Add "current" class to post type archive menu items when viewing single

post-type-current-menu-item.php

<?php

/*
Give the "current-menu-item" class to archive links 
in the menu when viewing singles of the post type.

1. Add the post type archive page to the menu (NOT as a custom link)
2. Add this filter.
*/

add_filter( 'nav_menu_css_class', function( $classes, $item ) {
    $type = get_post_type();
    $class_string = substr($item->classes[3], 17);
    if ( $type == $class_string ) {
      $classes[] = 'current-menu-item';
    }
    return $classes;
}, 10, 2 );

/*
This may not work if other custom menu classes are in play. The filter is looking
for the array of classes is the menu $item object.
It seems to always be in position [3], e.g.:
'menu-item-object-[post-type]'

The filter pulls the [post-type] from the class and compares it
to the post type currently being viewed.
*/