This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 110b. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2023-01-15


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

Section: 13.8.3.2  [temp.dep.type]     Status: CD6     Submitter: Richard Smith     Date: 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 type int[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

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