prop-types-logic.js

const trickyPropType = getPropTypeFromProps =>
  (props, propName, componentName) => {
    const error = getPropTypeFromProps(props)(props, propName, componentName)
    if (error) return error // WIP
  }

const Summary = ({ hasErrors, errors }) =>
  <div>
    {hasErrors &&
      errors.map(err => <span key={err}>{err}</span>)}
  </div>

Summary.propTypes = {
  hasErrors: bool.isRequired,
  errors: trickyPropType(
    props => props.hasErrors
      ? arrayOf(string).isRequired
      : arrayOf(string),
  ),
}

Summary.defaultProps = {
  errors: [],
}