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3724. decay-copy should be constrained

Section: 16.3.3.2 [expos.only.entity] Status: C++23 Submitter: Hui Xie Opened: 2022-06-23 Last modified: 2023-11-22

Priority: 3

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Discussion:

The spec of views::all 26.7.6.1 [range.all.general] p2 says:

Given a subexpression E, the expression views::all(E) is expression-equivalent to:

  1. (2.1) — decay-copy(E) if the decayed type of E models view.

  2. […]

If E is an lvalue move-only view, according to the spec, views::all(E) would be expression-equivalent to decay-copy(E).

However, [expos.only.func] p2 defines decay-copy as follows 

template<class T> constexpr decay_t<T> decay-copy(T&& v)
    noexcept(is_nothrow_convertible_v<T, decay_t<T>>)         // exposition only
  { return std::forward<T>(v); }

It is unconstrained. As a result, for the above example, views::all(E) is a well-formed expression and it would only error on the template instantiation of the function body of decay-copy, because E is an lvalue of move-only type and not copyable.

I think this behaviour is wrong, instead, we should make decay-copy(E) ill-formed if it is not copyable, so that views::all(E) can be SFINAE friendly.

[2022-07-08; Reflector poll]

Set priority to 3 after reflector poll.

[2022-07-11; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4910.

  1. Modify [expos.only.func] as indicated:

    -2- The following are defined for exposition only to aid in the specification of the library: 

    namespace std {
  template<class T> 
    requires convertible_to<T, decay_t<T>>
      constexpr decay_t<T> decay-copy(T&& v)
        noexcept(is_nothrow_convertible_v<T, decay_t<T>>) // exposition only
      { return std::forward<T>(v); }
  […]
}