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3041. Unnecessary decay in reference_wrapper

Section: [refwrap.const] Status: WP Submitter: Agustín K-ballo Bergé Opened: 2017-12-04 Last modified: 2018-03-18

Priority: 0

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Another occurrence of unnecessary decay (P0777) was introduced by the resolution of LWG 2993. A decayed function/array type will never be the same type as reference_wrapper.

[ 2018-01-09 Moved to Tentatively Ready after 9 positive votes on c++std-lib. ]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative to N4713.

  1. Change [refwrap.const] as indicated:

    template<class U>
    reference_wrapper(U&& u) noexcept(see below);

    -1- Remarks: Let FUN denote the exposition-only functions

    void FUN(T&) noexcept;
    void FUN(T&&) = delete;

    This constructor shall not participate in overload resolution unless the expression FUN(declval<U>()) is well-formed and is_same_v<decay_tremove_cvref_t<U>, reference_wrapper> is false. The expression inside noexcept is equivalent to noexcept(FUN(declval<U>())).