See the testcase.  bar starts as -1, but should be 42 after calling foo(), even
though its const.  You could argue that bar shouldn't be const since we are
modifying it, but even so, the behavior should not be different at -O0 than -O1/2.

# gcc quux.c -O0 -o quux
# ./quux
Success: bar is 42

# gcc quux.c -O2 -o quux
# ./quux
Error: bar is not 42!  (it is -1)

Testcase:

#include <stdarg.h>
#include <stdio.h>

void foo (int n_args, ...)
{
    va_list qux;
    int *bar;

    va_start (qux, n_args);
    bar = va_arg (qux, int *);
    *bar = 42;
    va_end(qux);
}

int main (int argc, char **argv)
{
    const int bar = -1;

    foo (1, &bar);
    if (bar != 42)
        printf ("Error: bar is not 42!  (it is %d)\n", bar);
    else
        printf ("Success: bar is 42\n");
    return 0;
}