An opaque type is a name that denotes an unknown subtype of some given reference type. For example, an opaque subtype of REFANY is an unknown traced reference type; an opaque subtype of UNTRACED ROOT is an unknown untraced object type. The actual type denoted by an opaque type name is called its concrete type.
Different scopes can reveal different information about an opaque type. For example, what is known in one scope only to be a subtype of REFANY could be known in another scope to be a subtype of ROOT.
An opaque type declaration has the form:
TYPE T <: U
where T is an identifier and U an expression denoting a
reference type. It introduces the name T as an opaque type and
reveals that U is a supertype of T. The concrete type of
T must be revealed elsewhere in the program.