= 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.
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