Updated: Comment #0
Problem/Motivation
\Drupal::entityQueryAggregate() typehints \Drupal\Core\Entity\Query\QueryInterface, but returns \Drupal\Core\Entity\Query\QueryAggregateInterface.
Proposed resolution
Typehint \Drupal\Core\Entity\Query\QueryAggregateInterface in \Drupal::entityQueryAggregate()
See line 382 of core/lib/Drupal.php.