title: PropTypes.instanceOf of functional component not working
body: <p>I'm trying for set <code>propTypes</code> of <code>children</code> for a specific type, so when passing children of the wrong type, it throws a warning, but currently, even after saying that <code>children: PropTypes.arrayOf(Option)</code> it gets completely ignored and accepts other types too.</p>
  
  <p>I want SortBy to accept only a list of Option, but it receives native html <code>option</code> without warnings.</p>
  
  <p>Here is the sandbox:
  <a href="https://codesandbox.io/s/sleepy-currying-fwdlq" rel="nofollow noreferrer">https://codesandbox.io/s/sleepy-currying-fwdlq</a></p>
  
  <p>And here is the code:</p>
  
  <p>sort-by.js</p>
  
  <pre><code>import React from 'react';
  import PropTypes from 'prop-types';
  import {Option} from './option';
  
  const SortBy = ({children, onChange}) =&gt; (
    &lt;select onChange={onChange}&gt;
      &lt;Option&gt;---&lt;/Option&gt;
      {children}
    &lt;/select&gt;
  );
  
  SortBy.Option = Option;
  
  export {
    SortBy,
  };
  
  SortBy.propTypes = {
    onChange: PropTypes.func.isRequired,
    children: PropTypes.arrayOf(Option).isRequired,
  };
  </code></pre>
  
  <p>option.js</p>
  
  <pre><code>import React from 'react';
  import PropTypes from 'prop-types';
  
  export function Option({ children }) {
    return &lt;option&gt;{children}&lt;/option&gt;;
  }
  
  Option.propTypes = {
    children: PropTypes.oneOf(['id', 'state', '---']).isRequired,
  };
  </code></pre>
  
  <p>app.js</p>
  
  <pre><code>import React from 'react';
  import {List, Issue, SortBy} from './components';
  
  
  function App() {
    return (
      &lt;div&gt;
        &lt;SortBy onChange={() =&gt; {}}&gt;
          &lt;SortBy.Option&gt;
            id
          &lt;/SortBy.Option&gt;
          &lt;SortBy.Option&gt;
            state
          &lt;/SortBy.Option&gt;
  
          {/* this one should throw a warning saying that
          option is not an instance of Option, but it doesn't */}
          &lt;option&gt;---&lt;/option&gt;
        &lt;/SortBy&gt;
      &lt;div/&gt;
    );
  }
  
  export default App;
  
  </code></pre>