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3348.__cpp_lib_unwrap_ref in wrong header

Section: 17.3.2[version.syn], 99 [refwrap.unwrapref]Status:C++20Submitter: Barry RevzinOpened: 2019-12-03Last modified: 2021-02-25

Priority:2

View otheractive issues in [version.syn].

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Discussion:

cpplearner points out inthis github comment that:

Sinceunwrap_reference andunwrap_ref_decay are defined in<functional> ([functional.syn]), their feature test macro should also be defined there.

P1902R1 adds this feature test macro in<type_traits> instead. The feature test macro and the type traits should go into the same header: either both in<functional> or both in<type_traits>.

The smallest diff is just to move the macro into<functional>.

[2019-12-12 Issue Prioritization]

Priority to 2 after reflector discussion.

This wording is relative toN4842.

1. Modify 17.3.2[version.syn] p2 as indicated:

   […]#define __cpp_lib_unordered_map_try_emplace 201411L// also in <unordered_map>#define __cpp_lib_unwrap_ref                201811L// also in <type_traitsfunctional>#define __cpp_lib_variant                   201606L// also in <variant>[…]

[2020-02-13, Prague]

During LWG discussions it had been suggested that they considered it is an improvement to move the definitions ofunwrap_reference andunwrap_ref_decay from<functional> to<type_traits>.This is what the alternative wording tries to accomplish.

[Status to Immediate on Thursday night in Prague.]

Proposed resolution:

This wording is relative toN4849.

1. Modify 22.10.2[functional.syn], header<functional> synopsis, as indicated:

   namespace std {[…]template<class T> struct unwrap_reference;template<class T> using unwrap_reference_t = typename unwrap_reference<T>::type;template<class T> struct unwrap_ref_decay;template<class T> using unwrap_ref_decay_t = typename unwrap_ref_decay<T>::type;[…]}

2. Delete sub-clause 99 [refwrap.unwrapref] completely, as indicated:

   20.14.5.6 Transformation type traitunwrap_reference [refwrap.unwrapref]

   template<class T>  struct unwrap_reference;

   -1- IfT is a specializationreference_wrapper<X> for some typeX, the member typedeftype ofunwrap_reference<T> isX&, otherwise it isT.

   template<class T>  struct unwrap_ref_decay;

   -2- The member typedeftype ofunwrap_ref_decay<T> denotes the typeunwrap_reference_t<decay_t<T>>.

3. Modify 21.3.3[meta.type.synop], header<type_traits> synopsis, as indicated:

   namespace std {[…]// 21.3.9.7[meta.trans.other], other transformations[…]template<class T> struct underlying_type;template<class Fn, class... ArgTypes> struct invoke_result;template<class T> struct unwrap_reference;template<class T> struct unwrap_ref_decay;template<class T>  using type_identity_t = typename type_identity<T>::type;[…]template<class Fn, class... ArgTypes>  using invoke_result_t = typename invoke_result<Fn, ArgTypes...>::type;template<class T>   using unwrap_reference_t = typename unwrap_reference<T>::type;template<class T>   using unwrap_ref_decay_t = typename unwrap_ref_decay<T>::type;template<class...>  using void_t = void;[…]}

4. Modify 21.3.9.7[meta.trans.other], Table 55 — "Sign modifications" in [tab:meta.trans.sign] as indicated:

   Table 52 — Other transformations [tab:meta.trans.other]

   Template	Comments
   […]
   template <class T> struct unwrap_reference;	IfT is a specializationreference_wrapper<X> for some typeX, the member typedeftype ofunwrap_reference<T> isX&, otherwise it isT.
   template <class T> struct unwrap_ref_decay;	The member typedeftype ofunwrap_ref_decay<T> denotes the typeunwrap_reference_t<decay_t<T>>.

5. Insert between 21.3.9.7[meta.trans.other] p1 and p2 as indicated:

   In addition to being available via inclusion of the<type_traits> header, the templatesunwrap_reference,unwrap_ref_decay,unwrap_reference_t, andunwrap_ref_decay_t are available when the header<functional> (22.10.2[functional.syn]) is included.