Issue #3557055 by alexpott, andypost:...

    // ("statically") valid. Not all types have a fallback type.

    // @see \Drupal\Core\Config\TypedConfigManager::getDefinitionWithReplacements()

    $fallback_type = $typed_data_manager->findFallback($original_mapping_type);

    $valid_keys_everywhere = array_intersect_key(

      $valid_keys_per_type,

      [$fallback_type => NULL],

    );

    assert(count($valid_keys_everywhere) <= 1);

    if ($fallback_type !== NULL && isset($valid_keys_per_type[$fallback_type])) {

      $valid_keys_everywhere = [$fallback_type => $valid_keys_per_type[$fallback_type]];

      $statically_required_keys = NestedArray::mergeDeepArray($valid_keys_everywhere);

    // Now that statically valid keys are known, determine which valid keys are

    // only valid in *some* cases: remove the statically valid keys from every

    // per-type array of valid keys.

    $valid_keys_some = array_diff_key($valid_keys_per_type, $valid_keys_everywhere);

    $valid_keys_some_processed = array_map(

      // Now that statically valid keys are known, determine which valid keys

      // are only valid in *some* cases: remove the statically valid keys from

      // every per-type array of valid keys.

      unset($valid_keys_per_type[$fallback_type]);

      $valid_keys_per_type = array_map(

        fn (array $keys) => array_values(array_filter($keys, fn (string $key) => !in_array($key, $statically_required_keys, TRUE))),

      $valid_keys_some

        $valid_keys_per_type

      );

    return $valid_keys_some_processed;

    }

    return $valid_keys_per_type;

  }

  /**