UML25 — UML association semantics vis-a-vis Interfaces (and other types of classifiers)

The semantics of associations are specified in terms of links, which the UML 2.4 spec defines as: "a tuple with one value for each end of the association, where each value is an instance of the type of the end" and the 2.5 spec as "a tuple with one value for each memberEnd of the Association, where each value is an instance of the type of the end". However, the specs include several examples of associations between interfaces. Since interfaces are abstract classifiers they cannot have instances ("Interfaces may not be instantiated"), the semantics of associations whose ends terminate on interfaces cannot be described by the above semantics.

Some other semantics need to be provided for such cases.

Furthermore, the issue extends to other types of classifiers (e.g., collaborations) where the meaning of "instance" is not straightforward.

With regard to Interfaces, the issue relates to the wording “an instance of the type at the end”, which should
be loosened to include “an instance of a type that implements the type at the end”.