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

2954. Specialization of the convenience variable templates should be prohibited

Section: 15.5.4 [constraints] Status: WP Submitter: Tim Song Opened: 2017-03-31 Last modified: 2017-07-30

Priority: Not Prioritized

View all issues with WP status.

Discussion:

There's currently no rule against specializing the various _v convenience variable templates outside of <type_traits>.

There should be; foo_v<T> should be always equal to foo<T>::value. The correct way to influence, say, is_placeholder_v<T> should be to specialize is_placeholder, not is_placeholder_v. Otherwise, the editorial changes to use the _v form to the specification would no longer be editorial but have normative impact.

Adding a global prohibition in 15.5.4.2.1 [namespace.std] seems preferable to adding individual prohibitions to each affected template; the PR below carves out an exception for variable templates that are intended to be specialized by users. As far as I know there are no such templates in the current WP, but the Ranges TS does use them.

[2017-06-14, Moved to Tentatively Ready after 6 positive votes on c++std-lib]

Proposed resolution:

This wording is relative to N4659.

  1. Add a paragraph to 15.5.4.2.1 [namespace.std], before p2:

    -1- The behavior of a C++ program is undefined if it adds declarations or definitions to namespace std or to a namespace within namespace std unless otherwise specified. A program may add a template specialization for any standard library template to namespace std only if the declaration depends on a user-defined type and the specialization meets the standard library requirements for the original template and is not explicitly prohibited.(footnote: […])

    -?- 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.

    -2- The behavior of a C++ program is undefined if it declares […]