CWG Issue 2457 (original) (raw)

This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 117a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2025-04-13

2457. Unexpanded parameter packs don't make a function type dependent

Section: 13.8.3.2 [temp.dep.type]Status: CD6Submitter: Richard SmithDate: 2020-07-28

[Accepted at the November, 2020 meeting.]

Consider the following example:

template<typename ...T> auto f() { using F = int(*)(int (...p)[sizeof(sizeof(T))]); // ... }

F is not covered in the list of cases in 13.8.3.2 [temp.dep.type] paragraph 9, because the types from which the function type is constructed are not dependent types. (The parameter pack p is of typeint[sizeof(size_t)].) Similar situations arise with non-injective alias templates.

Proposed resolution (August, 2020):

Change 13.8.3.2 [temp.dep.type] paragraph 9 as follows:

A type is dependent if it is

• ...
• an array type whose element type is dependent or whose bound (if any) is value-dependent,
• a function type whose parameters include one or more function parameter packs,
• a function type whose exception specification is value-dependent,
• ...

(We do have the relevant wording for pack expansions in _simple-template-id_s in bullet 9.8, so that similar case is already handled.)