Source

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

public function sanitize_query( $queries ) {
		$cleaned_query = array();

		$defaults = array(
			'taxonomy'         => '',
			'terms'            => array(),
			'field'            => 'term_id',
			'operator'         => 'IN',
			'include_children' => true,
		);

		foreach ( $queries as $key => $query ) {
			if ( 'relation' === $key ) {
				$cleaned_query['relation'] = $this->sanitize_relation( $query );

				// First-order clause.
			} elseif ( self::is_first_order_clause( $query ) ) {

				$cleaned_clause          = array_merge( $defaults, $query );
				$cleaned_clause['terms'] = (array) $cleaned_clause['terms'];
				$cleaned_query[]         = $cleaned_clause;

				/*
				 * Keep a copy of the clause in the flate
				 * $queried_terms array, for use in WP_Query.
				 */
				if ( ! empty( $cleaned_clause['taxonomy'] ) && 'NOT IN' !== $cleaned_clause['operator'] ) {
					$taxonomy = $cleaned_clause['taxonomy'];
					if ( ! isset( $this->queried_terms[ $taxonomy ] ) ) {
						$this->queried_terms[ $taxonomy ] = array();
					}

					/*
					 * Backward compatibility: Only store the first
					 * 'terms' and 'field' found for a given taxonomy.
					 */
					if ( ! empty( $cleaned_clause['terms'] ) && ! isset( $this->queried_terms[ $taxonomy ]['terms'] ) ) {
						$this->queried_terms[ $taxonomy ]['terms'] = $cleaned_clause['terms'];
					}

					if ( ! empty( $cleaned_clause['field'] ) && ! isset( $this->queried_terms[ $taxonomy ]['field'] ) ) {
						$this->queried_terms[ $taxonomy ]['field'] = $cleaned_clause['field'];
					}
				}

				// Otherwise, it's a nested query, so we recurse.
			} elseif ( is_array( $query ) ) {
				$cleaned_subquery = $this->sanitize_query( $query );

				if ( ! empty( $cleaned_subquery ) ) {
					// All queries with children must have a relation.
					if ( ! isset( $cleaned_subquery['relation'] ) ) {
						$cleaned_subquery['relation'] = 'AND';
					}

					$cleaned_query[] = $cleaned_subquery;
				}
			}
		}

		return $cleaned_query;
	}