A customer reported their Encounter profile created in Forge gave the following error in the Java validator:

Error @ StructureDefinition.snapshot.element[39].binding : The element Encounter.length has a binding, but no bindable types are present [Duration]

eld-11:

eld-11 Binding can only be present for coded elements, string, and uribinding.empty() or type.code.empty() or type.select((code = 'code') or (code = 'Coding') or (code='CodeableConcept') or (code = 'Quantity') or (code = 'string') or (code = 'uri')).exists()

 
Example resource created by Matthijs attached, which only puts a must support constraint on Encounter.identifier, with Snapshot generated in Forge: MyEncounter.StructureDefinition.xml
 
Ewout Kramer says:
And Duration is a subclass of Quantity - so this is bindeable.
This is very subtle. Duration is a subclass of Quantity, but in FHIR this kind of polymorphism does not work: you cannot use a Duration where a Quantity is specified in the definition.  However, I think the idea here is that the bindeability aspect of Quantity is inherited by Duration. However, eld-11 would not allow Duration to be bindeable. (edited)