From 07a674dc1f00b1e94d47a55bd43c1c9df34cd7b6 Mon Sep 17 00:00:00 2001
From: "Stephan T. Lavavej" <stl@nuwen.net>
Date: Tue, 16 Mar 2021 20:20:16 -0700
Subject: [PATCH 1/3] Use SFINAE for _Is_allocator.

Followup to GH 919. Addresses DevCom-1367531.
---
 stl/inc/sstream  | 8 ++++----
 stl/inc/xutility | 5 -----
 2 files changed, 4 insertions(+), 9 deletions(-)

diff --git a/stl/inc/sstream b/stl/inc/sstream
index 90ce15f8ea..51763b89b6 100644
--- a/stl/inc/sstream
+++ b/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);
     }
diff --git a/stl/inc/xutility b/stl/inc/xutility
index 2179e1251f..28a06fabfc 100644
--- a/stl/inc/xutility
+++ b/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) {

From 224702060c9163ddab735290df494be3824624d2 Mon Sep 17 00:00:00 2001
From: "Stephan T. Lavavej" <stl@nuwen.net>
Date: Tue, 16 Mar 2021 22:07:23 -0700
Subject: [PATCH 2/3] Use SFINAE for is_clock.

---
 stl/inc/chrono | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/stl/inc/chrono b/stl/inc/chrono
index 69d0f848fc..c6ae44da15 100644
--- a/stl/inc/chrono
+++ b/stl/inc/chrono
@@ -58,20 +58,18 @@ namespace chrono {
     };
 
 #if _HAS_CXX20
+    template <class _Clock, class = void>
+    struct _Is_clock : false_type {};
+
     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();
-    };
+    struct _Is_clock<_Clock, void_t<typename _Clock::rep, typename _Clock::period, typename _Clock::duration,
+                                 typename _Clock::time_point, decltype(_Clock::is_steady), decltype(_Clock::now())>>
+        : true_type {}; // TRANSITION, GH-602
 
     template <class _Clock>
-    struct is_clock : bool_constant<_Is_clock<_Clock>> {};
+    struct is_clock : _Is_clock<_Clock>::type {};
     template <class _Clock>
-    inline constexpr bool is_clock_v = _Is_clock<_Clock>;
+    inline constexpr bool is_clock_v = _Is_clock<_Clock>::value;
 #endif // _HAS_CXX20
 
     // CLASS TEMPLATE duration

From 482df350e4dbd40750c44bd4732a210b86e3c325 Mon Sep 17 00:00:00 2001
From: "Stephan T. Lavavej" <stl@nuwen.net>
Date: Wed, 17 Mar 2021 02:03:19 -0700
Subject: [PATCH 3/3] Use a variable template for throughput.

Co-authored-by: Michael Schellenberger Costa <mschellenbergercosta@googlemail.com>
---
 stl/inc/chrono | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/stl/inc/chrono b/stl/inc/chrono
index c6ae44da15..0744254385 100644
--- a/stl/inc/chrono
+++ b/stl/inc/chrono
@@ -59,17 +59,18 @@ namespace chrono {
 
 #if _HAS_CXX20
     template <class _Clock, class = void>
-    struct _Is_clock : false_type {};
+    inline constexpr bool _Is_clock_v = false;
 
     template <class _Clock>
-    struct _Is_clock<_Clock, void_t<typename _Clock::rep, typename _Clock::period, typename _Clock::duration,
-                                 typename _Clock::time_point, decltype(_Clock::is_steady), decltype(_Clock::now())>>
-        : true_type {}; // TRANSITION, GH-602
+    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>
-    struct is_clock : _Is_clock<_Clock>::type {};
+    struct is_clock : bool_constant<_Is_clock_v<_Clock>> {};
     template <class _Clock>
-    inline constexpr bool is_clock_v = _Is_clock<_Clock>::value;
+    inline constexpr bool is_clock_v = _Is_clock_v<_Clock>;
 #endif // _HAS_CXX20
 
     // CLASS TEMPLATE duration