Description of the issue
Assignment describes itself as:

A common super-class that represents all varieties of assignments.

However, it does not actually include UnaryAssignExpr; for example the following cannot be compiled:

import java

from Assignment a
// Error: test is always false, as UnaryAssignExpr is incompatible with the expression type Assignment.
where a instanceof UnaryAssignExpr
select a

In case that is the intended behavior it would be good to clarify this in the Assignment documentation.
If not, then Assignment should probably include UnaryAssignExpr as well. Assignment.getRhs() cannot have a result in this case, so its documentation would have to be adjusted to describe this behavior.