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

[Christopher Di Bella via libcxx-commits]

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@@ -0,0 +1,94 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// Many templates require their type parameters to be cv-unqualified objects.
+template <template <class...> class _Template, class _Tp, bool = is_same<__decay_t<_Tp>, _Tp>::value>
+struct __requires_cv_unqualified_object_type : true_type {};
+
+#define _LIBCPP_DEFINE__REQUIRES_CV_UNQUALIFIED_OBJECT_TYPE(_Template, _Verb)                                          \
+  template <class _Tp>                                                                                                 \
+  struct __requires_cv_unqualified_object_type<_Template, _Tp, false>                                                  \
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cjdb wrote:

Ah, I get where you're coming from now, thank you. This is a metaprogramming hack to preserve performance for valid use.
* In the case where `decay_t<_Tp>` matches `_Tp`: `__requires_cv_unqualified_type` should only need to instantiate two templates: `decay_t<_Tp>` and `__requires_cv_unqualified_type<_Template, _Tp, true>`.
* In case where `decay_t<_Tp>` is different to `_Tp`: `__requires_cv_unqualified_type` then instantiates all the checks to see why that isn't the case. This is indeed slower, but we're now going for a failed build, and tooling tends to burn compile times in favour of helpful diagnostics.

Neither `__allocator_requirements`, nor `__container_requirements` have the same effect, so I'm happy to revert those without further convincing.

Chromium is a good candidate for benchmarking the worst-case scenario, because it has well over 100k utterances of `std::vector`. Would providing benchmarks be helpful? (It'll take ~a day to get those as I discarded the results from last week, and I tend to run quite a few trials to reduce the impact of noise.)

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