Index: libcxx/include/__threading_support
===================================================================
--- libcxx/include/__threading_support
+++ libcxx/include/__threading_support
@@ -26,18 +26,12 @@
 #if defined(_LIBCPP_HAS_THREAD_API_PTHREAD)
 # include <pthread.h>
 # include <sched.h>
-// FIXME: On Apple, <semaphore.h> transitively includes <sys/param.h>, which
-//        is not well behaved because it defines several macros. This causes
-//        name collisions in some downstream projects. Instead, <sys/semaphore.h>
-//        gives us the declarations we need without the bad stuff.
-#ifdef __APPLE__
-# include <sys/semaphore.h>
-#else
-# include <semaphore.h>
-#endif
 # ifdef __APPLE__
 #  define _LIBCPP_NO_NATIVE_SEMAPHORES
 # endif
+# ifndef _LIBCPP_NO_NATIVE_SEMAPHORES
+# include <semaphore.h>
+# endif
 #elif defined(_LIBCPP_HAS_THREAD_API_C11)
 # include <threads.h>
 #endif
@@ -77,9 +71,11 @@
 typedef pthread_cond_t __libcpp_condvar_t;
 #define _LIBCPP_CONDVAR_INITIALIZER PTHREAD_COND_INITIALIZER
 
+#ifndef _LIBCPP_NO_NATIVE_SEMAPHORES
 // Semaphore
 typedef sem_t __libcpp_semaphore_t;
-#define _LIBCPP_SEMAPHORE_MAX SEM_VALUE_MAX
+# define _LIBCPP_SEMAPHORE_MAX SEM_VALUE_MAX
+#endif
 
 // Execute once
 typedef pthread_once_t __libcpp_exec_once_flag;
@@ -210,6 +206,8 @@
 _LIBCPP_THREAD_ABI_VISIBILITY
 int __libcpp_condvar_destroy(__libcpp_condvar_t* __cv);
 
+#ifndef _LIBCPP_NO_NATIVE_SEMAPHORES
+
 // Semaphore
 _LIBCPP_THREAD_ABI_VISIBILITY
 bool __libcpp_semaphore_init(__libcpp_semaphore_t* __sem, int __init);
@@ -226,6 +224,8 @@
 _LIBCPP_THREAD_ABI_VISIBILITY
 bool __libcpp_semaphore_wait_timed(__libcpp_semaphore_t* __sem, chrono::nanoseconds const& __ns);
 
+#endif // _LIBCPP_NO_NATIVE_SEMAPHORES
+
 // Execute once
 _LIBCPP_THREAD_ABI_VISIBILITY
 int __libcpp_execute_once(__libcpp_exec_once_flag *flag,
Index: libcxx/include/semaphore
===================================================================
--- libcxx/include/semaphore
+++ libcxx/include/semaphore
@@ -172,6 +172,8 @@
 using __semaphore_base =
     __atomic_semaphore_base;
 
+#define _LIBCPP_SEMAPHORE_MAX (numeric_limits<ptrdiff_t>::max())
+
 #endif //_LIBCPP_NO_NATIVE_SEMAPHORES
 
 template<ptrdiff_t __least_max_value = _LIBCPP_SEMAPHORE_MAX>
Index: libcxx/src/barrier.cpp
===================================================================
--- libcxx/src/barrier.cpp
+++ libcxx/src/barrier.cpp
@@ -26,13 +26,21 @@
         } __tickets[64];
     };
 
-    ptrdiff_t&              __expected;
-    unique_ptr<__state_t[]> __state;
+    ptrdiff_t&         __expected;
+    unique_ptr<char[]> __state_allocation;
+    __state_t*         __state;
 
     _LIBCPP_HIDDEN
     __barrier_algorithm_base(ptrdiff_t& __expected)
-        : __expected(__expected), __state(new __barrier_algorithm_base::__state_t[(__expected + 1) >> 1])
+        : __expected(__expected)
     {
+        size_t const __count = (__expected + 1) >> 1;
+        size_t const __size = sizeof(__state_t) * __count;
+        size_t __allocation_size = __size + alignof(__state_t);
+        __state_allocation = unique_ptr<char[]>(new char[__allocation_size]);
+        void* __allocation = __state_allocation.get();
+        void* const __state_ = align(alignof(__state_t), __size, __allocation, __allocation_size);
+        __state = new (__state_) __barrier_algorithm_base::__state_t[__count];
     }
     _LIBCPP_HIDDEN
     bool __arrive(__barrier_phase_t __old_phase)
Index: libcxx/test/std/thread/thread.semaphore/max.pass.cpp
===================================================================
--- libcxx/test/std/thread/thread.semaphore/max.pass.cpp
+++ libcxx/test/std/thread/thread.semaphore/max.pass.cpp
@@ -21,7 +21,6 @@
   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;