UMLR — Inconsistent approach to type conformance

The specification makes inconsistent statements about checking that communication between objects takes place according to the parameters of the communication mechanisms used.

1. In some places the specification says that UML does not attempt to define a type-safe conformance relationship:

1.1 On p128, in Semantics of Operations, it says:

"Different type-conformance systems adopt different schemes for how the types of parameters and results may vary when an Operation is redefined in a specialization. When the type may not vary, it is called invariance. When the parameter type may be specialized in a specialized type, it is called covariance. When the parameter type may be generalized in a specialized type, it is called contravariance. In UML, such rules for type conformance are intentionally not specified."

1.2 On p308-309 it says:

"A method of an Operation shall have Parameters corresponding to the Parameters of the Operation. ... The data values associated with a request - input Operation parameter values or Signal attribute values - are then passed to a method invoked due to the request via the method parameters. ...

However, no one approach is defined for matching the Parameters of the method to the Parameters of the BehavioralFeature. Possible approaches include exact match (i.e., the type of the corresponding Parameters, order, must be the same), co-variant match (the type of a Parameter of the method may be a subtype of the type of the Parameter of the BehavioralFeature), contra-variant match (the type of a Parameter of the method may be a supertype of the type of the Parameter of the BehavioralFeature), or a combination thereof."

2. On the other hand, in several places the specification does try to define rules for type conformance, with varying levels of success:

2.1 On p158 isConsistentWith is defined on operations, apparently in an attempt to test whether a operation can be redefined in a type-safe way. While the isConsistentWith semantics are broken on at least two counts (see separate issue report), this is an attempt to define a rule for type conformance on operations (albeit with a different name).

2.2 p187: "By declaring a Reception associated to a given Signal, a Classifier specifies that its instances will be able to receive that Signal, or a subtype thereof, and will respond to it with the designated Behavior." i.e. Signals must be passed in a type-conformant way.

2.3 p252: "A Port may be redefined when its containing EncapsulatedClassifier is specialized. The redefining Port may have additional Interfaces to those that are associated with the redefined Port or it may replace an Interface by one of its subtypes." Again, this is type conformance on Ports. There's similar language for redefining Connectors in a type-conformant way on p248.

2.4 p424: "If the ObjectNode is untyped then the Parameters shall also be untyped. Otherwise, the input Parameter shall have either the same type as or a supertype of the ObjectNode, while the output Parameter shall have either the same type or a subtype of the ObjectNode." Again, this is type-conformant.

Conclusion - there are several places where UML does intentionally define rules for type conformance. For consistency, either these type conformance rules should be removed or (preferably), the unhelpful statements (1.1 & 1.2 above) that UML does not enforce type conformance should be removed, and replaced by appropriate definitions of type conformance on Operations (perhaps based on a debugged version of isConsistentWith).