[sv-ec] casting between class handles

8.16 (Casting) says,

 

"It is always legal to assign an expression of subclass type to a variable
of a class type higher in the inheritance tree (a superclass or ancestor of
the expression type). 

It shall be illegal to directly assign avariable of a superclass type to a
variable of one of its subclass types. 

However, $cast may be used to assign a superclass handle to a variable of a
subclass type provided the superclass handle refers to an object that is
assignment compatible with the subclass variable."

 

Similarly, it says below,

 

"When $cast is applied to class handles, it succeeds in only three cases:

.

2) The type of the source expression is cast compatible with the destination
type, that is, either:

- the type of the source expression is a superclass of the destination type,
or

.

and the source is an object that is assignment compatible with the
destination type."

 

Question: where is the definition of assignment compatibility between class
objects?

 

Thanks,

Shalom