= Solution
Give the two components independent <upper type bounds> and immutable fields:
``
final case class BoundedPair[+A <: Base, +B <: Base](first: A, second: B)
``
Each component type must be a <subtype> of `Base`, but the two need not be the same <subtype>. The pair's fields cannot be reassigned. Their read-only use permits covariant <type parameters>; the separate <upper type bounds> constrain admissible instantiations.<Scala 2.13 basic declarations and definitions specification>[Type Parameters and Variance Annotations: upper bounds restrict admissible arguments; immutable output positions permit covariance]{ref}
\b[The component accessors retain their precise static types.] For example, a `BoundedPair[Derived, Base]` exposes its first component as `Derived`, so derived-specific operations are available without a cast. A pair whose two fields are simply declared `Base` discards that distinction. Immutability here concerns the two stored references; it does not guarantee that the referenced <software objects> have immutable internal state.
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