[libcxx-commits] [libcxx] [libc++] P2770R0: "Stashing stashing iterators for proper flattening" (PR #66033)

[Louis Dionne via libcxx-commits]

================
@@ -0,0 +1,39 @@
+// -*- 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___UTILITY_AS_LVALUE_H
+#define _LIBCPP___UTILITY_AS_LVALUE_H
+
+#include <__config>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 20
+
+namespace ranges {
+template <class _Tp>
+_LIBCPP_HIDE_FROM_ABI constexpr _Tp& __as_lvalue(_LIBCPP_LIFETIMEBOUND _Tp&& __t) {
+  return static_cast<_Tp&>(__t);
+}
+} // namespace ranges
+
+#endif // _LIBCPP_STD_VER >= 20
----------------
ldionne wrote:

```suggestion
#ifndef _LIBCPP_CXX03_LANG

template <class _Tp>
_LIBCPP_HIDE_FROM_ABI constexpr _Tp& __as_lvalue(_LIBCPP_LIFETIMEBOUND _Tp&& __t) {
  return static_cast<_Tp&>(__t);
}

#endif // !_LIBCPP_CXX03_LANG
```

This is a general-purpose utility, so I think it makes sense to have it in `std::`, not `std::ranges::`. Also, I think this works in all standard modes > C++03 so let's not restrict it to C++20 artificially.

https://github.com/llvm/llvm-project/pull/66033