ReactComponentPropType.js

function ReactComponentPropType(props, propName, componentName, ...rest) {
  const Component = props[propName];
  if (Component === undefined) return null;
  if (typeof Component === 'function' && Component.prototype.isReactComponent) {
    // it's a real react component
    return null;
  } else {
    return new Error(`${componentName}.${propName} is expected to be a React component.`);
  }
}