Past exam of the year 1 of the computer science course of the University of Oxford / 2017 / Imperative programming / 8 / b

Hypothetical covariance would allow the following unsound alias:

val narrow: MSet[Int] = new MSet[Int](Nil)
val wide: MSet[Any] = narrow       // hypothetical covariant conversion
wide.add("not an integer")
val n: Int = narrow.delete()      // would return the inserted string

The last value cannot satisfy its promised type. This is the covariance failure for a mutable collection: a broader consuming interface could insert values that violate a narrower alias's guarantee. The actual invariant MSet[T] rejects the conversion, and declaring +T while retaining add(x: T) is itself rejected by the variance-position rule. The mutable read-and-write interface must remain invariant in T.