Covariant type parameter ($S<:T⟹F[S]<:F[T]$)

A parameter position for which $S<:T$ implies $F[S]<:F[T]$. In Scala, it is annotated +. An immutable producer can return narrower values where broader values are expected. A method consuming that parameter directly would generally make covariance unsafe, since a widened reference could then receive unsupported inputs.