prepending to the singleton class of a module breaks instance_method

[doudou]

Environment

jruby 9.1.7.0 (2.3.1) 2017-01-11 68056ae Java HotSpot(TM) 64-Bit Server VM 25.121-b13 on 1.8.0_121-b13 +jit [linux-x86_64]

Ubuntu 16.04 LTS, jruby installed from rbenv with no special flags.

Expected Behavior

I'm the maintainer of flexmock, and during some of the partial mock operations, flexmock prepends to the singleton class of the object that is being mocked. It also stores the original instance method before mocking it so as to be able to call it as part of the mock process. On both MRI and JRuby, this works fine for plain objects. However, when the object is itself a module, it works on MRI but fails on JRuby

The problem can be reproduced with:

module Base
    def setup_submodel; end
end
module P; end

module M
    extend Base
    class << self
        prepend P
    end
end

m = M.singleton_class.instance_method(:setup_submodel)
m.bind(M).call

MRI executes it without any problem, while JRuby fails with

TypeError: bind argument must be an instance of #<Class:0x8646db9>
    bind at org/jruby/RubyUnboundMethod.java:105
  <main> at jruby_prepend_bind_bug.rb:17

[doudou]

The problem seems to have been introduced between jruby 9.1.5.0 and 9.1.6.0

[headius]

Works ok with a plain class and module, so it's something specific to singleton/meta classes.