diff --git a/libcxx/include/__config b/libcxx/include/__config
index 5454f8d..df3ce8a 100644
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -1432,6 +1432,8 @@
      _Pragma("clang attribute pop")
 #  define _LIBCPP_AVAILABILITY_TO_CHARS                                        \
      _LIBCPP_AVAILABILITY_FILESYSTEM
+#  define _LIBCPP_AVAILABILITY_SYNC                                            \
+     __attribute__((availability(unavailable, message="The C++20 Synchronization Library requires runtime support that hasn't been shipped on Apple platforms yet!")))
 #else
 #  define _LIBCPP_AVAILABILITY_SHARED_MUTEX
 #  define _LIBCPP_AVAILABILITY_BAD_VARIANT_ACCESS
@@ -1447,6 +1449,7 @@
 #  define _LIBCPP_AVAILABILITY_FILESYSTEM_PUSH
 #  define _LIBCPP_AVAILABILITY_FILESYSTEM_POP
 #  define _LIBCPP_AVAILABILITY_TO_CHARS
+#  define _LIBCPP_AVAILABILITY_SYNC
 #endif
 
 // Define availability that depends on _LIBCPP_NO_EXCEPTIONS.
diff --git a/libcxx/include/atomic b/libcxx/include/atomic
index 02784df..0cbd7c0 100644
--- a/libcxx/include/atomic
+++ b/libcxx/include/atomic
@@ -1471,15 +1471,15 @@
 
 #ifndef _LIBCPP_HAS_NO_PLATFORM_WAIT
 
-_LIBCPP_EXPORTED_FROM_ABI void __cxx_atomic_notify_one(void const volatile*);
-_LIBCPP_EXPORTED_FROM_ABI void __cxx_atomic_notify_all(void const volatile*);
-_LIBCPP_EXPORTED_FROM_ABI __cxx_contention_t __libcpp_atomic_monitor(void const volatile*);
-_LIBCPP_EXPORTED_FROM_ABI void __libcpp_atomic_wait(void const volatile*, __cxx_contention_t);
+_LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI void __cxx_atomic_notify_one(void const volatile*);
+_LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI void __cxx_atomic_notify_all(void const volatile*);
+_LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI __cxx_contention_t __libcpp_atomic_monitor(void const volatile*);
+_LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI void __libcpp_atomic_wait(void const volatile*, __cxx_contention_t);
 
-_LIBCPP_EXPORTED_FROM_ABI void __cxx_atomic_notify_one(__cxx_atomic_contention_t const volatile*);
-_LIBCPP_EXPORTED_FROM_ABI void __cxx_atomic_notify_all(__cxx_atomic_contention_t const volatile*);
-_LIBCPP_EXPORTED_FROM_ABI __cxx_contention_t __libcpp_atomic_monitor(__cxx_atomic_contention_t const volatile*);
-_LIBCPP_EXPORTED_FROM_ABI void __libcpp_atomic_wait(__cxx_atomic_contention_t const volatile*, __cxx_contention_t);
+_LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI void __cxx_atomic_notify_one(__cxx_atomic_contention_t const volatile*);
+_LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI void __cxx_atomic_notify_all(__cxx_atomic_contention_t const volatile*);
+_LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI __cxx_contention_t __libcpp_atomic_monitor(__cxx_atomic_contention_t const volatile*);
+_LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI void __libcpp_atomic_wait(__cxx_atomic_contention_t const volatile*, __cxx_contention_t);
 
 template <class _Atp, class _Fn>
 _LIBCPP_INLINE_VISIBILITY bool __cxx_atomic_wait(_Atp* __a, _Fn && __test_fn)
diff --git a/libcxx/include/barrier b/libcxx/include/barrier
index bf2b9a8..e6dc0a1 100644
--- a/libcxx/include/barrier
+++ b/libcxx/include/barrier
@@ -91,14 +91,14 @@
 
 class __barrier_algorithm_base;
 
-_LIBCPP_EXPORTED_FROM_ABI
+_LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI
 __barrier_algorithm_base* __construct_barrier_algorithm_base(ptrdiff_t& __expected);
 
-_LIBCPP_EXPORTED_FROM_ABI
+_LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI
 bool __arrive_barrier_algorithm_base(__barrier_algorithm_base* __barrier,
                                      __barrier_phase_t __old_phase);
 
-_LIBCPP_EXPORTED_FROM_ABI
+_LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI
 void __destroy_barrier_algorithm_base(__barrier_algorithm_base* __barrier);
 
 template<class _CompletionF>
diff --git a/libcxx/lib/abi/CHANGELOG.TXT b/libcxx/lib/abi/CHANGELOG.TXT
index daea0a1..ca81722 100644
--- a/libcxx/lib/abi/CHANGELOG.TXT
+++ b/libcxx/lib/abi/CHANGELOG.TXT
@@ -12,6 +12,34 @@
 
 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
index 2e405c9..db06a4c 100644
--- 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
index f1205a0..e8cb526 100644
--- 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'}
\ No newline at end of file
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 8bd262a..ea1d7a8 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
@@ -9,6 +9,15 @@
 // 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>
diff --git a/libcxx/test/std/thread/thread.barrier/arrive.pass.cpp b/libcxx/test/std/thread/thread.barrier/arrive.pass.cpp
index 943fc84..3283bf2 100644
--- a/libcxx/test/std/thread/thread.barrier/arrive.pass.cpp
+++ b/libcxx/test/std/thread/thread.barrier/arrive.pass.cpp
@@ -9,6 +9,15 @@
 // 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>
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 8724a1d..a0428a7 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 +9,15 @@
 // 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>
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 8596e8a..66f46ec 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 +9,15 @@
 // 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>
diff --git a/libcxx/test/std/thread/thread.barrier/completion.pass.cpp b/libcxx/test/std/thread/thread.barrier/completion.pass.cpp
index 67ae526..1cce370 100644
--- a/libcxx/test/std/thread/thread.barrier/completion.pass.cpp
+++ b/libcxx/test/std/thread/thread.barrier/completion.pass.cpp
@@ -9,6 +9,15 @@
 // 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>
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 156e913..07b3c76 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 +9,15 @@
 // 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>
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 81f0605..9374a6e 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 +9,15 @@
 // 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>
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 7284849..0524271 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 +9,15 @@
 // 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>
diff --git a/libcxx/test/std/thread/thread.semaphore/acquire.pass.cpp b/libcxx/test/std/thread/thread.semaphore/acquire.pass.cpp
index bb5f7fb..66e1375 100644
--- a/libcxx/test/std/thread/thread.semaphore/acquire.pass.cpp
+++ b/libcxx/test/std/thread/thread.semaphore/acquire.pass.cpp
@@ -9,6 +9,15 @@
 // 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>
diff --git a/libcxx/test/std/thread/thread.semaphore/binary.pass.cpp b/libcxx/test/std/thread/thread.semaphore/binary.pass.cpp
index d46d6ae..fc867be 100644
--- a/libcxx/test/std/thread/thread.semaphore/binary.pass.cpp
+++ b/libcxx/test/std/thread/thread.semaphore/binary.pass.cpp
@@ -9,6 +9,15 @@
 // 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>
diff --git a/libcxx/test/std/thread/thread.semaphore/release.pass.cpp b/libcxx/test/std/thread/thread.semaphore/release.pass.cpp
index 0660feb..fe3c70a 100644
--- a/libcxx/test/std/thread/thread.semaphore/release.pass.cpp
+++ b/libcxx/test/std/thread/thread.semaphore/release.pass.cpp
@@ -9,6 +9,15 @@
 // 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>
diff --git a/libcxx/test/std/thread/thread.semaphore/timed.pass.cpp b/libcxx/test/std/thread/thread.semaphore/timed.pass.cpp
index d0bbc91..39badb0 100644
--- a/libcxx/test/std/thread/thread.semaphore/timed.pass.cpp
+++ b/libcxx/test/std/thread/thread.semaphore/timed.pass.cpp
@@ -9,6 +9,15 @@
 // 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>
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 7f06567..7c8ded0 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 +9,15 @@
 // 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>