React - How to pass `ref` from child to parent component?

As per DOC:

You may not use the ref attribute on functional components because they don't have instances. You should convert the component to a class if you need a ref to it, just like you do when you need lifecycle methods or state.

So i think, if you want to use the ref, you need to use class.

Check this: https://github.com/facebook/react/issues/4936

With react^16.0.0 you would use React.createRef(). Using @Timo's answer, it would look like this:

// Dumb:
export default props =>
    <input type='number' ref={props.setRef} />

// Smart:
class Parent extends Component {
    constructor(props) {
        super(props);
        this.ref1 = React.createRef()
    }

    render(){
        return <Dumb {...this.props} setRef={this.ref1} />
    }
}

You could use the callback syntax for refs:

// Dumb:
export default props =>
    <input type='number' ref={props.setRef} />

// Smart:
class Parent extends Component {
    constructor(props) {
        super(props);
    }

    setRef(ref) {
        this.inputRef = ref;
    }

    render(){
        return <Dumb {...this.props} setRef={this.setRef} />
    }
}