into core expression.   we need to make a sql.elements.Slice object,
which isn't a SQL standard thing; however there's no other place to
put this as long as we want slice handling to have some meaning
in Core.  Basically any other backend that ever wants to use the []
operator, I don't want to re-implement everything for PG on that
backend as well.   this will need to have backend-agnostic tests
in tests/sql/test_operator.py.

- test for .cast() method has no good place now except for
test_cast in test_compiler.py

in conjunction with :func:`.and_` or :func:`.or_` would fail
with an AttributeError.
fixes #3490

of :class:`.FunctionElement` or other column element that incorrectly
states 'None' or any other invalid object as the ``.type``
attribute will report this exception instead of recursion overflow.
fixes #3485

object would fail to accommodate the labeled SQL expression
in all cases, such that any SQL operation that made use of
:meth:`.Label.self_group` would use the original unadapted
expression.  One effect of this would be that an ORM :func:`.aliased`
construct would not fully accommodate attributes mapped by
:obj:`.column_property`, such that the un-aliased table could
leak out when the property were used in some kinds of SQL
comparisons.
fixes #3445

(hence becoming two regressions); reports that
SELECT statements would GROUP BY a label name and fail was misconstrued
that certain backends such as SQL Server should not be emitting
ORDER BY or GROUP BY on a simple label name at all; when in fact,
we had forgotten that 0.9 was already emitting ORDER BY on a simple
label name for all backends, as described in :ref:`migration_1068`,
as 1.0 had rewritten this logic as part of :ticket:`2992`.

In 1.0.2, the bug is fixed both that SQL Server, Firebird and others
will again emit ORDER BY on a simple label name when passed a
:class:`.Label` construct that is expressed in the columns clause,
and no backend will emit GROUP BY on a simple label name in this case,
as even Postgresql can't reliably do GROUP BY on a simple name
in every case.
fixes #3338, fixes #3385