[class.conv.fct] Conversion function templates with deduced return type

[eisenwave]

Full name of submitter: Brian Bi

Reference (section label): [class.conv.fct]

See also cplusplus/draft#8490 for discussion.

Issue description

Following CWG1670, the ban of conversion function templates with deduced return type in [class.conv.fct] paragraph 8 has become redundant, except that removing this paragraph would make such uses of placeholder type specifiers IFNDR, not ill-formed.

Suggested resolution

Delete [class.conv.fct] paragraph 8:

-A conversion function template shall not have a deduced return type ([dcl.spec.auto]).
-[Example:
-struct S {
-  operator auto() const { return 10; }      // OK
-  template<class T>
-  operator auto() const { return 1.2; }     // error: conversion function template
-};
-— end example]

Change [dcl.spec.auto.general] paragraph 4 as follows: SUPERSEDED, SEE BELOW

 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
+either
 a function other than a conversion function\iref{class.conv.fct}
+or a conversion function template
 ;
 ...

[t3nsor]

I parse this as "either (a function other than a conversion function) or (a conversion function template)", meaning that the suggested wording would allow a templated operator auto.

[eisenwave]

Actually yeah, the current suggestion is plain wrong, and I'm getting the impression that there should be a new paragraph that says you shall not use a placeholder type in a conversion function template. We have plenty of those in [dcl.spec.auto.general] and it's a bit weird that [class.conv.fct] imposes a semantic requirement related to deduced return types instead of just talking about "this is syntactically not allowed" like we usually do.

Maybe

+A conversion function template shall not be declared
+with a return type that uses a placeholder type.

We could also attach something to https://eel.is/c++draft/dcl.spec.auto.general#16:

 If a function or function template F has a declared return type
 that uses a placeholder type,
 redeclarations or specializations of F shall use that placeholder type,
 not a deduced type;
 otherwise, they shall not use a placeholder type.
+Furthermore, F shall not be a conversion function
+or conversion function template.