Queries like:

  SELECT ROW(1, 2) IN (SELECT t1.a, 2)
    FROM t1 GROUP BY t1.a

or 

  SELECT ROW(1, 2) IN (SELECT t1.a, 2 FROM t2)
    FROM t1 GROUP BY t1.a

lead to assertion failure in the
Item_in_subselect::row_value_transformer method in debugging
build, or to unexpected error message in release build:

  ERROR 1247 (42S22): Reference '<list ref>' not supported (forward
                      reference in item list)

Unexpected error message and assertion failure have been
eliminated.