3819. reference_meows_from_temporary should not use is_meowible

Section: 21.3.5.4 [meta.unary.prop] Status: C++23 Submitter: Tim Song Opened: 2022-11-08 Last modified: 2023-11-22

Priority: Not Prioritized

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Discussion:

The intent of P2255R2 is for the reference_meows_from_temporary traits to fully support cases where a prvalue is used as the source. Unfortunately the wording fails to do so because it tries to use the is_meowible traits to say "the initialization is well-formed", but those traits only consider initialization from xvalues, not prvalues. For example, given: 

struct U {
  U();
  U(U&&) = delete;
};

struct T {
  T(U);
};

reference_constructs_from_temporary_v<const T&, U> should be true, but is currently defined as false. We need to spell out the "is well-formed" condition directly.

[Kona 2022-11-08; Move to Tentatively Ready]

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

[Drafting note: The note is already repeated every time we talk about "immediate context".]

  1. Modify 21.3.3 [meta.type.synop], Table 46 ([tab:meta.unary.prop]) — "Type property predicates" — as indicated:

    Table 46: Type property predicates [tab:meta.unary.prop]
    Template Condition Preconditions
    …
    template<class T, class U>
    struct reference_constructs_from_temporary;         		conjunction_v<is_reference<T>, is_constructible<T, U>> is trueT is a reference type, and the initialization T t(VAL<U>); is well-formed and binds t to a temporary object whose lifetime is extended (6.7.7 [class.temporary]). Access checking is performed as if in a context unrelated to T and U. Only the validity of the immediate context of the variable initialization is considered. [Note ?: The initialization can result in effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on. Such effects are not in the "immediate context" and can result in the program being ill-formed. — end note] T and U shall be complete types, cv void, or arrays of unknown bound.
    template<class T, class U>
    struct reference_converts_from_temporary;         		conjunction_v<is_reference<T>, is_convertible<U, T>> is trueT is a reference type, and the initialization T t = VAL<U>; is well-formed and binds t to a temporary object whose lifetime is extended (6.7.7 [class.temporary]). Access checking is performed as if in a context unrelated to T and U. Only the validity of the immediate context of the variable initialization is considered. [Note ?: The initialization can result in effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on. Such effects are not in the "immediate context" and can result in the program being ill-formed. — end note] T and U shall be complete types, cv void, or arrays of unknown bound.