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


2608. Omitting an empty template argument list

Section: 13.10.2  [temp.arg.explicit]     Status: CD6     Submitter: Anoop Rana     Date: 2022-07-03

[Accepted at the July, 2022 meeting.]

Subclause 13.10.2 [temp.arg.explicit] paragraph 4 specifies:

Trailing template arguments that can be deduced (13.10.3 [temp.deduct]) or obtained from default template-arguments may be omitted from the list of explicit template-arguments.
[Note 1: A trailing template parameter pack (13.7.4 [temp.variadic]) not otherwise deduced will be deduced as an empty sequence of template arguments. —end note]
If all of the template arguments can be deduced, they may all be omitted; in this case, the empty template argument list <> itself may also be omitted.

The wording does not allow omitting the empty template argument list <> if all of the template arguments have been obtained from default template-arguments. For example:

  template<typename T = int>
  int f();

  int x = f();      // ill-formed per the wording
  int (*y)() = f;   // ditto

Proposed resolution (approved by CWG 2022-07-15):

Change in 13.10.2 [temp.arg.explicit] paragraph 4 as follows:

... If all of the template arguments can be deduced or obtained from default template-arguments, they may all be omitted; in this case, the empty template argument list <> itself may also be omitted.