4 comments

1. Tested on woocommerce 3.0.4

   function my_woocommerce_taxonomy_args_product_tag( $array ) {
       $array['hierarchical'] = true;
       return $array;
   };
   add_filter( 'woocommerce_taxonomy_args_product_tag', 'my_woocommerce_taxonomy_args_product_tag', 10, 1 );
   

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2. Found the solution, here it is if someone is looking to do the same:

   function reregister_taxonomy_pro_tags() {
       # the post types that the taxonomy is registered to
       $post_types = array('product');
       # set this to the taxonomy name
       $tax_name = 'product_tag';
       # load the already created taxonomy as array so we can
       # pass it back in as $args to register_taxonomy
       $tax = (array)get_taxonomy($tax_name);
   
       if ($tax) {
           # adjust the hierarchical necessities
           $tax['hierarchical'] = true;
           $tax['rewrite']['hierarchical'] = true;
   
           # adjust the hierarchical niceties (these could be ignored)
           $tax['labels']['parent_item'] = sprintf(__("Parent %s"),
           $tax->labels->singular_name);
           $tax['labels']['parent_item_colon'] = sprintf(__("Parent %s:"),
           $tax->labels->singular_name);
   
           # cast caps to array as expected by register_taxonomy
           $tax['capabilities'] = (array)$tax['cap'];
           # cast labels to array
           $tax['labels'] = (array)$tax['labels'];
           # register the taxonomy with our new settings
           register_taxonomy($tax_name, array('product'), $tax);
       }
   }
   

   init action with a late priority so other taxonomies are loaded

   add_action(‘init’, ‘reregister_taxonomy_pro_tags’, 9999);

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3. Thanks for the above answer saved me a ton of research but you should replace

       $tax['labels']['parent_item'] = sprintf(__("Parent %s"),
       $tax->labels->singular_name);
       $tax['labels']['parent_item_colon'] = sprintf(__("Parent %s:"),
       $tax->labels->singular_name);
   

   to

   $tax['labels']->parent_item = sprintf(__("Parent %s"),
   $tax['labels']->singular_name);
   $tax['labels']->parent_item_colon = sprintf(__("Parent %s:"),
   $tax['labels']->singular_name);
   

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4. Here’s how I did it for WooCommerce 3.4.5 (and WordPress 4.9.8). This is based on @user2293554’s answer.

   <?php
   function add_hierarchical_product_tags( $taxonomy, $object_type, $args ) {
       if ( $taxonomy == 'product_tag' && $args['hierarchical'] != true ) {
           $args['hierarchical'] = true;
           $args['rewrite']['hierarchical'] = true;
   
           // Update CMS labels
           $args['labels']->parent_item = sprintf(__("Parent %s"), $args['labels']->singular_name);
           $args['labels']->parent_item_colon = sprintf(__("Parent %s:"), $args['labels']->singular_name);
   
           // Convert to array for register_taxonomy()
           $args['labels'] = (array)$args['labels'];
           $args['capabilities'] = (array)$args['cap'];
           unset( $args['cap'] );  // remove the cap object
   
           register_taxonomy( $taxonomy, $object_type, $args );
       }
   }
   
   add_action( 'registered_taxonomy', 'add_hierarchical_product_tags', 10, 3 );
   

   The args['hierarchical'] != true condition is needed so we don’t get stuck in a loop, as the registered_taxonomy action will get called again.

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