Failure to do so caused a failure in rocksdb.rocksdb test.
When test_if_cheaper_ordering computes is_covering= ...,
- MySQL calls table->file->primary_key_is_clustered()
- MariaDB calls (table->file->index_flags(nr, 0, 1) &
                          HA_CLUSTERED_INDEX)

The first produces true, the second used to produce false.