[committed] libstdc++: Invert relationship between std::is_clock and std::is_clock_v

[Jonathan Wakely via Gcc-patches]

Tested powerpc64le-linux, pushed to trunk.

-- >8 --

This redefines std::is_clock in terms of std::is_clock_v, instead of the
other way around. This avoids instantiatng the class template for code
that only uses the variable template.

libstdc++-v3/ChangeLog:

        * include/bits/chrono.h (is_clock_v): Define to false.
        (is_clock_v<T>): Define partial specialization for true cases.
        (is_clock): Define in terms of is_clock_v.
---
 libstdc++-v3/include/bits/chrono.h | 49 ++++++++++++------------------
 1 file changed, 20 insertions(+), 29 deletions(-)

diff --git a/libstdc++-v3/include/bits/chrono.h 
b/libstdc++-v3/include/bits/chrono.h
index 745f9a81357..05987ca09df 100644
--- a/libstdc++-v3/include/bits/chrono.h
+++ b/libstdc++-v3/include/bits/chrono.h
@@ -273,16 +273,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #endif // C++17
 
 #if __cplusplus > 201703L
-    template<typename _Tp>
-      struct is_clock;
-
-    template<typename _Tp>
-      inline constexpr bool is_clock_v = is_clock<_Tp>::value;
-
 #if __cpp_lib_concepts
     template<typename _Tp>
-      struct is_clock : false_type
-      { };
+      inline constexpr bool is_clock_v = false;
 
     template<typename _Tp>
       requires requires {
@@ -298,32 +291,30 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        requires same_as<typename _Tp::time_point::duration,
                         typename _Tp::duration>;
       }
-      struct is_clock<_Tp> : true_type
-      { };
+    inline constexpr bool is_clock_v<_Tp> = true;
 #else
     template<typename _Tp, typename = void>
-      struct __is_clock_impl : false_type
-      { };
+      inline constexpr bool is_clock_v = false;
 
     template<typename _Tp>
-      struct __is_clock_impl<_Tp,
-                            void_t<typename _Tp::rep, typename _Tp::period,
-                                   typename _Tp::duration,
-                                   typename _Tp::time_point::duration,
-                                   decltype(_Tp::is_steady),
-                                   decltype(_Tp::now())>>
-      : __and_<is_same<typename _Tp::duration,
-                      duration<typename _Tp::rep, typename _Tp::period>>,
-              is_same<typename _Tp::time_point::duration,
-                      typename _Tp::duration>,
-              is_same<decltype(&_Tp::is_steady), const bool*>,
-              is_same<decltype(_Tp::now()), typename _Tp::time_point>>::type
-      { };
-
-    template<typename _Tp>
-      struct is_clock : __is_clock_impl<_Tp>::type
-      { };
+      inline constexpr bool
+      is_clock_v<_Tp, void_t<typename _Tp::rep, typename _Tp::period,
+                            typename _Tp::duration,
+                            typename _Tp::time_point::duration,
+                            decltype(_Tp::is_steady),
+                            decltype(_Tp::now())>>
+       = __and_v<is_same<typename _Tp::duration,
+                         duration<typename _Tp::rep, typename _Tp::period>>,
+                 is_same<typename _Tp::time_point::duration,
+                         typename _Tp::duration>,
+                 is_same<decltype(&_Tp::is_steady), const bool*>,
+                 is_same<decltype(_Tp::now()), typename _Tp::time_point>>;
 #endif
+
+    template<typename _Tp>
+      struct is_clock
+      : bool_constant<is_clock_v<_Tp>>
+      { };
 #endif // C++20
 
 #if __cplusplus >= 201703L
-- 
2.36.1