Avoid concepts when __cpp_lib_concepts isn't defined (#1749)

Co-authored-by: Michael Schellenberger Costa <mschellenbergercosta@googlemail.com>

stl/inc/chrono

@@ -58,20 +58,19 @@ namespace chrono {
     };
 
 #if _HAS_CXX20
-    template <class _Clock>
-    concept _Is_clock = requires {
-        typename _Clock::rep;
-        typename _Clock::period;
-        typename _Clock::duration;
-        typename _Clock::time_point;
-        _Clock::is_steady;
-        _Clock::now();
-    };
+    template <class _Clock, class = void>
+    inline constexpr bool _Is_clock_v = false;
 
     template <class _Clock>
-    struct is_clock : bool_constant<_Is_clock<_Clock>> {};
+    inline constexpr bool
+        _Is_clock_v<_Clock, void_t<typename _Clock::rep, typename _Clock::period, typename _Clock::duration,
+                                typename _Clock::time_point, decltype(_Clock::is_steady), decltype(_Clock::now())>> =
+            true; // TRANSITION, GH-602
+
     template <class _Clock>
-    inline constexpr bool is_clock_v = _Is_clock<_Clock>;
+    struct is_clock : bool_constant<_Is_clock_v<_Clock>> {};
+    template <class _Clock>
+    inline constexpr bool is_clock_v = _Is_clock_v<_Clock>;
 #endif // _HAS_CXX20
 
     // CLASS TEMPLATE duration

stl/inc/sstream

@@ -168,7 +168,7 @@ public:
     }
 
 #if _HAS_CXX20
-    template <_Allocator _Alloc2>
+    template <class _Alloc2, enable_if_t<_Is_allocator<_Alloc2>::value, int> = 0>
     _NODISCARD basic_string<_Elem, _Traits, _Alloc2> str(const _Alloc2& _Al) const {
         return basic_string<_Elem, _Traits, _Alloc2>{view(), _Al};
     }
@@ -632,7 +632,7 @@ public:
     }
 
 #if _HAS_CXX20
-    template <_Allocator _Alloc2>
+    template <class _Alloc2, enable_if_t<_Is_allocator<_Alloc2>::value, int> = 0>
     _NODISCARD basic_string<_Elem, _Traits, _Alloc2> str(const _Alloc2& _Al) const {
         return _Stringbuffer.str(_Al);
     }
@@ -752,7 +752,7 @@ public:
     }
 
 #if _HAS_CXX20
-    template <_Allocator _Alloc2>
+    template <class _Alloc2, enable_if_t<_Is_allocator<_Alloc2>::value, int> = 0>
     _NODISCARD basic_string<_Elem, _Traits, _Alloc2> str(const _Alloc2& _Al) const {
         return _Stringbuffer.str(_Al);
     }
@@ -878,7 +878,7 @@ public:
     }
 
 #if _HAS_CXX20
-    template <_Allocator _Alloc2>
+    template <class _Alloc2, enable_if_t<_Is_allocator<_Alloc2>::value, int> = 0>
     _NODISCARD basic_string<_Elem, _Traits, _Alloc2> str(const _Alloc2& _Al) const {
         return _Stringbuffer.str(_Al);
     }

stl/inc/xutility

@@ -1459,11 +1459,6 @@ using _Enable_if_execution_policy_t = typename remove_reference_t<_ExPo>::_Stand
 
 #endif // _HAS_CXX17
 
-#if _HAS_CXX20
-template <class _Ty>
-concept _Allocator = _Is_allocator<_Ty>::value;
-#endif // _HAS_CXX20
-
 // FUNCTION TEMPLATE _Idl_distance
 template <class _Checked, class _Iter>
 _NODISCARD constexpr auto _Idl_distance(const _Iter& _First, const _Iter& _Last) {