Deduction guides using abbreviated function syntax
Section [temp.deduct.guide]

Created on 2023-02-11.00:00:00 last changed 2 months ago


Date: 2023-05-12.20:29:43

Proposed resolution (approved by CWG 2023-05-12):

Change in [temp.deduct.guide] paragraph 3 as follows:

The same restrictions apply to the parameter-declaration-clause of a deduction guide as in a function declaration ( [dcl.fct]), except that a generic parameter type placeholder ( [dcl.spec.auto]) shall not appear in the parameter-declaration-clause of a deduction guide. The simple-template-id shall name a class template specialization. The template-name shall be the same identifier as the template-name of the simple-template-id. A deduction-guide shall inhabit the scope to which the corresponding class template belongs and, for a member class template, have the same access. Two deduction guide declarations for the same class template shall not have equivalent parameter-declaration-clauses if either is reachable from the other.
Date: 2023-05-12.20:29:43

CWG 2023-02-11

CWG solicits input from EWG whether abbreviated function syntax is intended to be used for deduction guides. See cplusplus/papers#1465.

EWG 2023-05-11

CWG should clarify that abbreviated function syntax should not be permitted in deduction guides.

Date: 2023-06-15.00:00:00

[Accepted as a DR at the June, 2023 meeting.]

It is unclear whether deduction guides can be expressed using abbreviated function syntax. Subclause [temp.deduct.guide] paragraph 3 refers to the restrictions of a function's parameter-declaration-clause:

The same restrictions apply to the parameter-declaration-clause of a deduction guide as in a function declaration ( [dcl.fct]). ...

However, that subclause is silent on the meaning of abbreviated function syntax when used for deduction guides. Furthermore, [dcl.fct] paragraph 22 explicitly restricts the definition to function templates, which deduction guides are not:

An abbreviated function template is a function declaration that has one or more generic parameter type placeholders ( [dcl.spec.auto]). ...

Arguably, the lack of template parameter names in abbreviated function syntax makes it less suitable to specifiy deduction guides.

Date User Action Args
2023-07-16 13:00:43adminsetstatus: ready -> dr
2023-05-12 20:29:43adminsetmessages: + msg7276
2023-05-12 20:29:43adminsetstatus: review -> ready
2023-05-11 20:09:57adminsetstatus: open -> review
2023-02-12 17:00:44adminsetmessages: + msg7197
2023-02-11 00:00:00admincreate