20869c5ba0
For an lvalue reference to a move only view x, views::all(x) gives hard error because the expression inside noexcept is not well formed and it is not SFINAE friendly.
Given a move only view type `V`, and a concept
```
template <class R>
concept can_all = requires {
std::views::all(std::declval<R>());
};
```
The expression `can_all<V&>` returns
libstdc++: false
msvc stl : false
libc++ : error: static_cast from 'V' to 'typename decay<decltype((std::forward<V &>(__t)))>::type' (aka 'V') uses deleted function
noexcept(noexcept(_LIBCPP_AUTO_CAST(std::forward<_Tp>(__t))))
The standard spec has its own problem, the spec says it is expression equivalent to `decay-copy(E)` but the spec of `decay-copy` does not have any constraint, which means the expression `decay-copy(declval<V&>())` is well-formed and the concept `can_all<V&>` should return true and should error when instantiating the function body of decay-copy. This is clearly wrong behaviour in the spec and we will probably create an LWG issue. But the libc++'s behaviour is clearly not correct. The `noexcept` is an "extension" in libc++ which is not in the spec, but the expression inside `noexpect` triggers hard error, which is not right.
Reviewed By: #libc, ldionne, var-const
Differential Revision: https://reviews.llvm.org/D128281
84 lines
2.6 KiB
C++
84 lines
2.6 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___RANGES_ALL_H
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#define _LIBCPP___RANGES_ALL_H
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#include <__config>
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#include <__iterator/concepts.h>
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#include <__iterator/iterator_traits.h>
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#include <__ranges/access.h>
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#include <__ranges/concepts.h>
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#include <__ranges/owning_view.h>
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#include <__ranges/range_adaptor.h>
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#include <__ranges/ref_view.h>
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#include <__utility/auto_cast.h>
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#include <__utility/declval.h>
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#include <__utility/forward.h>
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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 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
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namespace ranges::views {
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namespace __all {
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struct __fn : __range_adaptor_closure<__fn> {
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template<class _Tp>
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requires ranges::view<decay_t<_Tp>>
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[[nodiscard]] _LIBCPP_HIDE_FROM_ABI
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constexpr auto operator()(_Tp&& __t) const
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noexcept(noexcept(_LIBCPP_AUTO_CAST(std::forward<_Tp>(__t))))
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-> decltype(_LIBCPP_AUTO_CAST(std::forward<_Tp>(__t)))
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{
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return _LIBCPP_AUTO_CAST(std::forward<_Tp>(__t));
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}
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template<class _Tp>
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requires (!ranges::view<decay_t<_Tp>>) &&
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requires (_Tp&& __t) { ranges::ref_view{std::forward<_Tp>(__t)}; }
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[[nodiscard]] _LIBCPP_HIDE_FROM_ABI
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constexpr auto operator()(_Tp&& __t) const
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noexcept(noexcept(ranges::ref_view{std::forward<_Tp>(__t)}))
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{
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return ranges::ref_view{std::forward<_Tp>(__t)};
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}
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template<class _Tp>
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requires (!ranges::view<decay_t<_Tp>> &&
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!requires (_Tp&& __t) { ranges::ref_view{std::forward<_Tp>(__t)}; } &&
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requires (_Tp&& __t) { ranges::owning_view{std::forward<_Tp>(__t)}; })
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[[nodiscard]] _LIBCPP_HIDE_FROM_ABI
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constexpr auto operator()(_Tp&& __t) const
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noexcept(noexcept(ranges::owning_view{std::forward<_Tp>(__t)}))
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{
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return ranges::owning_view{std::forward<_Tp>(__t)};
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}
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};
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} // namespace __all
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inline namespace __cpo {
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inline constexpr auto all = __all::__fn{};
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} // namespace __cpo
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template<ranges::viewable_range _Range>
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using all_t = decltype(views::all(declval<_Range>()));
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} // namespace ranges::views
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#endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
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_LIBCPP_END_NAMESPACE_STD
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#endif // _LIBCPP___RANGES_ALL_H
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