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2838. is_literal_type specification needs a little cleanup

Section: D.19 [depr.meta.types] Status: C++17 Submitter: Tim Song Opened: 2016-12-09 Last modified: 2020-09-06

Priority: 0

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Discussion:

D.19 [depr.meta.types]/3 currently reads:

Effects: is_literal_type has a base-characteristic of true_type if T is a literal type ([basic.types]), and a base-characteristic of false_type otherwise.

First, this doesn't seem like an Effects clause. Second, this wording fails to say that is_literal_type is an UnaryTypeTrait, and misspells BaseCharacteristic — which is only defined for UnaryTypeTraits and BinaryTypeTraits. Third, moving this to Annex D means that the general prohibition against specializing type traits in [meta.type.synop]/1 no longer applies, which is presumably unintended.

[2017-01-27 Telecon]

Priority 0

Proposed resolution:

This wording is relative to N4618.

1. Edit D.19 [depr.meta.types] as indicated:

   The header <type_traits> has the following addition:

   namespace std {
     template <class T> struct is_literal_type;
     
     template <class T> constexpr bool is_literal_type_v = is_literal_type<T>::value;
   }
   

   -2- Requires: remove_all_extents_t<T> shall be a complete type or (possibly cv-qualified) void.

   -3- Effects: is_literal_type has a base-characteristic of true_type if T is a literal type (3.9), and a basecharacteristic of false_type otherwiseis_literal_type<T> is a UnaryTypeTrait (21.3.2 [meta.rqmts]) with a BaseCharacteristic of true_type if T is a literal type (6.8 [basic.types]), and false_type otherwise.

   -?- The behavior of a program that adds specializations for is_literal_type or is_literal_type_v is undefined.