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<?php
/**
  * Display Past Events in List View if there are no Upcoming Events -- from https://cl.ly/29222x003e22 to https://cl.ly/1z3V0s0Q282Y
  * By Barry 2016-08-23
  * From https://gist.github.com/cliffordp/295379c665a27090151ad28fd2bf6a96
  */
function show_past_events_if_no_future_events( $posts, WP_Query $query ) {
    if (    ! $query->is_main_query()
         || ! tribe_is_event_query()
         || ! tribe_is_upcoming()
         || ! tribe_is_list_view()
         || ! empty( $posts )
        ) {
            return $posts;
    }

    return tribe_get_events( array(
        'eventDisplay' => 'past',
        'posts_per_page' => 4
    ) );
}

add_filter( 'posts_results', 'show_past_events_if_no_future_events', 10, 2 );