https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61864
--- Comment #4 from Eric Gallager <egall at gwmail dot gwu.edu> ---
Actually, after giving this some more thought, and reading the GCC
documentation some more, I came up with some ideas for a compromise that could
allow both -Wswitch-default and -Wcovered-switch-default to be used without
conflicting:
* Seeing as -Wcovered-switch-default (as clang implements it) makes certain
assumptions about the nature of enums, its implementation in GCC should be
restricted to only being active when the -fstrict-enums flag is also in use. As
-fstrict-enums is a C++-only flag, this would effectively make
-Wcovered-switch-default a C++-only warning, which would allow C-only
programmers (such as myself) to avoid the flag.
* Besides general philosophical issues with the -Wcovered-switch-default flag,
there's also two specific implementation issues I have with clang's
implementation of it that I would want to see GCC fix in its own
implementation. I wrote a quick source file to demonstrate:
$ cat switches.c
#include <stdlib.h> /* for rand() */
int main(void)
{
int swindex0 = rand();
enum values {
FIRST_ENUMERATOR,
SECOND_ENUMERATOR,
THIRD_ENUMERATOR
} swindex1 = SECOND_ENUMERATOR; /* silence unitialized warnings */
/* do the non-enumerator switch first: */
switch (swindex0) {
case 1:
swindex1 = FIRST_ENUMERATOR;
break;
case 2:
swindex1 = SECOND_ENUMERATOR;
break;
case 3:
swindex1 = THIRD_ENUMERATOR;
break;
/* -Wswitch-default: usual warning about switch with no default */
}
if (swindex0 > 3) {
swindex1 = THIRD_ENUMERATOR;
} else if (swindex0 < 1) {
swindex1 = FIRST_ENUMERATOR;
}
switch (swindex1) {
case FIRST_ENUMERATOR:
break;
case SECOND_ENUMERATOR:
break;
case THIRD_ENUMERATOR:
break;
/* -Wcovered-switch-default: print a warning here like clang's,
* but only if also used with -fstrict-enums: */
default:
break;
}
switch (swindex1) {
case FIRST_ENUMERATOR:
break;
case SECOND_ENUMERATOR:
break;
case THIRD_ENUMERATOR:
break;
/* -Wswitch-default: still warns:
* * if by itself, the usual message, as in the first switch.
* * if with -Wcovered-switch-default, use a different message,
* like:
* "Warning: switch missing default case which would be covered if it
had one"
* "Note: use -fstrict-enums to silence this warning"
* * if with -fstrict-enums, do like mentioned in the previous,
* and print no warning. */
}
switch (swindex1) {
case FIRST_ENUMERATOR:
break;
case SECOND_ENUMERATOR:
break;
case THIRD_ENUMERATOR:
break;
/* Ideally there should be no warning here whatsoever; clang still
* warns about it though: */
default: __builtin_unreachable(); /*NOTREACHED*/
}
switch (swindex1) {
case FIRST_ENUMERATOR:
break;
case SECOND_ENUMERATOR:
break;
case THIRD_ENUMERATOR: /*FALLTHROUGH*/
/* Ideally there should be no warning from -Wcovered-switch-default
* here, because even though there are labels for all enum values,
* the 3rd case still falls through to the default case: */
default: /*REACHEDBYFALLTHROUGH*/
break;
/* (clang still warns here though) */
}
}
$ clang --version && clang -c -Weverything switches.c
clang version 3.6.0 (tags/RELEASE_360/final)
Target: x86_64-apple-darwin10.8.0
Thread model: posix
switches.c:38:9: warning: default label in switch which covers all enumeration
values [-Wcovered-switch-default]
default:
^
switches.c:66:9: warning: default label in switch which covers all enumeration
values [-Wcovered-switch-default]
default: __builtin_unreachable(); /*NOTREACHED*/
^
switches.c:77:9: warning: default label in switch which covers all enumeration
values [-Wcovered-switch-default]
default: /*REACHEDBYFALLTHROUGH*/
^
3 warnings generated.
$ g++-6_git --version && g++-6_git -c -fstrict-enums -Wall -Wextra -Wswitch
-Wswitch-default -Wswitch-enum switches.c
g++-6_git (GCC) 6.0.0 20150604 (experimental)
Copyright (C) 2015 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.
switches.c: In function 'int main()':
switches.c:11:12: warning: switch missing default case [-Wswitch-default]
switch (swindex0) {
^
switches.c:41:12: warning: switch missing default case [-Wswitch-default]
switch (swindex1) {
^
I'll add the file as an attachment, too.
