c++03, c++11
 
+// Until we drop support for the synchronization library in C++11/14/17
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
+
 // XFAIL: availability-synchronization_library-missing
 
 // <semaphore>