BUG: assuming signed overflow does not occur when simplifying conditional to constant

[Bruce Korb]

I wrote a loop that figures out how many items are in a list,
counts down from that count to -1, changes direction and counts
up from 0 to the limit, a la:


    inc  = -1;
    int idx = 0;
    while (opts->papzHomeList[idx+1] != NULL)
        idx++;
    for (;;) {
        if (idx < 0) {  <<<=== line 1025
            inc = 1;
            idx = 0;
        }
        char const * path = opts->papzHomeList[ idx ];
        if (path == NULL)
            break;
        idx += inc;
        //  do a bunch of stuff
    }

../../autoopts/configfile.c: In function 'intern_file_load':
../../autoopts/configfile.c:1025:12: error: assuming signed overflow does not occur \
    when simplifying conditional to constant [-Werror=strict-overflow]

I do not think GCC should be "simplifying" the expression "idx < 0" to a constant.
The number of entries in "papzHomeList" is known to be less than INT_MAX (64K actually),
but not by the code in question.  My guess is that some code somewhere presumes
that "idx" never gets decremented.  Not true.