Regression with `withComponent` and `defaultProps`

There is a regression between Styled Components 3.4.10 and 4.2.0 in which the behavior of defaultProps changes when used in combination with withComponent.

Steps to reproduce

The following code works in 3.4.10 and crashes in 4.2.0.

import React from "react";
import { render } from "react-dom";
import styled from 'styled-components';

const STYLES = {
  fallback: {
    color: 'red',
  },
}

const A = styled.div`
  color: ${({styling}) => STYLES[styling].color};
`

A.defaultProps = {
  styling: 'fallback',
}

const B = ({...props}) => <div {...props} />
const C = styled(A.withComponent(B))``; 

render((<C>text</C>), document.getElementById("root"));

Expected Behavior

The component to render text in a red font.

https://codesandbox.io/s/0m1zp6k7pw

Actual Behavior

The execution crashes with error STYLES[styling] is undefined. This is because styling is undefined instead of fallback (as specified in the defaultProps of A), leading to an access of STYLES[undefined].color.

https://codesandbox.io/s/7j12406oyj

[clentfort]

@konojunya This is an undocumented regression in version 4 of styled-components. A regression in the world of software describes the appearance of a bug which was absent in a previous revision.

I am aware that withComponent is a candidate for deprecation, it is not deprecated yet. Therefore it should work in v4 as it did in v3.

[probablyup]

Closing, feel free to make a documentation PR against the website!