This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 116a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
2024-12-19
[Accepted as a DR at the June, 2023 meeting.]
It is unclear whether deduction guides can be expressed using abbreviated function syntax. Subclause 13.7.2.3 [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 (9.3.4.6 [dcl.fct]). ...
However, that subclause is silent on the meaning of abbreviated function syntax when used for deduction guides. Furthermore, 9.3.4.6 [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 (9.2.9.7 [dcl.spec.auto]). ...
Arguably, the lack of template parameter names in abbreviated function syntax makes it less suitable to specifiy deduction guides.
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.
Proposed resolution (approved by CWG 2023-05-12):
Change in 13.7.2.3 [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 (9.3.4.6 [dcl.fct]), except that a generic parameter type placeholder (9.2.9.7 [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.