still check for specialization

Author: yuxuanchen1997

libcxx/include/__chrono/is_clock.h

@@ -32,6 +32,14 @@ class duration;
 template <class _Clock, class _Duration>
 class time_point;
 
+// Helper to check that _Tp::time_point has the form time_point<_, typename _Tp::duration>.
+template <class _TimePoint, class _ClockType>
+constexpr bool __is_valid_clock_time_point_v = false;
+
+template <class _TimePointClock, class _ClockType>
+constexpr bool __is_valid_clock_time_point_v<time_point<_TimePointClock, typename _ClockType::duration>, _ClockType> =
+    true;
+
 // Check if a clock satisfies the Cpp17Clock requirements as defined in [time.clock.req]
 template <class _Tp>
 _LIBCPP_NO_SPECIALIZATIONS inline constexpr bool is_clock_v = requires {
@@ -45,7 +53,7 @@ _LIBCPP_NO_SPECIALIZATIONS inline constexpr bool is_clock_v = requires {
   requires _IsSame<typename _Tp::duration, duration<typename _Tp::rep, typename _Tp::period>>::value;
 
   typename _Tp::time_point;
-  requires _IsSame<typename _Tp::time_point::duration, typename _Tp::duration>::value;
+  requires __is_valid_clock_time_point_v<typename _Tp::time_point, _Tp>;
 
   _Tp::is_steady;
   requires _IsSame<decltype(_Tp::is_steady), const bool>::value;