[Bug c/49120] New: bogus "value computed is not used" warning (variable-length array in compound statement)

[kari.nurmela at stonesoft dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49120

           Summary: bogus "value computed is not used" warning
                    (variable-length array in compound statement)
           Product: gcc
           Version: 4.5.1
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: kari.nurmela@stonesoft.com


When declaring a variable-length array in the beginning of a compound
statement, so that the length expression contains at least one operator,
the warning "value computed is not used" is incorrectly given for the computed
length of the array (with -Wall). For example:

~$ cat kok1.c
int main(void)
{
    int foo = 1;
    ({
        char bar[foo + 1]; /* <==== */
        bar[0] = 0;
    });
    return 0;
}
~$ gcc-4.5 -Wall kok1.c -o kok1
kok1.c: In function ‘main’:
kok1.c:5:22: warning: value computed is not used
~$ gcc-4.5 --version
gcc-4.5 (Ubuntu/Linaro 4.5.1-7ubuntu2) 4.5.1
...

If the compound statement is replaced with a block in this example, the warning
goes away:

~$ cat kok2.c
int main(void)
{
    int foo = 1;
    {
        char bar[foo + 1];
        bar[0] = 0;
    }
    return 0;
}
~$ gcc-4.5 -Wall kok2.c -o kok2
~$

In gcc 4.4 the bogus warning does not appear:

~$ gcc-4.4 -Wall kok1.c -o kok1
~$ gcc-4.4 --version
gcc-4.4 (Ubuntu/Linaro 4.4.4-14ubuntu5) 4.4.5
...