extended methods don't play nice

singleton_extend.rb

module A
  def a_method
    puts "a method"
  end
end
# => :a_method

class B
  extend A
  def b_method
    puts "b method"
  end
  def self.c_method
    puts "c_method"
  end
end
# => :c_method

B.singleton_methods
# => [:c_method, :a_method]

B.a_method
# a method
# => nil

B.c_method
# c_method
# => nil

B.singleton_methods(:all).select { |sm| begin; B.singleton_method(sm); false; rescue; true; end }
# => [:a_method]

B.singleton_method(:c_method)
# => #<Method: B.c_method>

B.singleton_method(:a_method)
# NameError: undefined singleton method `a_method' for `B'
# from (pry):30:in `singleton_method'