it doesn't select.

This bug was fixed along with bug #16861: User defined variable can 
have a wrong value if a tmp table was used.

There the fix consisted of Item_func_set_user_var overloading the method
Item::save_in_field. Consider the query from the test case:


INSERT INTO foo( bar, baz )
SELECT 
  bar,
  @newBaz := 1 + baz
FROM 
  foo
WHERE 
  quux <= 0.1;

Here the assignment expression '@newBaz := 1 + baz' is represented by an 
Item_func_set_user_var. Its member method save_in_field, which writes the 
value of this assignment into the result field, writes the val_xxx() value, 
which is not updated at this point. In the fix introduced by the patch,
the save_in_field method reads the actual variable value instead.

See also comment for 
ChangeSet@1.2368.1.3, 2007-01-09 23:24:56+03:00, evgen@moonbone.local +4 -0
and comment for
Item_func_set_user_var::save_in_field (item_func.cc)