This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 117b. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
2025-08-12
(See also submission #736.)
The current wording allows something like
struct S {
operator auto() { return 0; }
};
If it is intended to be permitted, the details of its handling are not clear. Also, a similar syntax has been discussed as a possible future extension for dealing with proxy types in deduction which, if adopted, could cause confusion.
Additional note, November, 2013:
Doubt was expressed during the 2013-11-25 drafting review teleconference as to the usefulness of this provision. It is therefore being left open for further consideration after C++14 is finalized.
Notes from the February, 2014 meeting:
CWG continued to express doubt as to the usefulness of this construct but felt that if it is permitted, the rules need clarification.
Additional note (December, 2021):
See duplicate issue 2493 for additional details.
Additional note (August, 2025):
CWG appears to be leaning towards making declarations of operator auto ill-formed. Forwarding to EWG for approval of that direction via paper issue #2409, by decision of the CWG chair.
Possible resolution:
Change in 9.2.9.7.1 [dcl.spec.auto.general] paragraph 4 as follows:
A placeholder type can appear in the decl-specifier-seq or type-specifier-seq in the declared return type of a function declarator that declares a function other than a conversion function (11.4.8.3 [class.conv.fct]); the return type of the function is deduced from non-discarded return statements, if any, in the body of the function (8.5.2 [stmt.if]).