There is probably a lack in the current [dcl.spec.auto.general] (§9.2.9.7.1)

I use the text from https://eel.is/c++draft/#dcl.dcl (generated on 9th of Nov, 2024).

Parts p3 and p4 read:

p3: A placeholder type can appear in the decl-specifier-seq for a function declarator that includes a trailing-return-type ([dcl.fct]).

p4: 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; the return type of the function is deduced from non-discarded return statements, if any, in the body of the function ([stmt.if]).

This is not much different compared to the original text I studied from draft n4849 (last before official C++20), where [decl.spec.auto]-p3 reads:

"
The placeholder type can appear with a function declarator in the decl-specifier-seq, type-specifier-seq, conversion-function-id, or trailing-return-type, in any context where such a declarator is valid. If the function declarator includes a trailing-return-type ([dcl.fct]), that trailing-return-type specifies the declared return type of the function. Otherwise, the function declarator shall declare a function. If the declared return type of the function contains a placeholder type, the return type of the function is deduced from non-discarded return statements, if any, in the body of the function ([stmt.if]).
"

The intent appears to force function declarators to declare a function in case a placeholder is used for returning type, otherwise it is not possible to deduce in case of definitions of function types, function pointer types, or function pointers.
However, it should be mentioned that if the trailing-return-type itself contains a placeholder type, then we should fall back to the constraint that a function has to be declared.

I see this lack in both the text of draft n4849 and the current draft text.
e.g.
auto (*p)() -> auto; // ill-formed (as all major compilers act)

A possible solution is modifying p3 as follows:
A placeholder type can appear in the decl-specifier-seq for a function declarator that includes a trailing-return-type ([dcl.fct]) that does not contain, in turn, another placeholder type.

On the other hand, current p1 recalls the fact that the trailing-return-type, if present, is the one that effectively replaces the declared returnd type, therefore the context cannot implicitly be without a placeholder for the trailing type.


As second point, I appreciate some redundancy was removed (compared to draft n4849), for example the fact that type-specifier-seq already includes the case of conversion-function-id.
However, current p1 is likely too specialized when it reads "... or as one of the type-specifiers in a trailing-return-type ...", as I guess this excludes the case of conversion function id,

unless I am wrong supposing that conversion function ids do not fall in the case of "... as one of the decl-specifiers in the decl-specifier-seq  ...".

Of coruse, conversion functions are still allowed to be auto() [class.conv.ftc] (§14.8.3-p3 & p8).


Last detail: In [class.conv.fct]-p3 there is "trailing return type" in commented code without dashes. In my pdf version of n4849, it is the only occurrence, as all others both in text and commented code for the examples, appear with dashes.