The optimizer removes expressions from GROUP BY/DISTINCT
  if they happen to participate in a <expression> = <const>
  predicates of the WHERE clause (the idea being that if
  it's always equal to a constant it can't have multiple 
  values).
  However for predicates where the expression and the 
  constant item are of different result type this is not
  valid (e.g. a string column compared to 0).
  Fixed by additional check of the result types of the 
  expression and the constant and if they differ the 
  expression don't get removed from the group by list.


mysql-test/r/distinct.result:
  Bug #15881: cast problems
    - test case
mysql-test/t/distinct.test:
  Bug #15881: cast problems
    - test case
sql/sql_select.cc:
  Bug #15881: cast problems
    - can't use <expr>=<const> to remove GROUP BY/DISTINCT cols
      if they're not of the same type.