Source

File: wp-includes/link-template.php

function get_the_post_navigation( $args = array() ) {
	// Make sure the nav element has an aria-label attribute: fallback to the screen reader text.
	if ( ! empty( $args['screen_reader_text'] ) && empty( $args['aria_label'] ) ) {
		$args['aria_label'] = $args['screen_reader_text'];
	}

	$args = wp_parse_args(
		$args,
		array(
			'prev_text'          => '%title',
			'next_text'          => '%title',
			'in_same_term'       => false,
			'excluded_terms'     => '',
			'taxonomy'           => 'category',
			'screen_reader_text' => __( 'Post navigation' ),
			'aria_label'         => __( 'Posts' ),
			'class'              => 'post-navigation',
		)
	);

	$navigation = '';

	$previous = get_previous_post_link(
		'<div class="nav-previous">%link</div>',
		$args['prev_text'],
		$args['in_same_term'],
		$args['excluded_terms'],
		$args['taxonomy']
	);

	$next = get_next_post_link(
		'<div class="nav-next">%link</div>',
		$args['next_text'],
		$args['in_same_term'],
		$args['excluded_terms'],
		$args['taxonomy']
	);

	// Only add markup if there's somewhere to navigate to.
	if ( $previous || $next ) {
		$navigation = _navigation_markup( $previous . $next, $args['class'], $args['screen_reader_text'], $args['aria_label'] );
	}

	return $navigation;
}