[Bug middle-end/88991] New: missing warning on a strcpy and strlen from a zero-length array

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88991

            Bug ID: 88991
           Summary: missing warning on a strcpy and strlen from a
                    zero-length array
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

GCC diagnoses the undefined behavior in function f() in the test case below but
fails to diagnose the same bug in g() or h().

However, the warning issued for f() should be (also) for the call to strlen(s)
which is also undefined.  Worse, in g() neither the call to strlen(s) nor
memcpy() is diagnosed.

$ cat t.c && gcc -O2 -S -Wall t.c

void f (char *d)
{
  __builtin_memcpy (d, s, __builtin_strlen (s) + 1);
}

void g (char *d)
{
  unsigned n = __builtin_strlen (s) + 1;   // missing warning
  __builtin_memcpy (d, s, n);              // same here
}

void h (char *d)
{
  __builtin_strcpy (d, s);                 // missing warning here too
}
t.c: In function ‘f’:
t.c:5:3: warning: ‘__builtin_memcpy’ forming offset [1, 9223372036854775805] is
out of the bounds [0, 0] of object ‘s’ with type ‘const char[]’
[-Warray-bounds]
    5 |   __builtin_memcpy (d, s, __builtin_strlen (s) + 1);
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
t.c:1:12: note: ‘s’ declared here
    1 | const char s[0] = { };
      |            ^