[Bug c++/96120] New: Ambiguity diagnostic message of "invalid use of type 'void' in parameter declaration"

[Bug c++/96120] New: Ambiguity diagnostic message of "invalid use of type 'void' in parameter declaration"

[haoxintu at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96120

            Bug ID: 96120
           Summary: Ambiguity diagnostic message of "invalid use of type
                    'void' in parameter declaration"
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: haoxintu at gmail dot com
  Target Milestone: ---

Hi, all.

This code, test.cc, GCC might emit the ambiguity diagnostic message on it.

$cat test.cc
void foo1 (void) {}
void foo2 (void, int){}

$g++ -c test.cc
test.cc:2:12: error: invalid use of type ‘void’ in parameter declaration
    2 | void foo2 (void, int){}
      |            ^~~~

Apparently, "void" can be used in parameter declaration like line 1, but GCC
emits the ambiguity message on it. The output information conflicts with valid
grammar, so I guess users might be confused with this diagnostic.

I think clang handles better in this case

$clang++ -c test.cc
test.cc:2:11: error: 'void' must be the first and only parameter if specified
void foo2 (void, int){}
          ^
1 error generated.

Thanks,
Haoxin

[Bug c++/96120] Ambiguity diagnostic message of "invalid use of type 'void' in parameter declaration"

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96120

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Haoxin Tu from comment #0)
> GCC might emit the ambiguity diagnostic message on it.

What does that mean?

[Bug c++/96120] Ambiguity diagnostic message of "invalid use of type 'void' in parameter declaration"

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96120

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
GCC's diagnostic seems fine to me. Using 'void' as the type of a parameter is
invalid. There's a special case for (void) but that isn't relevant to your
declaration, which has two parameters.

[Bug c++/96120] Ambiguity diagnostic message of "invalid use of type 'void' in parameter declaration"

[haoxintu at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96120

--- Comment #3 from Haoxin Tu <haoxintu at gmail dot com> ---
(In reply to Jonathan Wakely from comment #1)
> (In reply to Haoxin Tu from comment #0)
> > GCC might emit the ambiguity diagnostic message on it.
> 
> What does that mean?

Apologize for my expression. I mean the meaning of the error message "invalid
use of type ‘void’ in parameter declaration" is opposite with the valid
grammar(it should be allowed in the first parameter).

This might be not a bug, might be just an improvement?

Thanks.

[Bug c++/96120] Ambiguity diagnostic message of "invalid use of type 'void' in parameter declaration"

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96120

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Haoxin Tu from comment #3)
> Apologize for my expression. I mean the meaning of the error message
> "invalid use of type ‘void’ in parameter declaration" is opposite with the
> valid grammar(it should be allowed in the first parameter).

No, it's not the opposite at all.

void is only allowed when there is a single, unnamed non-dependent parameter of
type void. Any other use is void is invalid, including the case where you have
(void, int). And so GCC's error message is absolutely correct. Using void in a
parameter list with more than one parameter is invalid.