On the single view of any WordPress custom post type item, make the archive menu item for that post type active

active-cpt-menu-trail.php

<?php
// on the single view of any custom post type item, make the archive menu item for that post type active

// set the global scope variables
$current_menu_parent = null;
$current_menu_item   = null;

// STEP 1: loop through all the menu items and find the current and parent items
function squarecandy_active_items_prepare( $classes ) {

	// get all the menu items
	$menu = wp_get_nav_menu_items( 'main-menu' ); // insert your menu slug here

	global $post;
	global $current_menu_parent;
	global $current_menu_item;

	// loop through the menu items
	foreach ( $menu as $menu_item ) {
		$archive_link  = get_post_type_archive_link( $post->post_type );
		$menu_item_url = $menu_item->url;
		$base_url      = get_site_url();

		// when you hit the item that matches the post type archive, save the current menu item and parent menu item.
		if ( $archive_link === $menu_item_url || $archive_link === $base_url . $menu_item_url ) {
			$current_menu_parent = (int) $menu_item->menu_item_parent;
			$current_menu_item   = (int) $menu_item->ID;
		}
	}

	// unrelated to the above, but required to keep body_class working correctly
	return $classes;
}
add_filter( 'body_class', 'squarecandy_active_items_prepare', 99, 1 );

// STEP 2: apply the current and parent classes as appropriate
function squarecandy_active_item_classes( $classes = array(), $menu_item = false ) {
	global $post;
	global $current_menu_parent;
	global $current_menu_item;

	if ( $menu_item->ID === $current_menu_parent ) {
		$classes[] = 'current-menu-ancestor';
	}
	if ( $menu_item->ID === $current_menu_item ) {
		$classes[] = 'current-menu-item active';
	}
	return $classes;
}
add_filter( 'nav_menu_css_class', 'squarecandy_active_item_classes', 10, 2 );