A marker for annotations that, when applied to a type,
should be treated as a constraint on the annotated type.
A proper constraint should restrict the type based only
on information mentioned within the type. A Scala compiler
can use this assumption to rewrite the contents of the
constraint as necessary. To contrast, a type annotation
whose meaning depends on the context where it is written
down is not a proper constrained type, and this marker
should not be applied. A Scala compiler will drop such
annotations in cases where it would rewrite a type constraint.