2016-11-03-investigating-ruby-define-method-behaviour.md

I've been investigating why the behaviour of ClassMethod#restore_original_method is different between Ruby 1.8.x and all others versions. See line 77 of ClassMethod.

It turns out that there appears to be a problem in 1.8.7 with using define_method when the method argument is a singleton method. This can be illustrated with the following code:

class Foo
  class << self
    def my_class_method
      'Foo.my_class_method'
    end
  end
end

class Bar < Foo; end

p Foo.my_class_method #=> 'Foo.my_class_method'
p Bar.my_class_method #=> 'Foo.my_class_method'

class Foo
  M = method(:my_class_method)

  class << self
    define_method :my_class_method, M
  end
end

p Foo.my_class_method #=> 'Foo.my_class_method'
p Bar.my_class_method #=> singleton method bound for a different object (TypeError)

Other reports of this behaviour:

• https://www.ruby-forum.com/topic/72973
• http://kagemusha.rubyforge.org/svn/trunk/kagemusha/test/test_bugs.rb

It's not clear whether this was a bug that's now been fixed.