4152. The primary template of std::char_traits is totally underspecified

Section: 27.2.2 [char.traits.require] Status: New Submitter: Jiang An Opened: 2024-09-08 Last modified: 2025-02-07

Priority: 4

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Discussion:

Currently, only explicit specializations of std::char_traits are specified in the standard. Nothing is specified for the primary template except that it's not marked "not defined".

Given some implementations provide the definition of the primary template (perhaps as an extension), it might be better to explicitly say that instantiation of the primary template has unspecified effect. Also, it seems outdated to say "explicit specializations" as partial specializations are possible since C++20.

[2025-02-07; Reflector poll]

Set priority to 4 after reflector poll.

27.2.1 [char.traits.general]/1 says that the subclause "defines a class template char_traits<charT>", but we never provided a definition.

"Libc++ started to warn that the primary template is deprecated and will be removed. The proposed resolution doesn't improve matters though."

"The definition should be cross-platform. Unspecified doesn't help."

Proposed resolution:

This wording is relative to N4988.

1. Modify 27.2.2 [char.traits.require] as indicated:

   -2- The class template

   template<class charT> struct char_traits;
   

   is provided in the header <string> as a basis for explicit and partial specializations. The effect of instantiating the primary template of char_traits is unspecified and possibly makes the program ill-formed.