[Bug middle-end/77606] abort in __memcpy_chk on an in-bounds copy with type-2 builtin_object_size

["msebor at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77606

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
The following test case reduces the problem to __builtin_object_size failing
for pointers to objects that are the result of a __builtin_malloc call (or VLA
definition) with an argument that's a runtime conditional expression.

$ cat z.c && /build/gcc-trunk-git/gcc/xgcc -B /build/gcc-trunk-git/gcc -O2
-Wall -Wextra -Wpedantic z.c && ./a.out 
#define P(x) \
    __builtin_printf ("%2zd %2zd %2zd %2zd\n", \
                      __builtin_object_size (x, 0), \
                      __builtin_object_size (x, 1), \
                      __builtin_object_size (x, 2), \
                      __builtin_object_size (x, 3)) \

int main (int argc, char *argv[])
{
  (void)argv;

  {
    char b [1 < argc ? 13 : 17 ];
    P (b);
  }

  {
    char a [13];
    char b [17];
    char *p = 1 < argc ? a : b;
    P (p);
  }

  {
    char *p = 1 < argc ? __builtin_malloc (13) : __builtin_malloc (17);
    P (p);
  }

  {
    char *p = __builtin_malloc (1 < argc ? 13 : 17);
    P (p);
  }
}
-1 -1  0  0
17 17 13 13
17 17 13 13
-1 -1  0  0