Why when I JOIN to a casted field in an index view, the execution plan warns about "type conversion in expression"?
An indexed view is expanded during query compilation unless the NOEXPAND table hint is used. The warning is generated on that expansion. In Enterprise Edition, the optimizer may later match the expansion back to the materialized view. This is a cost-based decision.

In anything other than Enterprise Edition (or equivalent) you must use a NOEXPAND hint to use the index(es) on a materialized view.

In Enterprise Edition, there are still good reasons to use the NOEXPAND hint:


Indexed Views and Statistics
Another Reason to Use NOEXPAND hints in Enterprise Edition