Source

File: wp-includes/class-wp-tax-query.php

private function clean_query( &$query ) {
		if ( empty( $query['taxonomy'] ) ) {
			if ( 'term_taxonomy_id' !== $query['field'] ) {
				$query = new WP_Error( 'invalid_taxonomy', __( 'Invalid taxonomy.' ) );
				return;
			}

			// So long as there are shared terms, 'include_children' requires that a taxonomy is set.
			$query['include_children'] = false;
		} elseif ( ! taxonomy_exists( $query['taxonomy'] ) ) {
			$query = new WP_Error( 'invalid_taxonomy', __( 'Invalid taxonomy.' ) );
			return;
		}

		$query['terms'] = array_unique( (array) $query['terms'] );

		if ( is_taxonomy_hierarchical( $query['taxonomy'] ) && $query['include_children'] ) {
			$this->transform_query( $query, 'term_id' );

			if ( is_wp_error( $query ) ) {
				return;
			}

			$children = array();
			foreach ( $query['terms'] as $term ) {
				$children   = array_merge( $children, get_term_children( $term, $query['taxonomy'] ) );
				$children[] = $term;
			}
			$query['terms'] = $children;
		}

		$this->transform_query( $query, 'term_taxonomy_id' );
	}