This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
if (__DEV__) {
      validateFunctionComponentInDev(workInProgress, Component);
    }
    return workInProgress.child;
  }
}
function validateFunctionComponentInDev(workInProgress: Fiber, Component: any) {
  if (__DEV__) {
    if (Component) {
      if (Component.childContextTypes) {
        console.error(
          '%s(...): childContextTypes cannot be defined on a function component.',
          Component.displayName || Component.name || 'Component',
        );
      }
    }
    if (workInProgress.ref !== null) {
      let info = '';
      const ownerName = getCurrentFiberOwnerNameInDevOrNull();
      if (ownerName) {
        info += '\n\nCheck the render method of `' + ownerName + '`.';
      }
      let warningKey = ownerName || workInProgress._debugID || '';
      const debugSource = workInProgress._debugSource;
      if (debugSource) {
        warningKey = debugSource.fileName + ':' + debugSource.lineNumber;
      }
      if (!didWarnAboutFunctionRefs[warningKey]) {
        didWarnAboutFunctionRefs[warningKey] = true;
        console.error(