Query: fix edge cases around Cast/OfType result operator lifting

[maumar]

There are some issues around result operator lifting, specifically Cast and OfType:

1.) cast used as type validation doesn't always throw when it should, e.g.:

from g in ctx.Gears.Cast<Officer>().Cast<Gear>()
select g.Nickname

when run on the client it will throw, because not every gear is an officer. However it works fine on sql sever, even though during QM optimization we turn this to:

from g in ctx.Gears
select ((Gear)((Officer)g)).Nickname

2.) Before we try to lift cast, we handle OfType, by updating the type on EntityQueryable directly, e.g:

from o in (from g in EntityQueryable<Gear> select g).OfType<Officer>()
select o.NumberOfReports

will get converted to:

from o in EntityQueryable<Officer>
select o.NumberOfReports

This however breaks down if Cast is introduced to the mix:

from o in (from g in EntityQueryable<Gear> select g).Cast<Gear>().OfType<Officer>()
select o.NumberOfReports

will fail, because we process OfType first:

from o in (from g in EntityQueryable<Officer> select g).Cast<Gear>()
select o.NumberOfReports

which then gets lifted to:

from o in (from g in EntityQueryable<Officer> select g)
select ((Gear)o).NumberOfReports

which is wrong

[maumar]

2.) can be addressed by removing all Cast operators that come before the given OfType, but that potentially makes 1) worse

[ajcvickers]

@maumar Can you write down some of the ideas we discussed in triage?

[maumar]

We should force client eval on Cast operator to mimic its type asserting functionality. However, some queries inject unnecessary casts, e.g.

from Customer c in ctx.Customers
select c.Name

we should strip casts for those cases (where the cast type is the same as element type of EntityQueryable

[smitpatel]

1. would be irrelevant now that we don't have client eval.

[smitpatel]

2nd case is fixed in 3.1 release.