A routine in class A does an assignment attempt whose target is an
attribute of type B and is the routine's class.  A descendant A1 of
class A redefines the type of the attribute to be a descendant of B.
When the routine is executed in A1 with a passed-in argument that is
used as the source of the assignment attempt, it incorrectly allows
the assignment attempt to succeed.

Reported by Manu on Release 5.1.12.