[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 +1,49 @@
+// -*- 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>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if _LIBCPP_STD_VER >= 20
+
+#  include <__type_traits/integral_constant.h>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+namespace chrono {
+
+template <class>
+struct is_clock : std::false_type {};
+
+template <class _Tp>
+  requires requires {
+    typename _Tp::rep;
+    typename _Tp::period;
+    typename _Tp::duration;
+    typename _Tp::time_point;
+    _Tp::is_steady;
+    _Tp::now();
+  }
+struct is_clock<_Tp> : std::true_type {};
+
+template <class _Tp>
+_LIBCPP_NO_SPECIALIZATIONS inline constexpr bool is_clock_v = is_clock<_Tp>::value;
----------------
frederick-vs-ja wrote:

It seems better to directly implement `is_clock_v` with the requires-expression.

```suggestion
template <class _Tp>
_LIBCPP_NO_SPECIALIZATIONS inline constexpr bool is_clock_v = requires {
  typename _Tp::rep;
  typename _Tp::period;
  typename _Tp::duration;
  typename _Tp::time_point;
  _Tp::is_steady;
  _Tp::now();
};

template <class _Tp>
struct _LIBCPP_NO_SPECIALIZATIONS is_clock : bool_constant<is_clock_v<_Tp>> {};
```

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