Re: gcc 13.2 is missing warnings?

[Jakub Jelinek <jakub@redhat.com>]

On Thu, Oct 19, 2023 at 07:39:43AM -0400, Eric Sokolowsky via Gcc wrote:
> I am using gcc 13.2 on Fedora 38. Consider the following program.
> 
> #include <stdio.h>
> int main(int argc, char **argv)
> {
>     printf("Enter a number: ");
>     int num = 0;
>     scanf("%d", &num);
> 
>     switch (num)
>     {
>     case 1:
>         int a = num + 3;
>         printf("The new number is %d.\n", a);
>         break;
>     case 2:
>         int b = num - 4;
>         printf("The new number is %d.\n", b);
>         break;
>     default:
>         int c = num * 3;
>         printf("The new number is %d.\n", c);
>         break;
>     }
> }
> 
> I would expect that gcc would complain about the declaration of
> variables (a, b, and c) within the case statements. When I run "gcc
> -Wall t.c" I get no warnings. When I run "g++ -Wall t.c" I get
> warnings and errors as expected. I do get warnings when using MinGW on
> Windows (gcc version 6.3 specifically). Did something change in 13.2?

C isn't C++.

In particular, the above is valid C23, which is why it is accepted as an
extension in older C language versions starting with GCC 11.
It is warned about with -pedantic/-Wpedantic and errored on with
-pedantic-errors/-Werror=pedantic unless -std=c2x or -std=gnu2x is used.

The C++ case is completely different.  There labels are allowed before
declarations already in C++98, but it is invalid to cross initialization
of some variable using the jump to case 2 or default labels above.
If you rewrite it as:
     case 1:
         int a;
	 a = num + 3;
         printf("The new number is %d.\n", a);
         break;
     case 2:
         int b;
	 b = num - 4;
         printf("The new number is %d.\n", b);
         break;
     default:
         int c;
	 c = num * 3;
         printf("The new number is %d.\n", c);
         break;
it is valid C++ and it won't be diagnosed.

Note, this should have been posted to gcc-help instead.

	Jakub