WHERE clause not accepted on derived expression columns

A very simple query involving an expression column with a derived name in the WHERE clause fails:

ij> select a+b as s from t where s > 2;
ERROR 42X04: Column 'S' is either not in any table in the FROM list or appears within a join specification and is outside the scope of the join specification or appears in a HAVING clause and is not in the GROUP BY list. If this is a CREATE or ALTER TABLE statement then 'S' is not a column in the target table.

Noticed this while working on DERBY-2998, and AFAIK this is a valid query and a limitation in derby.

There is a workaround for this using a nested select (output from a test table):
ij> select * from (select a+b as s from t) as t(s) where s > 2;
S
-----------
10
10
10

3 rows selected