[Bug c/115516] New: constexpr use before C23 could give a better error message

[Bug c/115516] New: constexpr use before C23 could give a better error message

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115516

            Bug ID: 115516
           Summary: constexpr use before C23 could give a better error
                    message
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: enhancement
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---

Take:
```
constexpr int j = 1;
```

Currently we give:
```
<source>:1:10: error: expected ';' before 'int'
    1 | constexpr int j = 1;
      |          ^~~~
      |          ;
```

This could give a better error message just like the C++ front-end does for
C++98:
```
<source>:1:1: error: 'constexpr' does not name a type
    1 | constexpr int j = 1;
      | ^~~~~~~~~
<source>:1:1: note: C++11 'constexpr' only available with '-std=c++11' or
'-std=gnu++11'
```

[Bug c/115516] constexpr use before C23 could give a better error message

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115516

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2024-06-18
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.

[Bug c/115516] constexpr use before C23 could give a better error message

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115516

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
It will be hard, because constexpr before C++11 is not a keyword.
And it can't be, int constexpr = 1; is completely valid C++98.

[Bug c/115516] constexpr use before C23 could give a better error message

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115516

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Sorry, this was about C, not C++.  And apparently C++ manages to warn about
that in
cp_parser_diagnose_invalid_type_name through
if (cxx_dialect < cxx11 && id == ridpointers[(int)RID_CONSTEXPR])
check.