4e0ea2cf2e
I just ran into a compiler error involving __bind_back and some overloads
that were being disabled with _EnableIf. I noticed that the error message
was quite bad and did not mention the reason for the overload being
excluded. Specifically, the error looked like this:
candidate template ignored: substitution failure [with _Args =
<ContiguousView>]: no member named '_EnableIfImpl' in 'std::_MetaBase<false>'
Instead, when using enable_if or enable_if_t, the compiler is clever and
can produce better diagnostics, like so:
candidate template ignored: requirement 'is_invocable_v<
std::__bind_back_op<1, std::integer_sequence<unsigned long, 0>>,
std::ranges::views::__transform::__fn &, std::tuple<PlusOne> &,
ContiguousView>' was not satisfied [with _Args = <ContiguousView>]
Basically, it tries to do a poor man's implementation of concepts, which
is already a lot better than simply complaining about substitution failure.
Hence, this commit uses enable_if_t instead of _EnableIf whenever
possible. That is both more straightforward than using the internal
helper, and also leads to better error messages in those cases.
I understand the motivation for _EnableIf's implementation was to improve
compile-time performance, however I believe striving to improve error
messages is even more important for our QOI, hence this patch. Furthermore,
it is unclear that _EnableIf actually improved compile-time performance
in any noticeable way (see discussion in the review for details).
Differential Revision: https://reviews.llvm.org/D108216
86 lines
2.4 KiB
C++
86 lines
2.4 KiB
C++
// -*- C++ -*-
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//===----------------------------------------------------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#ifndef _LIBCPP___MEMORY_CONSTRUCT_AT_H
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#define _LIBCPP___MEMORY_CONSTRUCT_AT_H
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#include <__config>
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#include <__debug>
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#include <__iterator/access.h>
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#include <__memory/addressof.h>
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#include <__utility/forward.h>
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#include <type_traits>
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#include <utility>
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#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
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#pragma GCC system_header
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#endif
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_LIBCPP_BEGIN_NAMESPACE_STD
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// construct_at
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#if _LIBCPP_STD_VER > 17
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template<class _Tp, class ..._Args, class = decltype(
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::new (declval<void*>()) _Tp(declval<_Args>()...)
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)>
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_LIBCPP_INLINE_VISIBILITY
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constexpr _Tp* construct_at(_Tp* __location, _Args&& ...__args) {
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_LIBCPP_ASSERT(__location, "null pointer given to construct_at");
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return ::new ((void*)__location) _Tp(_VSTD::forward<_Args>(__args)...);
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}
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#endif
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// destroy_at
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#if _LIBCPP_STD_VER > 14
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template <class _ForwardIterator>
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_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17
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void destroy(_ForwardIterator, _ForwardIterator);
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template <class _Tp, enable_if_t<!is_array_v<_Tp>, int> = 0>
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_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17
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void destroy_at(_Tp* __loc) {
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_LIBCPP_ASSERT(__loc, "null pointer given to destroy_at");
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__loc->~_Tp();
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}
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#if _LIBCPP_STD_VER > 17
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template <class _Tp, enable_if_t<is_array_v<_Tp>, int> = 0>
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_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17
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void destroy_at(_Tp* __loc) {
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_LIBCPP_ASSERT(__loc, "null pointer given to destroy_at");
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_VSTD::destroy(_VSTD::begin(*__loc), _VSTD::end(*__loc));
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}
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#endif
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template <class _ForwardIterator>
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_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17
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void destroy(_ForwardIterator __first, _ForwardIterator __last) {
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for (; __first != __last; ++__first)
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_VSTD::destroy_at(_VSTD::addressof(*__first));
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}
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template <class _ForwardIterator, class _Size>
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_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17
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_ForwardIterator destroy_n(_ForwardIterator __first, _Size __n) {
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for (; __n > 0; (void)++__first, --__n)
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_VSTD::destroy_at(_VSTD::addressof(*__first));
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return __first;
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}
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#endif
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_LIBCPP_END_NAMESPACE_STD
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#endif // _LIBCPP___MEMORY_CONSTRUCT_AT_H
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