■cocoon-child-master/functions.php　に定義
function get_the_category_ids($post_id){
    $categories = array();
    if ( get_post_type() === 'hogehoge' ) {
      $categories = get_the_terms($post_id,'hoge_cat');
    }
    else
    {
      $categories = get_the_category($post_id);
    }
  $category_IDs = array();
  $cat_count = 0;
  foreach($categories as $category):
    $cat_count += (intval($category->count) - 1);
  endforeach ;

  if ($cat_count > 0) {
    foreach($categories as $category):
    if ( get_post_type() === 'hogehoge' ) {
      array_push( $category_IDs, $category->term_id);
    }
    else
    {
      array_push( $category_IDs, $category->cat_ID);
    }
    endforeach ;
  }
  return $category_IDs;
}

function own_search_filter($query) {
    if (is_singular( 'hogehoge' ) ) {
        $query->set( 'post_type', array( 'post', 'hogehoge' ) ); 
    }
}
add_action( 'pre_get_posts','own_search_filter' );




■実行ダンプ

【cocoon-master/tmp/related-list.php　$args】
array
  'post_type' => string 'hogehoge'
  'post__not_in' => 
    array
      0 => int 1560
  'posts_per_page' => int 4
  'orderby' => string 'rand' 
  'no_found_rows' => boolean true
  'date_query' => 
    array
      0 => 
        array
          'after' => string '2020-01-25 0:0:0'
          'inclusive' => boolean true
  'category__in' => 
    array
      0 => int 8

【cocoon-master/tmp/related-list.php　$query->query_vars】
array (size=65)
  'post_type' => 
    array
      0 => string 'post' 
      1 => string 'hogehoge'
  'post__not_in' => 
    array
      0 => int 1560
  'posts_per_page' => int 4
  'orderby' => string 'rand' 
  'no_found_rows' => boolean true
  'date_query' => 
    array
      0 => 
        array
          'after' => string '2020-01-25 0:0:0'
          'inclusive' => boolean true
  'category__in' => 
    array
      0 => int 8
  'error' => string ''
  'm' => string ''
  'p' => int 0
  'post_parent' => string ''
  'subpost' => string ''
  'subpost_id' => string ''
  'attachment' => string ''
  'attachment_id' => int 0
  'name' => string ''
  'pagename' => string ''
  'page_id' => int 0
  'second' => string ''
  'minute' => string ''
  'hour' => string ''
  'day' => int 0
  'monthnum' => int 0
  'year' => int 0
  'w' => int 0
  'category_name' => string ''
  'tag' => string ''
  'cat' => string ''
  'tag_id' => string ''
  'author' => string ''
  'author_name' => string ''
  'feed' => string ''
  'tb' => string ''
  'paged' => int 0
  'meta_key' => string ''
  'meta_value' => string ''
  'preview' => string ''
  's' => string ''
  'sentence' => string ''
  'title' => string ''
  'fields' => string ''
  'menu_order' => string ''
  'embed' => string ''
  'category__not_in' => 
    array
      empty
  'category__and' => 
    array
      empty
  'post__in' => 
    array
      empty
  'post_name__in' => 
    array
      empty
  'tag__in' => 
    array
      empty
  'tag__not_in' => 
    array
      empty
  'tag__and' => 
    array
      empty
  'tag_slug__in' => 
    array
      empty
  'tag_slug__and' => 
    array
      empty
  'post_parent__in' => 
    array
      empty
  'post_parent__not_in' => 
    array
      empty
  'author__in' => 
    array
      empty
  'author__not_in' => 
    array
      empty
  'ignore_sticky_posts' => boolean false
  'suppress_filters' => boolean false
  'cache_results' => boolean true
  'update_post_term_cache' => boolean true
  'lazy_load_term_meta' => boolean true
  'update_post_meta_cache' => boolean true
  'nopaging' => boolean false
  'comments_per_page' => string '50' 
  'order' => string ''

【cocoon-master/tmp/related-list.php　$query->request】
SELECT   ID FROM posts  WHERE 1=1  AND ( 
  post_date >= '2020-01-25 00:00:00'
) AND ID NOT IN (1560) AND ( 
  0 = 1
) AND post_type IN ('post', 'hogehoge') AND (post_status = 'publish' OR post_author = 1 AND post_status = 'private') GROUP BY ID ORDER BY RAND() LIMIT 0, 4