mod <= other   →  true, false, or nil

Returns true if mod is a subclass of other or is the same as other. Returns nil if there’s no relationship between the two. (Think of the relationship in terms of the class definition: "class A<B" implies "A<B").

Source Code

/*
* call-seq:
*   mod <= other   =>  true, false, or nil
*
* Returns true if <i>mod</i> is a subclass of <i>other</i> or
* is the same as <i>other</i>. Returns 
* <code>nil</code> if there's no relationship between the two. 
* (Think of the relationship in terms of the class definition: 
* "class A<B" implies "A<B").
*
*/

VALUE
rb_class_inherited_p(mod, arg)
   VALUE mod, arg;
{
   VALUE start = mod;

   if (mod == arg) return Qtrue;
   switch (TYPE(arg)) {
     case T_MODULE:
     case T_CLASS:
       break;
     default:
       rb_raise(rb_eTypeError, "compared with non class/module");
   }

   if (FL_TEST(mod, FL_SINGLETON)) {
       if (RCLASS(mod)->m_tbl == RCLASS(arg)->m_tbl)
           return Qtrue;
       mod = RBASIC(mod)->klass;
   }
   while (mod) {
       if (RCLASS(mod)->m_tbl == RCLASS(arg)->m_tbl)
           return Qtrue;
       mod = RCLASS(mod)->super;
   }
   /* not mod < arg; check if mod > arg */
   while (arg) {
       if (RCLASS(arg)->m_tbl == RCLASS(start)->m_tbl)
           return Qfalse;
       arg = RCLASS(arg)->super;
   }
   return Qnil;
}