Bug #7475: Unexpected behavior of Module#append_features on singleton class

• Status: Open
• Priority: Normal
• Assignee: matz (Yukihiro Matsumoto)
• Target version: next minor
• ruby -v: ruby 1.9.3p327 (2012-11-10 revision 37606) [x86_64-darwin12.2.0]
• Backport: 2.2: UNKNOWN, 2.3: UNKNOWN, 2.4: UNKNOWN

[ernie (Ernie Miller)]

=begin
A more involved explanation is available at ((URL:http://erniemiller.org/2012/11/29/ruby-tidbit-include-vs-extend-with-module-class-variables/))

In short, the handling of class variables (and constants) when a module is extended vs included is not as expected.

Example:

module Foo
  @@foo = 'foo'
end

class Bar
  include Foo
end

class Baz
  extend Foo
end

Bar.class_variable_get :@@foo # => "foo"
Baz.singleton_class.class_variable_get :@@foo # => NameError: uninitialized class variable @@foo in Class

We would expect constant and class variable lookup on the singleton class to work, but it doesn't. Both Rubinius and JRuby seem to behave as expected in this case.

Thanks!
=end

[ernie (Ernie Miller)]

Another quick set of observations from this morning:

class Baz
  class << self
    Const = 'Const'
    @@foo = 'foo'
  end
end

Baz.class_variables.inspect # => [:@@foo]
Baz.singleton_class.class_variables.inspect # => []
Baz.singleton_class.class_variable_get :@@foo # => 'foo' ???
Baz.const_get(:Const, false) rescue "Nope." # => 'Nope.'
Baz.singleton_class.const_get(:Const, false) rescue "Nope." # => Const

# Let's try setting it explicitly.
Baz.singleton_class.class_variable_set :@@foo, 'foo'
Baz.singleton_class.class_variables.inspect # => [] -- still "empty"

However, if we extend Foo on Baz, vs opening the singleton class with "class << self", we can class_variable_set on the singleton and see it show up in the list of class variables, vs being empty.

All of this is to say that I think some unexpected weirdness is going on in rb_include_module and/or include_class_new.

[ko1 (Koichi Sasada)]

• Assignee set to matz (Yukihiro Matsumoto)
• Target version changed from 1.9.3 to next minor

Matz, could you check it?

[nobu (Nobuyoshi Nakada)]

• Description updated (diff)