This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 112c. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
[Accepted as a DR at the June, 2023 meeting.]
It is unclear whether deduction guides can be expressed using abbreviated function syntax. Subclause 184.108.40.206 [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 (220.127.116.11 [dcl.fct]). ...
However, that subclause is silent on the meaning of abbreviated function syntax when used for deduction guides. Furthermore, 18.104.22.168 [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 (22.214.171.124 [dcl.spec.auto]). ...
Arguably, the lack of template parameter names in abbreviated function syntax makes it less suitable to specifiy deduction guides.
CWG solicits input from EWG whether abbreviated function syntax is intended to be used for deduction guides. See cplusplus/papers#1465.
CWG should clarify that abbreviated function syntax should not be permitted in deduction guides.
Proposed resolution (approved by CWG 2023-05-12):
Change in 126.96.36.199 [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 (188.8.131.52 [dcl.fct]). 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.