Should not check/call `to_str` for immediate objects; ref #3515

src/array.c

@@ -1048,6 +1048,7 @@ join_ary(mrb_state *mrb, mrb_value ary, mrb_value sep, mrb_value list)
       break;
 
     default:
+      if (!mrb_immediate_p(val)) {
         tmp = mrb_check_string_type(mrb, val);
         if (!mrb_nil_p(tmp)) {
           val = tmp;
@@ -1058,6 +1059,7 @@ join_ary(mrb_state *mrb, mrb_value ary, mrb_value sep, mrb_value list)
           val = tmp;
           goto ary_join;
         }
+      }
       val = mrb_obj_as_string(mrb, val);
       goto str_join;
     }