Comparison semantic of int = string changes with creation of an index

MariaDB [test]> create table t1(id int primary key, c1 int);

Query OK, 0 rows affected (0.030 sec)

MariaDB [test]> insert into t1 values(1, 1),(2, 2);

Query OK, 2 rows affected (0.003 sec)

Records: 2  Duplicates: 0  Warnings: 0

MariaDB [test]> select * from t1;

+----+------+

| id | c1   |

+----+------+

|  1 |    1 |

|  2 |    2 |

+----+------+

2 rows in set (0.001 sec)

MariaDB [test]> select * from t1 where c1 = '1.5';

Empty set (0.001 sec)

-- Note: this is as expected because column values 1,2 converted to decimal are 1.0 and 2.0

-- and neither is equal to 1.5 (string '1.5' converted to decimal).

MariaDB [test]> create index idx1 on t1(c1);

Query OK, 0 rows affected (0.032 sec)

Records: 0  Duplicates: 0  Warnings: 0

MariaDB [test]> select * from t1 where c1 = '1.5';

+----+------+

| id | c1   |

+----+------+

|  2 |    2 |

+----+------+

1 row in set (0.001 sec)

-- Note: this was unexpected. It looks as if string '1.5' is converted to decimal first (as it should) but then is cast to int (causing the rounding to occur).

-- This seems wrong because index scan vs table scan should not change the semantics of the data type resolution. And it is possible to implement 

-- this so that index is still used but semantics is consistent with table scan path (and documentation) - hence sounds like an oversight? 

MariaDB [test]> select * from t1 ignore index(idx1) where c1 = '1.5';

Empty set (0.001 sec)

-- Note: back to expected when index is not picked (or can also drop the index).

MariaDB [test]> select * from t1 where c1 = '1.5A';

Empty set (Engine=InnoDB), 1 warning (0.001 sec)

MariaDB [test]> show warnings;

+---------+------+------------------------------------------+

| Level   | Code | Message                                  |

+---------+------+------------------------------------------+

| Warning | 1292 | Truncated incorrect DOUBLE value: '1.5A' |

+---------+------+------------------------------------------+