Make 'sequence' Typed Data instances return type of 'sequence' instead of 'list'

Oops! 😅

Discovered this in #3216089: Expose validation constraints (and validatability %) in Config Inspector UI and hence had to work around this.

How the hell did you run into this?

I worked around it in contrib like this:

  /**
   * Work-around for bug introduced in #2361539.
   *
   * @see \Drupal\Core\TypedData\ListDataDefinition::getDataType()
   * @see \Drupal\Core\Config\Schema\SequenceDataDefinition
   * @see https://www.drupal.org/project/drupal/issues/2361539
   * @todo Remove this when this module requires a Drupal core version that includes https://www.drupal.org/project/drupal/issues/3361034.
   */
  protected static function getDataType(TypedDataInterface $typed_data) : string {
    $data_type = $typed_data->getDataDefinition()->getDataType();
    if ($data_type === 'list') {
      $data_type = 'sequence';
    }
    return $data_type;
  }

… because without this, it's impossible to correctly traverse a typed data tree by using the return value of $typed_data->getDataDefinition()->getDataType() … since list is not actually a config schema type, \Drupal\Core\Config\TypedConfigManager::determineType() decides to make it automagically fall back to the undefined type 😳

… at which point traversing further down the three of course is impossible.

Why can this go undiscovered for a decade?

The reason this has not caused problems for Drupal core so far is that \Drupal\Core\TypedData\Validation\RecursiveContextualValidator::validateNode() does not use the full config schema tree, but instead relies on inspecting the typed data's object instances:

    // If the data is a list or complex data, validate the contained list items
    // or properties. However, do not recurse if the data is empty.
    // Next, we do not recurse if given constraints are validated against an
    // entity, since we should determine whether the entity matches the
    // constraints and not whether the entity validates.
    if (($data instanceof ListInterface || $data instanceof ComplexDataInterface) && !$data->isEmpty() && !($data instanceof EntityAdapter && $constraints_given)) {
      foreach ($data as $property) {
        $this->validateNode($property);
      }
    }

Are the other TypedData classes correct?

FieldConfigBase::getDataType() is wrong too:

  public function getDataType() {
    return 'list';
  }

… but I figured we'd keep this tightly scoped, because that is one of the most complex pieces of config 😱

#11 That's LOVELY to hear! 😊😊 Glad to read it helps and is appreciated!

New issue created, with a lot more detail than was present in #7: #3370179: Clarify why FieldConfigBase::getDataType() is 'list' and not 'field_config_base'.