<?php

add_action('init', function() {
	register_post_type( 'foo', array(
		'has_archive' => true,
		'public' => true,
		'label' => 'Foo',
	) );

	global $wp_rewrite;

	var_dump( wp_unique_post_slug( $wp_rewrite->pagination_base, 1, 'publish', 'foo', 0 ) );
});