2021-08-10 03:41:26 +08:00
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// -*- 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___FUNCTIONAL_BIND_BACK_H
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#define _LIBCPP___FUNCTIONAL_BIND_BACK_H
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#include <__config>
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#include <__functional/invoke.h>
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#include <__functional/perfect_forward.h>
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#include <__utility/forward.h>
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#include <__utility/integer_sequence.h>
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#include <tuple>
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#include <type_traits>
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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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#if _LIBCPP_STD_VER > 17
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template <size_t _NBound, class = make_index_sequence<_NBound>>
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struct __bind_back_op;
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template <size_t _NBound, size_t ..._Ip>
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struct __bind_back_op<_NBound, index_sequence<_Ip...>> {
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template <class _Fn, class _Bound, class ..._Args>
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_LIBCPP_HIDE_FROM_ABI
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constexpr auto operator()(_Fn&& __f, _Bound&& __bound, _Args&& ...__args) const
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noexcept(noexcept(_VSTD::invoke(_VSTD::forward<_Fn>(__f), _VSTD::forward<_Args>(__args)..., _VSTD::get<_Ip>(_VSTD::forward<_Bound>(__bound))...)))
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-> decltype( _VSTD::invoke(_VSTD::forward<_Fn>(__f), _VSTD::forward<_Args>(__args)..., _VSTD::get<_Ip>(_VSTD::forward<_Bound>(__bound))...))
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{ return _VSTD::invoke(_VSTD::forward<_Fn>(__f), _VSTD::forward<_Args>(__args)..., _VSTD::get<_Ip>(_VSTD::forward<_Bound>(__bound))...); }
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};
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template <class _Fn, class _BoundArgs>
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struct __bind_back_t : __perfect_forward<__bind_back_op<tuple_size_v<_BoundArgs>>, _Fn, _BoundArgs> {
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using __perfect_forward<__bind_back_op<tuple_size_v<_BoundArgs>>, _Fn, _BoundArgs>::__perfect_forward;
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};
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[libc++] Use enable_if_t instead of _EnableIf
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
2021-08-18 00:26:09 +08:00
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template <class _Fn, class ..._Args, class = enable_if_t<
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2021-08-10 03:41:26 +08:00
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_And<
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is_constructible<decay_t<_Fn>, _Fn>,
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is_move_constructible<decay_t<_Fn>>,
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is_constructible<decay_t<_Args>, _Args>...,
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is_move_constructible<decay_t<_Args>>...
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>::value
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>>
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_LIBCPP_HIDE_FROM_ABI
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constexpr auto __bind_back(_Fn&& __f, _Args&&... __args)
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noexcept(noexcept(__bind_back_t<decay_t<_Fn>, tuple<decay_t<_Args>...>>(_VSTD::forward<_Fn>(__f), _VSTD::forward_as_tuple(_VSTD::forward<_Args>(__args)...))))
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-> decltype( __bind_back_t<decay_t<_Fn>, tuple<decay_t<_Args>...>>(_VSTD::forward<_Fn>(__f), _VSTD::forward_as_tuple(_VSTD::forward<_Args>(__args)...)))
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{ return __bind_back_t<decay_t<_Fn>, tuple<decay_t<_Args>...>>(_VSTD::forward<_Fn>(__f), _VSTD::forward_as_tuple(_VSTD::forward<_Args>(__args)...)); }
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#endif // _LIBCPP_STD_VER > 17
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_LIBCPP_END_NAMESPACE_STD
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#endif // _LIBCPP___FUNCTIONAL_BIND_BACK_H
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