Ruby special behaviour: `respond_to_missing?` method gets hidden

test_respond_to_missing.rb

# When you implement `method_missing` you should also implement `respond_to_missing?`
# But the later doesn't show up as instance methods on the class, and can not be called directly! Or does it???

class Foo
  def method_missing(name, *)
    "called #{name}"
  end

  def bar?
    respond_to_missing?(:bar)
  end

  # Defined as private in `Object`
  def respond_to_missing?(*)
    true
  end
end

Foo.instance_methods(false) # => [:method_missing, :bar?]
foo = Foo.new
foo.bar # => "called bar"
foo.respond_to? :bar # => true
foo.respond_to_missing? :bar # => "called respond_to_missing?"

foo.private_methods(false) # => [:respond_to_missing?]
foo.send(:respond_to_missing?) # => true

foo.bar? # => true

Updated: I had some weird issues where I wasn't able to call respond_to_missing? even from other methods in the class. But it all seems to work now 😕

Updated update: I must have been calling self.respond_to_missing?!
See e.g. http://www.skorks.com/2010/04/ruby-access-control-are-private-and-protected-methods-only-a-guideline/