[Share slug between sub page and post type]

share-slug.php

add_action( 'parse_request', 'my_parse_request' );
function my_parse_request( $wp ) {
    $post_type = 'whatever'; // set the post type slug
    // and the "whatever" below is the Page slug

    // This code checks if the path of the current request/page URL begins with
    // "whatever/" as in https://example.com/whatever/child-page-name and also
    // https://example.com/whatever/child-page-name/page/2 (paginated request).
    // We also check if the post_type var is the post type set above.
    if ( preg_match( '#^whatever/#', $wp->request ) &&
        isset( $wp->query_vars['post_type'], $wp->query_vars['name'] ) &&
        $post_type === $wp->query_vars['post_type']
    ) {
        $posts = get_posts( array(
            'post_type' => 'page',
            'name'      => $wp->query_vars['name'],
        ) );

        // If a (child) Page with the same name/slug exists, we load the Page,
        // regardless the post type post exists or not.
        if ( ! empty( $posts ) ) {
            $wp->query_vars['pagename'] = get_page_uri( $posts[0] );

            unset( $wp->query_vars['post_type'], $wp->query_vars['name'],
                $wp->query_vars[ $post_type ] );
        }
    }
}