 Covariance failure for a mutable collection

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.