[libcxx-commits] [libcxx] [libcxx] Implement `std::constant_wrapper` (PR #191695)

[A. Jiang via libcxx-commits]

================
@@ -0,0 +1,338 @@
+//===----------------------------------------------------------------------===//
+//
+// 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___UTILITY_CONSTANT_WRAPPER_H
+#define _LIBCPP___UTILITY_CONSTANT_WRAPPER_H
+
+#include <__config>
+#include <__cstddef/size_t.h>
+#include <__functional/invoke.h>
+#include <__type_traits/is_constructible.h>
+#include <__type_traits/remove_cvref.h>
+#include <__utility/forward.h>
+#include <__utility/integer_sequence.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 26
+
+template <class _Tp>
+struct __cw_fixed_value {
+  using __type _LIBCPP_NODEBUG = _Tp;
+  _LIBCPP_HIDE_FROM_ABI constexpr __cw_fixed_value(__type __v) noexcept : __data(__v) {}
+  _Tp __data;
+};
+
+template <class _Tp, size_t _Extent>
+struct __cw_fixed_value<_Tp[_Extent]> {
+  using __type _LIBCPP_NODEBUG = _Tp[_Extent];
+  _Tp __data[_Extent];
+
+  _LIBCPP_HIDE_FROM_ABI constexpr __cw_fixed_value(_Tp (&__arr)[_Extent]) noexcept
+      : __cw_fixed_value(__arr, make_index_sequence<_Extent>{}) {}
+
+private:
+  template <size_t... _Idxs>
+  _LIBCPP_HIDE_FROM_ABI constexpr __cw_fixed_value(_Tp (&__arr)[_Extent], index_sequence<_Idxs...>) noexcept
+      : __data{__arr[_Idxs]...} {}
+};
+
+template <class _Tp, size_t _Extent>
+__cw_fixed_value(_Tp (&)[_Extent]) -> __cw_fixed_value<_Tp[_Extent]>;
+
+template <__cw_fixed_value _Xp,
+#  ifdef _LIBCPP_COMPILER_GCC
+          // gcc bug:  https://gcc.gnu.org/PR117392
+          class = typename decltype(__cw_fixed_value(_Xp))::__type
+#  else
+          class = typename decltype(_Xp)::__type
+#  endif
+          >
+struct constant_wrapper;
+
+template <class _Tp>
+concept __constexpr_param = requires { typename constant_wrapper<_Tp::value>; };
+
+template <__cw_fixed_value _Xp>
+constexpr auto cw = constant_wrapper<_Xp>{};
+
+struct __cw_operators {
+  // unary operators
+  template <__constexpr_param _Tp>
+  _LIBCPP_HIDE_FROM_ABI friend constexpr auto operator+(_Tp) noexcept -> constant_wrapper<(+_Tp::value)> {
----------------
frederick-vs-ja wrote:

As noted by @H-G-Hristov, I think we should mark most of these operators `[[nodiscard]]` (because they don't have side effects, even for `operator=`).

Exceptions, IMO, are:
- the `operator decltype(value)` conversion function,
- the deleted `operator,` overload, and
- `operator[]` and `operator()` because their underlying (unwrapped) operations may have side effects.

The test file could be `libcxx/test/libcxx/utilities/const.wrap.class/nodiscard.verify.cpp`.

https://github.com/llvm/llvm-project/pull/191695