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.

Suppose a mutable collection of narrow-type values could be viewed as a mutable collection of a wider type. Inserting a wider-only value through that alias would make a subsequent narrow-type read invalid. For example, hypothetical covariance would let one insert a string through an Any-typed alias of an integer multiset. Generic type invariance prevents that substitution; making a parameter covariant does not authorize an unsound consuming method.