Source

File: wp-includes/post-template.php

function _wp_link_page( $i ) {
	global $wp_rewrite;
	$post       = get_post();
	$query_args = array();

	if ( 1 == $i ) {
		$url = get_permalink();
	} else {
		if ( ! get_option( 'permalink_structure' ) || in_array( $post->post_status, array( 'draft', 'pending' ), true ) ) {
			$url = add_query_arg( 'page', $i, get_permalink() );
		} elseif ( 'page' === get_option( 'show_on_front' ) && get_option( 'page_on_front' ) == $post->ID ) {
			$url = trailingslashit( get_permalink() ) . user_trailingslashit( "$wp_rewrite->pagination_base/" . $i, 'single_paged' );
		} else {
			$url = trailingslashit( get_permalink() ) . user_trailingslashit( $i, 'single_paged' );
		}
	}

	if ( is_preview() ) {

		if ( ( 'draft' !== $post->post_status ) && isset( $_GET['preview_id'], $_GET['preview_nonce'] ) ) {
			$query_args['preview_id']    = wp_unslash( $_GET['preview_id'] );
			$query_args['preview_nonce'] = wp_unslash( $_GET['preview_nonce'] );
		}

		$url = get_preview_post_link( $post, $query_args, $url );
	}

	return '<a href="' . esc_url( $url ) . '" class="post-page-numbers">';
}