[Bug c++/98031] New: missing the error message of undeclared label

[Bug c++/98031] New: missing the error message of undeclared label

[tangyixuan at mail dot dlut.edu.cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98031

            Bug ID: 98031
           Summary: missing the error message of undeclared label
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tangyixuan at mail dot dlut.edu.cn
  Target Milestone: ---

Hi, the following code has two errors. However, gcc only reports the second
error while ignoring the first one.

$ cat s.cpp
template<class T>
int fun(T i)
{
  static void* labs[2] = { &&lab1, &&lab2 };
  goto *(labs[i==0]);
    lab1:   return 1;
    labs:   return 2;      //should report an error
    return 0;
}

int main()
{  return lab1(1); }       //error here

$ g++ -c s.cpp
s.cpp: In function ‘int main()’:
s.cpp:12:11: error: ‘lab1’ was not declared in this scope
   12 | {  return lab1(1); }
      |   


$ clang++ -c s.cpp
s.cpp:4:38: error: use of undeclared label 'lab2'
  static void* labs[2] = { &&lab1, &&lab2 };
                                     ^
s.cpp:12:11: error: use of undeclared identifier 'lab1'
{  return lab1(1); }
          ^
2 errors generated.

[Bug c++/98031] missing the error message of undeclared label

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98031

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

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

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
This is again diagnostics on uninstantiated template, invalid, no diagnostics
required.
Instantiate the template and it will be diagnosed.

[Bug c++/98031] missing the error message of undeclared label

[tangyixuan at mail dot dlut.edu.cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98031

--- Comment #2 from tangyixuan <tangyixuan at mail dot dlut.edu.cn> ---
(In reply to Jakub Jelinek from comment #1)
> This is again diagnostics on uninstantiated template, invalid, no
> diagnostics required.
> Instantiate the template and it will be diagnosed.

Yeah, I agree with you. If we correct the second error (i.e., lab1(1) ->
fun(1)), there is a error diagnostic for the first error as below:
s.cpp:4:36: error: label ‘lab2’ used but not defined
    4 |   static void* labs[2] = { &&lab1, &&lab2 };
      |                                    ^~~~~~

The convenient thing is that if the compiler reports the two programming
mistakes (real errors) at one compilation time could be better, so as to save
the developers' time I guess.

[Bug c++/98031] missing the error message of undeclared label

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98031

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
The compiler just can't try to instantiate random templates it would think the
user may wanted to instantiate but didn't.