[libcxx-commits] [libcxx] [libc++][tuple][utility] P2968R2 Make `std::ignore` a first-class object (PR #97401)

[A. Jiang via libcxx-commits]

================
@@ -0,0 +1,35 @@
+//===----------------------------------------------------------------------===//
+//
+// 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___TUPLE_IGNORE_H
+#define _LIBCPP___TUPLE_IGNORE_H
+
+#include <__config>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#ifndef _LIBCPP_CXX03_LANG
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+struct __ignore_type {
+  template <class _Tp>
+  _LIBCPP_HIDE_FROM_ABI constexpr const __ignore_type& operator=(const _Tp&) const noexcept {
+    return *this;
+  }
+};
+
+inline constexpr __ignore_type ignore;
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frederick-vs-ja wrote:

MSVC STL is currently marking these non-templated `constexpr` variables `_INLINE_VAR`, which expands to `inline` only since C++17. `_INLINE_VAR` expands to nothing in C++14 mode, which makes `constexpr` variables non-inline and have internal linkage. I think libc++ is effectively doing the same.

https://github.com/llvm/llvm-project/pull/97401