[gcc r13-1285] libstdc++: Invert relationship between std::is_clock and std::is_clock_v

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:48099f7dafe8d10e92ddfc88c0fd5bfbb435d3d6

commit r13-1285-g48099f7dafe8d10e92ddfc88c0fd5bfbb435d3d6
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Jun 23 18:17:30 2022 +0100

    libstdc++: Invert relationship between std::is_clock and std::is_clock_v
    
    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.

Diff:
---
 libstdc++-v3/include/bits/chrono.h | 45 +++++++++++++++-----------------------
 1 file changed, 18 insertions(+), 27 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
-      { };
+      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 : __is_clock_impl<_Tp>::type
+      struct is_clock
+      : bool_constant<is_clock_v<_Tp>>
       { };
-#endif
 #endif // C++20
 
 #if __cplusplus >= 201703L