This script:

DROP TABLE IF EXISTS t1;

CREATE TABLE t1 (a YEAR);

INSERT INTO t1 VALUES (2010),(2011);

SELECT a=10 AND NULLIF(a,2011.1)='2011' AS cond FROM t1;

returns

+------+

| cond |

+------+

|    0 |

|    0 |

+------+

Now if I put the same expression into WHERE:

SELECT * FROM t1 WHERE a=10 AND NULLIF(a,2011.1)='2011';

it erroneously returns one row:

+------+

| a    |

+------+

| 2010 |

+------+

The expected result is to have the expression evaluate into the same result in the SELECT list and in WHERE.

The problem is that Item_func_nullif::const_item() returns true and its val_real() is called from eval_const_cond().
In fact, it has constant items in args[0] and args[1]:

(gdb) p args[0]->const_item()

$14 = true

(gdb) p args[1]->const_item()

$15 = true

but the returned value referenced by m_args0_copy is not a constant item:

(gdb) p this->m_args0_copy

$17 = (Item_field *) 0x7fff98001668

The return argument should probably be stored in arg[2] instead, so the standard Item_func methods can see it and update Used_tables_cache taking into account the return value (not only the compared values).