On Wed, Nov 20, 2024 at 2:24 PM mauro russo via Std-Discussion <
std-discussion_at_[hidden]> wrote:

>
> 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*
> <https://eel.is/c++draft/dcl.spec.general#nt:decl-specifier-seq> for a function
> declarator that includes a *trailing-return-type*
> <https://eel.is/c++draft/dcl.decl.general#nt:trailing-return-type> ([dcl.
> fct] <https://eel.is/c++draft/dcl.fct>).
> <https://eel.is/c++draft/dcl.spec.auto#general-3.sentence-1>
>
> p4: A placeholder type can appear in the *decl-specifier-seq*
> <https://eel.is/c++draft/dcl.spec.general#nt:decl-specifier-seq> or
> *type-specifier-seq*
> <https://eel.is/c++draft/dcl.type.general#nt: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]
> <https://eel.is/c++draft/stmt.if>).
> <https://eel.is/c++draft/dcl.spec.auto#general-4.sentence-1>
>
>
> 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)
>

For this example we have to check whether both `auto`s are legal.

The first one is legal under p3. The second one is not legal: it appears in
the *type-specifier-seq* of the trailing return type, but the declarator in
this case doesn't declare a function. So, the entire declaration is
ill-formed.


>
> A possible solution is modifying p3 as follows:
> A placeholder type can appear in the *decl-specifier-seq*
> <https://eel.is/c++draft/dcl.spec.general#nt:decl-specifier-seq> for a function
> declarator that includes a *trailing-return-type*
> <https://eel.is/c++draft/dcl.decl.general#nt:trailing-return-type> ([dcl.
> fct] <https://eel.is/c++draft/dcl.fct>) that does not contain, in turn,
> another placeholder type.
> <https://eel.is/c++draft/dcl.spec.auto#general-3.sentence-1>
>
> 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-specifier*
> <https://eel.is/c++draft/dcl.type.general#nt:type-specifier>*s* in a
> *trailing-return-type*
> <https://eel.is/c++draft/dcl.decl.general#nt: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-specifier*
> <https://eel.is/c++draft/dcl.spec.general#nt:decl-specifier>*s* in the
> *decl-specifier-seq*
> <https://eel.is/c++draft/dcl.spec.general#nt: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.
>
>
>
>
>
>
>
>
>
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-- 
*Brian Bi*