[Bug c/125346] New: break of named loop suppresses -Wimplicit-fallthrough

[rasheeqhere at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125346

            Bug ID: 125346
           Summary: break of named loop suppresses -Wimplicit-fallthrough
           Product: gcc
           Version: 16.1.0
            Status: UNCONFIRMED
          Keywords: diagnostic, false-negative
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rasheeqhere at gmail dot com
  Target Milestone: ---

The following does not generate a warning:

$ cat >main.c <<"EOF"
extern int  decide_something();
extern void     do_something();
int main() {
  loop: while(true) switch(decide_something()) {
  case 42:
    do_something();
    // should warn here! 
  case -1:
    break loop;
  }
}
EOF

$ gcc -std=c2y -Wimplicit-fallthrough=5 -c main.c
# no output

$ uname -mo; gcc --version
x86_64 GNU/Linux
gcc (GCC) 16.1.0
Copyright (C) 2026 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

The behavior is the same on 15.2 and on trunk (07aa3ec). Compiler Explorer:
https://godbolt.org/z/aMqTvqrEq

This is probably the same behavior as in 91432: GCC doesn't want to warn if a
case falls through to a case that immediately does break;, because

  break;
case -1:
  break;

is the same as

  [[fallthrough]];
case -1:
  break;

But now, with named loops, there really is a difference between

  break;
case -1:
  break loop;

and

  [[fallthrough]];
case -1:
  break loop;

So there should be a warning if [[fallthrough]] is missing in the latter case.