[Bug c/52635] New: gcc fails to diagnose invalid type in unused sizeof() when optimizing

["mikpe at it dot uu.se" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52635

             Bug #: 52635
           Summary: gcc fails to diagnose invalid type in unused sizeof()
                    when optimizing
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: mikpe@it.uu.se


Created attachment 26930
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=26930
test case

> cat bug.c
extern int bar;
void foo(void)
{
    (void)sizeof(char[1 - 2 * !__builtin_constant_p(bar)]);
}
> objdir/gcc/xgcc -Bobjdir/gcc -O1 -S bug.c
> objdir/gcc/xgcc -Bobjdir/gcc -O0 -S bug.c
bug.c: In function 'foo':
bug.c:4:5: error: size of unnamed array is negative

The test case deliberately contains an invalid type expression, but gcc only
diagnoses it at -O0, at -O1 and above there is no error or warning.

Reproduced on x86_64-linux with gcc 4.8-20120318, 4.7.0-RC-20120314, every
4.x.latest release, 3.3.6, and 3.2.3.  gcc-3.4.6 fails to diagnose also at -O0.

The test case is based on the Linux kernel's BUILD_BUG_ON() macro.