forked from OSchip/llvm-project
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
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