Trace replay produces "Impossible WHERE noticed after reading const tables"

CREATE TABLE t1 (a INT, b INT, KEY(a)) engine=myisam;

INSERT INTO t1 VALUES (1,1), (2,2), (3,3);

analyze table t1;

set optimizer_record_context=1;

EXPLAIN FORMAT=JSON SELECT * FROM t1 WHERE a = 1;

select context into dumpfile

    "../../tmp/dump1.sql" from information_schema.optimizer_context;

drop table t1;

-- source "$MYSQLTEST_VARDIR/tmp/dump1.sql"

drop table t1;

EXPLAIN FORMAT=JSON SELECT * FROM t1 WHERE a = 1;

EXPLAIN

{

  "query_block": {

    "select_id": 1,

    "cost": 0.002024411,

    "nested_loop": [

      {

        "table": {

          "table_name": "t1",

          "access_type": "ref",

          "possible_keys": ["a"],

          "key": "a",

          "key_length": "5",

          "used_key_parts": ["a"],

          "ref": ["const"],

          "loops": 1,

          "rows": 1,

          "cost": 0.002024411,

          "filtered": 100

        }

      }

    ]

  }

}

select context into dumpfile

"../../tmp/dump1.sql" from information_schema.optimizer_context;

EXPLAIN FORMAT=JSON SELECT * FROM t1 WHERE a = 1;

EXPLAIN

{

  "query_block": {

    "select_id": 1,

    "table": {

      "message": "Impossible WHERE noticed after reading const tables"

    }

  }

}