[Bug c/79927] New: 5.4.0: exponential notation triggers bogus "variably modified" warning

[dtikhonov at litespeedtech dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79927

            Bug ID: 79927
           Summary: 5.4.0: exponential notation triggers bogus "variably
                    modified" warning
           Product: gcc
           Version: 5.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dtikhonov at litespeedtech dot com
  Target Milestone: ---

I use a compile-time check to make sure that a particular value is correct:

------ 8< ----------
struct my_struct {
    unsigned char   n_usecs[3];
};

#ifndef ONE_MILLION
#   define ONE_MILLION 1e6
#endif

/* Compile-time check that n_usecs holds at least 16 seconds worth. */
typedef char holds_at_least_16_seconds[
    ((1 << (sizeof(((struct my_struct *) 0)->n_usecs) * 8)) / ONE_MILLION
                                                                >= 16) - 1];

int
main (void)
{
    return 0;
}
------ 8< ----------

Using 1e6 instead of 1000000 produces the following warning:

1e6.c:15:65: warning: variably modified ‘holds_at_least_16_seconds’ at file
scope
                                                                 >= 16) - 1];
                                                                 ^

I reproduced this using 5.4.0 and 4.9.4 versions of gcc.