[Bug other/94165] New: Wrong "warning: this statement may fall through" after __builtin_unreachable().

[gjl at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94165

            Bug ID: 94165
           Summary: Wrong "warning: this statement may fall through" after
                    __builtin_unreachable().
           Product: gcc
           Version: 9.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: other
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gjl at gcc dot gnu.org
  Target Milestone: ---

Created attachment 48026
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48026&action=edit
C/C++ test case.

The compiler gcc, g++ incorrectly complain about fall-through case after
__builtin_unreachable().

Compiler test case with

$ g++ -c -W -Wall

w.c: In function ‘int func(int, int)’:
w.c:7:15: warning: this statement may fall through [-Wimplicit-fallthrough=]
     (__extension__({                            \
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
             int _r = error_at (msg);            \
             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
             __builtin_unreachable();            \
             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
             _r;                                 \
             ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
         }))
         ~~~    
w.c:15:9: note: in expansion of macro ‘error’
         error ("bug");
         ^~~~~
w.c:16:5: note: here
     case 1:     case 2:
     ^~~~

The only route to the fall-through is via __builtin_unreachable(), thus this
warning is wrong.