[libcxx-commits] [libcxx] [libcxx] Implement C++20 std::chrono::is_clock, std::chrono::is_clock_v (PR #160607)

[A. Jiang via libcxx-commits]

================
@@ -0,0 +1,73 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___CHRONO_IS_CLOCK_H
+#define _LIBCPP___CHRONO_IS_CLOCK_H
+
+#include <__config>
+#include <__type_traits/integral_constant.h>
+#include <__type_traits/is_arithmetic.h>
+#include <__type_traits/is_same.h>
+#include <ratio>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if _LIBCPP_STD_VER >= 20
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+namespace chrono {
+
+template <class _Rep, class _Period>
+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 {
+  typename _Tp::rep;
+  requires is_arithmetic_v<typename _Tp::rep>;
+
+  typename _Tp::period;
+  requires __is_ratio_v<typename _Tp::period>;
+
+  typename _Tp::duration;
+  requires _IsSame<typename _Tp::duration, duration<typename _Tp::rep, typename _Tp::period>>::value;
+
+  typename _Tp::time_point;
+  requires __is_valid_clock_time_point_v<typename _Tp::time_point, _Tp>;
+
+  _Tp::is_steady;
+  requires _IsSame<decltype(_Tp::is_steady), const bool>::value;
----------------
frederick-vs-ja wrote:

I possibly forgot one point - the row in [[tab:time.clock]](https://eel.is/c++draft/tab:time.clock) probably actually means that `remove_reference_t<decltype((_Tp::is_steady))>` is `const bool`. The standardese drops `&`/`&&` when simply talking about the type of an expression.

So
```suggestion
  requires same_as<decltype((_Tp::is_steady)), const bool&>;
```

However, there's possibly a defect as the intent seems to be that `_Tp::is_steady` should be a static constexpr data member of type `const bool` (or a plain `const` member but usable in constant expression). This should be detectible by `requires same_as<decltype(_Tp::is_steady), const bool>;` and `typename bool_constant<_Tp::is_steady>;`.

https://github.com/llvm/llvm-project/pull/160607