Remove Factory Components

We support a very unusual kind of components that look like functional components but return an instance.

function Hello(props) {
  return {
    componentDidMount() {
      alert('wow')
    }
    render() {
      return <div>Hi, {this.props.name}</div>
    }
  };
}

Note these are not functional components. They’re pretty exotic and don’t offer any practical benefits over classes. At the time they were added (I think 0.14?) it was because we thought other type systems might want to compile to a format like this instead of ES classes-like output. But that didn’t happen.

There’s some complexity associated with continuing to support this. Since this is more confusing than useful, I think we should deprecate them in 16.x and remove in 17.x.

[lustoykov]

Curious: Maybe some warning is needed for SSR too, in case a different renderer is utilized?

PS. Sorry for the pollution below, I’ve removed the issue id from the commit message so it doesn’t affect future rebases.

[prichodko]

Last answer sounded unclear, so I would love to take this over. 😃

I suppose warning could be placed somewhere here - https://github.com/facebook/react/blob/master/packages/react-reconciler/src/ReactFiberBeginWork.js#L715?