When is @uncheckedVariance needed in Scala, and why is it used in GenericTraversableTemplate?

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The problem is that GenericTraversableTemplate is used twice: once for mutable collections (where its type parameter should be invariant), and once for immutable collections (where covariance is invariably king).

GenericTraversableTemplate’s typechecks assuming either covariance or invariance for the A type parameter. However, when we inherit it in a mutable trait, we have to pick invariance. Conversely, we’d like covariance in an immutable subclass.

Since we can’t abstract over the variance annotation (yet ;-)) in GenericTraversableTemplate, so that we could have instantiated it to either one depending on the subclass, we have to resort to casting (@uncheckVariance is essentially a kind-cast). For further reading, I recommend my dissertation (sorry ;-)) or our recent bitrot paper

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