[libcxx-commits] [libcxx] [libc++] Avoid constructing additional objects when using map::at (PR #157866)

[Louis Dionne via libcxx-commits]

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@@ -0,0 +1,46 @@
+//===----------------------------------------------------------------------===//
+//
+// 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___TYPE_TRAITS_MAKE_TRANSPARENT_H
+#define _LIBCPP___TYPE_TRAITS_MAKE_TRANSPARENT_H
+
+#include <__config>
+#include <__type_traits/enable_if.h>
+#include <__type_traits/is_same.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+// __make_transparent tries to replace non-transparent comparator with their transparent counterparts. e.g. `less<T>` is
+// replaced with `less<>`. This is useful in cases where conversions can be avoided (e.g. a string literal to a
+// std::string).
+
+template <class _Comparator>
+struct __make_transparent {
+  using type _LIBCPP_NODEBUG = _Comparator;
+};
+
+template <class _Comparator>
+using __make_transparent_t _LIBCPP_NODEBUG = typename __make_transparent<_Comparator>::type;
+
+template <class _Comparator, __enable_if_t<is_same<_Comparator, __make_transparent_t<_Comparator> >::value, int> = 0>
+_LIBCPP_HIDE_FROM_ABI _Comparator& __as_transparent(_Comparator& __comp) {
+  return __comp;
+}
+
+template <class _Comparator, __enable_if_t<!is_same<_Comparator, __make_transparent_t<_Comparator> >::value, int> = 0>
+_LIBCPP_HIDE_FROM_ABI __make_transparent_t<_Comparator> __as_transparent(_Comparator&) {
----------------
ldionne wrote:

```suggestion
template <class _Comparator, __enable_if_t<!is_same<_Comparator, __make_transparent_t<_Comparator> >::value && is_empty_v<__make_transparent_t<_Comparator> >, int> = 0>
_LIBCPP_HIDE_FROM_ABI __make_transparent_t<_Comparator> __as_transparent(_Comparator&) {
```

That, or a `static_assert`. But it seems important since we default construct it.

https://github.com/llvm/llvm-project/pull/157866