[libcxx-commits] [libcxx] [llvm] [libcxx] improves diagnostics for containers with bad value types (PR #106296)

[Nikolas Klauser via libcxx-commits]

================
@@ -0,0 +1,80 @@
+//===----------------------------------------------------------------------===//
+//
+// 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_DIAGNOSTIC_UTILITIES_H
+#define _LIBCPP___TYPE_TRAITS_DIAGNOSTIC_UTILITIES_H
+
+#include <__config>
+#include <__type_traits/decay.h>
+#include <__type_traits/integral_constant.h>
+#include <__type_traits/is_bounded_array.h>
+#include <__type_traits/is_const.h>
+#include <__type_traits/is_function.h>
+#include <__type_traits/is_reference.h>
+#include <__type_traits/is_same.h>
+#include <__type_traits/is_unbounded_array.h>
+#include <__type_traits/is_void.h>
+#include <__type_traits/is_volatile.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+// Per https://eel.is/c++draft/containers#container.reqmts-64, allocator-aware containers must have an
+// allocator that meets the Cpp17Allocator requirements (https://eel.is/c++draft/allocator.requirements).
+// In particular, this means that containers should only accept non-cv-qualified object types, and
+// types that are Cpp17Erasable.
+template <template <class...> class _Template, class _Tp, bool = is_same<typename decay<_Tp>::type, _Tp>::value>
+struct __allocator_requirements : true_type {};
+
+#if _LIBCPP_STD_VER >= 20
+template <class _Tp>
+struct __bounded_arrays_allowed_only_after_cxx20 : false_type {};
+#else
+template <class _Tp>
+struct __bounded_arrays_allowed_only_after_cxx20 : integral_constant<bool, __libcpp_is_bounded_array<_Tp>::value> {};
+#endif
----------------
philnik777 wrote:

```suggestion
#if _LIBCPP_STD_VER >= 20
template <class _Tp>
inline const bool __bounded_arrays_allowed_only_after_cxx20 = false;
#else
template <class _Tp>
inline const bool __bounded_arrays_allowed_only_after_cxx20 = __libcpp_is_bounded_array<_Tp>::value;
#endif
```

https://github.com/llvm/llvm-project/pull/106296