Source

File: wp-includes/rest-api.php

function rest_handle_multi_type_schema( $value, $args, $param = '' ) {
	$allowed_types = array( 'array', 'object', 'string', 'number', 'integer', 'boolean', 'null' );
	$invalid_types = array_diff( $args['type'], $allowed_types );

	if ( $invalid_types ) {
		_doing_it_wrong(
			__FUNCTION__,
			/* translators: 1. Parameter. 2. List of allowed types. */
			wp_sprintf( __( 'The "type" schema keyword for %1$s can only contain the built-in types: %2$l.' ), $param, $allowed_types ),
			'5.5.0'
		);
	}

	$best_type = rest_get_best_type_for_value( $value, $args['type'] );

	if ( ! $best_type ) {
		if ( ! $invalid_types ) {
			return '';
		}

		// Backward compatibility for previous behavior which allowed the value if there was an invalid type used.
		$best_type = reset( $invalid_types );
	}

	return $best_type;
}