The ProfileSelect form element breaks value callbacks of contained form elements

Not sure where this belongs exactly.

I updated to 2.x-dev and tested my profile, which has a date field. And it stopped working and I got a validation error about an invalid date format when it was clearly valid.

Some fun debugging later, I tracked this down to the fact that ProfileSelect is a form element which means it has #input TRUE (not actually explicitly set, but all form elements get that automatically). That means it owns everything within it and the value is set in the form_state values.

When nested elements are then processed in \Drupal\Core\Form\FormBuilder::handleInputElement(), it sees that there already are values and doesn't update it:

    // Set the element's value in $form_state->getValues(), but only, if its key
    // does not exist yet (a #value_callback may have already populated it).
    if (!NestedArray::keyExists($form_state->getValues(), $element['#parents'])) {
      $form_state->setValueForElement($element, $element['#value']);
    }

And then the values for the date field are incomplete and everything falls apart :)

Not exactly sure what needs to happen. I see that form elements like Table are also form elements and it works fine for them obviously. They do have a value callback, so maybe what's needed is a custom value callback that only returns its own value?