Merging query args fails

[JuhG]

In multiple cases where I needed to search for some posts I specifically need to add the post type, otherwise there's no result. Which is an issue with WP itself afaik. For example:

{# doesn't work #}
{% for post in taxonomy.posts %}
    {# ... #}
{% endfor %}

{# works #}
{% for post in taxonomy.posts.type('post_type') %}
    {# ... #}
{% endfor %}

The problem with Rila appears when I try to add that post type, it can't merge with the default, which is set to any, as a string. I searched for a quick solution, which was type casting these strings as an array:

// trait-query-args.php - line 43
public function set( $key, $value, $merge = false ) {
    if( ! $this->allow_augumentation() ) {
        return false;
    }

    # Some properties expect arrays. Make sure to format values
    if( in_array( $key, array( 'post__in', 'post__not_in', 'author__in' ) ) ) {
        $value = (array) $value;
    }

    if( $merge && is_array( $value ) ) {
        $current = isset( $this->args[ $key ] ) ? $this->args[ $key ] : array();

                // here I added the (array) casting as a quick fix

        $current = array_merge( (array) $current, $value );
        $this->args[ $key ] = $current;
    } else {
        $this->args[ $key ] = $value;
    }

    return $this;
}

What do you think the proper solution to this problem?