UMLR — Optional values and evaluation of defaults

7.3.44 says that the defaultValue is "A ValueSpecification that is evaluated to give a default value for the Property when an object of the owning Classifier is instantiated."
This makes it a bit pointless to have such as property as optional since it will always be set to its default value (though in theory the property could be explicitly unset - in which case it will not use the default value either.
There is a need for dynamic evaluation of an expression associated with a Property if no value has been explicitly set.

Proposed resolution:

• if lower = 1 then defaultValue is assigned to the property when object is instantiated (and when the class is attached to an object using reclassify)
• if lower = 0 then no assignment is made but the defaultValue (if present) is evaluated each time the property value is read: in effect it is acting as a derived property until a value is explicitly assigned.

This behavior must also be included in the description of ReadStructurualFeatureAction (11.3.37) which does not mention default values at all.