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Showing content from https://cplusplus.github.io/LWG/issue3177 below:

Limit permission to specialize variable templates to program-defined types

This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of C++23 status.

Section: 16.4.5.2.1 [namespace.std] Status: C++23 Submitter: Johel Ernesto Guerrero Peña Opened: 2018-12-11 Last modified: 2023-11-22

Priority: 3

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Discussion:

The permission denoted by [namespace.std]/3 should be limited to program-defined types.

[2018-12-21 Reflector prioritization]

Set Priority to 3

Previous resolution [SUPERSEDED]:

This wording is relative to N4791.

1. Change 16.4.5.2.1 [namespace.std] as indicated:

   -2- Unless explicitly prohibited, a program may add a template specialization for any standard library class template to namespace std provided that (a) the added declaration depends on at least one program-defined type and (b) the specialization meets the standard library requirements for the original template.(footnote 174)

   -3- The behavior of a C++ program is undefined if it declares an explicit or partial specialization of any standard library variable template, except where explicitly permitted by the specification of that variable template , provided that the added declaration depends on at least one program-defined type.

[2022-08-24; LWG telecon]

Each variable template that grants permission to specialize already states requirements more precisely than proposed here anyway. For example, disable_sized_range only allows it for cv-unqualified program-defined types. Adding less precise wording here wouldn't be an improvement. Add a note to make it clear we didn't just forget to say something here, and to remind us to state requirements for each variable template in future.

[2022-08-25; Jonathan Wakely provides improved wording]

[2022-09-28; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4910.

• Change 16.4.5.2.1 [namespace.std] as indicated:

  -2- Unless explicitly prohibited, a program may add a template specialization for any standard library class template to namespace std provided that (a) the added declaration depends on at least one program-defined type and (b) the specialization meets the standard library requirements for the original template.(footnote 163)

  -3- The behavior of a C++ program is undefined if it declares an explicit or partial specialization of any standard library variable template, except where explicitly permitted by the specification of that variable template.

  [Note 1: The requirements on an explicit or partial specialization are stated by each variable template that grants such permission. — end note]

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