The problematic behavior of `and_return` without arguments

and_return_without_args_spec.rb

describe '`and_return` without arguments' do
  let(:obj) { double('obj') }

  context 'with `stub`' do
    context 'and `{ }` block' do
      it 'works properly' do
        obj.stub(:foo).and_return {
          'a return value'
        }

        expect(obj.foo).to eq('a return value')
      end
    end

    context 'and `do end` block' do
      it 'works properly' do
        obj.stub(:foo).and_return do
          'a return value'
        end

        expect(obj.foo).to eq('a return value')
      end
    end
  end

  context 'with `allow(...).to receive(...)`' do
    context 'and `{ }` block' do
      it 'works properly' do
        # The block is taken by #and_return.
        allow(obj).to receive(:foo).and_return {
          'a return value'
        }

        expect(obj.foo).to eq('a return value')
      end
    end

    context 'and `do end` block' do
      it 'returns nil unexpectedly' do
        # The block is taken by #to.
        allow(obj).to receive(:foo).and_return do
          'a return value'
        end

        # Describing the unexpected behavior,
        # this should not pass, but it passes.
        expect(obj.foo).to be_nil
      end
    end
  end

  context 'without block' do
    it 'returns nil' do
      # Shouldn't this raise error?
      allow(obj).to receive(:foo).and_return

      expect(obj.foo).to be_nil
    end
  end
end