GCC silently accepts array declarations with string initializers that contain an extra element (the terminating nul) that doesn't fit into the array, and also silently allows such arrays to be used as arguments to functions that require string arguments.

It would be helpful to diagnose the initialization.  Regardless of diagnostics for those, GCC should also diagnose uses of such arrays when possible (e.g., in the constant cases).  This should be done regardless of the form of the initialization (i.e., using a string literal or using a series of characters, as in const char a[3] = { '1', '2', '3' };).

const char a[3] = "123";   // no warning

int f (void)
{
  return __builtin_strlen (a);   // missing warning
}

void g (char *d)
{
  __builtin_strcpy (d, a);   // missing warning
}