[all-commits] [llvm/llvm-project] 20869c: [libc++] fix views::all hard error on lvalue move ...

[Hui via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 20869c5ba0692974a3ec76b935360aaa67d0218b
      https://github.com/llvm/llvm-project/commit/20869c5ba0692974a3ec76b935360aaa67d0218b
  Author: Hui Xie <hui.xie1990 at gmail.com>
  Date:   2022-06-22 (Wed, 22 Jun 2022)

  Changed paths:
    M libcxx/include/__ranges/all.h
    M libcxx/test/std/ranges/range.adaptors/range.all/all.pass.cpp

  Log Message:
  -----------
  [libc++] fix views::all hard error on lvalue move only views instead of SFINAE

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