diff --git a/libcxx/include/__threading_support b/libcxx/include/__threading_support
index 8ecc0ed12bff..13ab769ba93d 100644
--- a/libcxx/include/__threading_support
+++ b/libcxx/include/__threading_support
@@ -61,6 +61,8 @@ typedef pthread_once_t __libcpp_exec_once_flag;
 typedef pthread_t __libcpp_thread_id;
 
 // Thread
+#define _LIBCPP_NULL_THREAD 0U
+
 typedef pthread_t __libcpp_thread_t;
 
 // Thrad Local Storage
@@ -86,6 +88,8 @@ typedef INIT_ONCE __libcpp_exec_once_flag;
 typedef DWORD __libcpp_thread_id;
 
 // Thread
+#define _LIBCPP_NULL_THREAD 0U
+
 typedef HANDLE __libcpp_thread_t;
 
 // Thread Local Storage
@@ -157,6 +161,9 @@ _LIBCPP_THREAD_ABI_VISIBILITY
 bool __libcpp_thread_id_less(__libcpp_thread_id t1, __libcpp_thread_id t2);
 
 // Thread
+_LIBCPP_THREAD_ABI_VISIBILITY
+bool __libcpp_thread_isnull(const __libcpp_thread_t *__t);
+
 _LIBCPP_THREAD_ABI_VISIBILITY
 int __libcpp_thread_create(__libcpp_thread_t *__t, void *(*__func)(void *),
                            void *__arg);
@@ -309,6 +316,10 @@ bool __libcpp_thread_id_less(__libcpp_thread_id t1, __libcpp_thread_id t2)
 }
 
 // Thread
+bool __libcpp_thread_isnull(const __libcpp_thread_t *__t) {
+  return *__t == 0;
+}
+
 int __libcpp_thread_create(__libcpp_thread_t *__t, void *(*__func)(void *),
                            void *__arg)
 {
@@ -506,6 +517,10 @@ __libcpp_beginthreadex_thunk(void *__raw_data)
   return static_cast<unsigned>(reinterpret_cast<uintptr_t>(__func(__arg)));
 }
 
+bool __libcpp_thread_isnull(const __libcpp_thread_t *__t) {
+  return *__t == 0;
+}
+
 int __libcpp_thread_create(__libcpp_thread_t *__t, void *(*__func)(void *),
                            void *__arg)
 {
diff --git a/libcxx/include/thread b/libcxx/include/thread
index 479e3c08f809..602445644fc4 100644
--- a/libcxx/include/thread
+++ b/libcxx/include/thread
@@ -290,7 +290,7 @@ public:
     typedef __libcpp_thread_t native_handle_type;
 
     _LIBCPP_INLINE_VISIBILITY
-    thread() _NOEXCEPT : __t_(0) {}
+    thread() _NOEXCEPT : __t_(_LIBCPP_NULL_THREAD) {}
 #ifndef _LIBCPP_HAS_NO_VARIADICS
     template <class _Fp, class ..._Args,
               class = typename enable_if
@@ -306,7 +306,7 @@ public:
 
 #ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
     _LIBCPP_INLINE_VISIBILITY
-    thread(thread&& __t) _NOEXCEPT : __t_(__t.__t_) {__t.__t_ = 0;}
+    thread(thread&& __t) _NOEXCEPT : __t_(__t.__t_) {__t.__t_ = _LIBCPP_NULL_THREAD;}
     _LIBCPP_INLINE_VISIBILITY
     thread& operator=(thread&& __t) _NOEXCEPT;
 #endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
@@ -315,7 +315,7 @@ public:
     void swap(thread& __t) _NOEXCEPT {_VSTD::swap(__t_, __t.__t_);}
 
     _LIBCPP_INLINE_VISIBILITY
-    bool joinable() const _NOEXCEPT {return __t_ != 0;}
+    bool joinable() const _NOEXCEPT {return !__libcpp_thread_isnull(&__t_);}
     void join();
     void detach();
     _LIBCPP_INLINE_VISIBILITY
@@ -409,10 +409,10 @@ inline
 thread&
 thread::operator=(thread&& __t) _NOEXCEPT
 {
-    if (__t_ != 0)
+    if (!__libcpp_thread_isnull(&__t_))
         terminate();
     __t_ = __t.__t_;
-    __t.__t_ = 0;
+    __t.__t_ = _LIBCPP_NULL_THREAD;
     return *this;
 }