Query handling of CAST only caters for some situations, and doesn't always add a discriminator clause

[andyjefferson]

If we have something like
class Base (SINGLE_TABLE, with DISCRIMINATOR)
class Sub extends Base (SUPERCLASS_TABLE)

and we have a query like

SELECT b FROM Base b WHERE (TREAT(b) AS Sub).someField = value

then this currently ignore the TREAT (cast) whereas it should add a DISCRIMINATOR clause.

The problem is that we can only add the discriminator clause to a BooleanExpression ... i.e let it propagate back up to the
{...}.someField = value
and add the discriminator constraint there.

[andyjefferson]

See ObjectExpression.cast(). Near the end there is a block "if (castSqlTbl == table)". We need to allow ObjectExpression to have a BooleanExpression adding (when part of FILTER), and then when we evaluate a BooleanExpression from the ObjectExpression (e.g in ObjectExpression = value) we then add the BooleanExpression the ObjectExpression has attached.