refine_class_method_fixed.rb

class Foo
  def self.foo
    "foo"
  end
  def foo
    "ifoo"
  end
  def true_foo
    Foo.foo
  end
end

module Interceptor
  refine Foo.singleton_class do
    def foo
      "bar"
    end
  end
  refine Foo do
    def foo
      "ibar"
    end
  end
end

using Interceptor

p Foo.foo # "bar"
p Foo.new.foo # "ibar"
p Foo.new.true_foo # "foo"