From: "Florian Aßmann" <redmine@...>
Date: 2010-06-11T17:40:26+09:00
Subject: [ruby-core:30730] [Bug #3420] Module#method calling <=> causes SystemStackError

Issue #3420 has been updated by Florian A��mann.


Please let me ask, why as from 1.9.2:


Sat Oct 24 13:38:45 2009  Yukihiro Matsumoto  <matz@ruby-lang.org>

	* object.c (rb_obj_cmp): defines Object#<=>.  [ruby-core:24063]

Before that all ruby implementations did:

ruby-1.8.7-p249 >
  BrokenComparable = Module.new { def ==(other) self <=> other end }
  BrokenClass = Class.new { include BrokenComparable }
  BrokenClass.new == nil
NoMethodError: undefined method `<=>' for #<BrokenClass:0x10043ef08>
	from (irb):1:in `=='
	from (irb):3

Which is a lot more verbose on what happened and allowed people to hook into method missing and delegate it to a wrapped object as a SystemStackError. Now, if there is a global defined <=> for all classes which mix in the kernel module (all but BasicObject) I'd suspect it should either just work (not calling rb_equal from rb_obj_cmp) as long people are used to 1.8.x/1.9.1 or be more verbose on what happened.

Removing the call of rb_equal in rb_obj_cmp would not just save the extra test of obj identity but also work for all objects; which seems to me like the intended behavior when adding a method like <=> to almost all objects.
Otherwise it should be stated in the docs of Comparable what happens if one does not and what the currently undocumented Object#<=> actually does since a SystemStackError only adds confusion and isn't obvious at all.

Regards
Florian
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