3348. __cpp_lib_unwrap_ref in wrong header

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

Priority: 2

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Discussion:

cpplearner points out in this github comment that:

Since unwrap_reference and unwrap_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 to N4842.

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 of unwrap_reference and unwrap_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 to N4849.

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 trait unwrap_reference [refwrap.unwrapref]

   template<class T>
     struct unwrap_reference;
   

   -1- If T is a specialization reference_wrapper<X> for some type X, the member typedef type of unwrap_reference<T> is X&, otherwise it is T.

   template<class T>
     struct unwrap_ref_decay;
   

   -2- The member typedef type of unwrap_ref_decay<T> denotes the type unwrap_reference_t<decay_t<T>>.

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

   namespace std {
   […]
   // 21.3.8.7 [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 21.3.8.7 [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;	If T is a specialization reference_wrapper<X> for some type X, the member typedef type of unwrap_reference<T> is X&, otherwise it is T.
   template <class T> struct unwrap_ref_decay;	The member typedef type of unwrap_ref_decay<T> denotes the type unwrap_reference_t<decay_t<T>>.

5. Insert between 21.3.8.7 [meta.trans.other] p1 and p2 as indicated:

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