Table segment exists after error. Index valid after truncate

START WITH an unusable UNIQUE INDEX.
Attempt an INSERT, which fails.
The TABLE SEGMENT appears.

TRUNCATE the TABLE.
The INDEX SEGMENT appears, AND the INDEX IS usable.

Connected.
SQL> SET linesize 200
SQL> SET trimspool ON
SQL> COLUMN index_name format a30
SQL> COLUMN segment_name format a30
SQL> show parameter deferred_segment_creation

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
deferred_segment_creation            BOOLEAN     TRUE
SQL> DROP TABLE t;

TABLE dropped.

SQL> CREATE TABLE t ( n NUMBER );

TABLE created.

SQL> CREATE UNIQUE INDEX tu ON t ( n )
  2  unusable;

INDEX created.

SQL> TRUNCATE TABLE t;

TABLE truncated.

SQL> SELECT index_name, status FROM user_indexes
  2  WHERE table_name = 'T';

INDEX_NAME                     STATUS
------------------------------ --------
TU                             UNUSABLE

SQL> SELECT segment_name, segment_type FROM user_segments
  2  WHERE ( segment_name = 'TU' AND segment_type = 'INDEX'
  3          OR segment_name = 'T' AND segment_type = 'TABLE' );

no ROWS selected

SQL> INSERT INTO t ( n ) VALUES ( 0 );
INSERT INTO t ( n ) VALUES ( 0 )
*
ERROR AT line 1:
ORA-01502: INDEX 'U.TU' OR PARTITION OF such INDEX IS IN unusable state


SQL> SELECT segment_name, segment_type FROM user_segments
  2  WHERE ( segment_name = 'TU' AND segment_type = 'INDEX'
  3          OR segment_name = 'T' AND segment_type = 'TABLE' );

SEGMENT_NAME                   SEGMENT_TYPE
------------------------------ ------------------
T                              TABLE

SQL> TRUNCATE TABLE t;

TABLE truncated.

SQL> SELECT segment_name, segment_type FROM user_segments
  2  WHERE ( segment_name = 'TU' AND segment_type = 'INDEX'
  3          OR segment_name = 'T' AND segment_type = 'TABLE' );

SEGMENT_NAME                   SEGMENT_TYPE
------------------------------ ------------------
T                              TABLE
TU                             INDEX

SQL> SELECT index_name, status FROM user_indexes
  2  WHERE table_name = 'T';

INDEX_NAME                     STATUS
------------------------------ --------
TU                             VALID