How to invoke Module#constants on Module itself

constants.rb

# Module.constants returns constants accessible at the place where the
# method is called. 
class A
  X = 1
  p Module.constants.include?(:X) #=> true
end

# Module#constants returns constants defined in the receiver.  However,
# you need a trick to invoke Module#constants on Module itself.
p Module.instance_method(:constants).bind(Module).call #=> []

# Another way to invoke Module#constants on Module itself suggested by
# Marc-Andre Lafortune in [ruby-core:42091]
# It's available only in CRuby 1.9, and I doubt that Matz has accepted the
# feature because it has been introduced by nobu secretly.
p Module.constants(Module)

Good point. So I guess the clearest demonstration would be:

class A
  X = 1
  p Module.constants.include?(:X) #=> true
end
p Module.constants.include?(:X) #=> false

And I think Module.instance_method(:constants).bind(Module).call is actually a totally different function, which is defined in variable.c:1775 (from ruby-1.9.3-p0)
https://github.com/ruby/ruby/blob/v1_9_3_0/variable.c#L1775-1809 rb_mod_constants (Module#constants)
https://github.com/ruby/ruby/blob/v1_9_3_0/eval.c#L286-328 rb_mod_s_constants (Module.constants)

I guess rb_mod_s_constants is overriding rb_mod_constants inside Module,
and which will also call rb_mod_constants when argv > 0.

This is so confusing :o
Thanks for the discussion.

/*
 *  call-seq:
 *     mod.constants(inherit=true)    -> array
 *
 *  Returns an array of the names of the constants accessible in
 *  <i>mod</i>. This includes the names of constants in any included
 *  modules (example at start of section), unless the <i>all</i>
 *  parameter is set to <code>false</code>.
 *
 *    IO.constants.include?(:SYNC)        #=> true
 *    IO.constants(false).include?(:SYNC) #=> false
 *
 *  Also see <code>Module::const_defined?</code>.
 */

VALUE
rb_mod_constants(int argc, VALUE *argv, VALUE mod)
{
    VALUE inherit;
    st_table *tbl;

    if (argc == 0) {
    inherit = Qtrue;
    }
    else {
    rb_scan_args(argc, argv, "01", &inherit);
    }
    if (RTEST(inherit)) {
    tbl = rb_mod_const_of(mod, 0);
    }
    else {
    tbl = rb_mod_const_at(mod, 0);
    }
    return rb_const_list(tbl);
}