 Solution

A covariant type parameter preserves subtyping: if $A<:B$, then PartialFn[T,A] <: PartialFn[T,B] when its second parameter is covariant. In Scala the class annotation is +U. A mapping that produces a narrower output can be used where a broader output is expected. Covariance describes a type relationship; it does not convert stored values or make mutation through a widened reference safe.^[ref]