Is there a way to stub a method of an included module with Rspec?

Note you cannot directly call M.foo! Your code only seems to work because you mocked M.foo to return :bar.

When you open A metaclass (class << self) to include M, you have to mock any instance of M, that is adding to your before block:

allow_any_instance_of(M).to receive(:foo).and_return(:bar)

module M
  def foo
    :M
  end
end

module A
  class << self
    include M

    def foo
      super
    end
  end
end

describe "trying to stub the included method" do
  before do
    allow(M).to receive(:foo).and_return(:bar)
    allow_any_instance_of(M).to receive(:foo).and_return(:bar)
  end


  it "should be stubbed when calling M" do
    expect(M.foo).to eq :bar
  end

  it "should be stubbed when calling A" do
    expect(A.foo).to eq :bar
  end
end