[Bug c/110839] New: Bogus -Wreturn-type for unreachable switch default case

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110839

            Bug ID: 110839
           Summary: Bogus -Wreturn-type for unreachable switch default
                    case
           Product: gcc
           Version: 13.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rguenth at gcc dot gnu.org
  Target Milestone: ---

unsigned char random_ui8(void);
int f16RandomQOutP3 (void);
int f16RandomQOutPInf (void);
int f16RandomQInfP3 (void);
int f16RandomQInfPInf (void);
int f16Random( void )
{   

    switch ( random_ui8() & 7 ) {
     case 0:
     case 1:
     case 2:
        return f16RandomQOutP3();
     case 3:
        return f16RandomQOutPInf();
     case 4:
     case 5:
     case 6:
        return f16RandomQInfP3();
     case 7:
        return f16RandomQInfPInf();
    }
}

diagnoses

gcc -S t.c -O2 -Wreturn-type
t.c: In function 'f16Random':
t.c:23:1: warning: control reaches end of non-void function [-Wreturn-type]
   23 | }
      | ^

this is because pass_warn_function_return runs after lowering before any
optimization is done and the gimplifier adds a default: case that
falls through to the function end.