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Editorial Team
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Asked: 2026-05-14T01:10:34+00:00

@uncheckedVariance can be used to bridge the gap between Scala’s declaration site variance annotations

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@uncheckedVariance can be used to bridge the gap between Scala’s declaration site variance annotations and Java’s invariant generics.

scala> import java.util.Comparator    
import java.util.Comparator

scala> trait Foo[T] extends Comparator[T]
defined trait Foo

scala> trait Foo[-T] extends Comparator[T]     
<console>:5: error: contravariant type T occurs in invariant position in type [-T]java.lang.Object with java.util.Comparator[T] of trait Foo
       trait Foo[-T] extends Comparator[T]
             ^

scala> import annotation.unchecked._    
import annotation.unchecked._

scala> trait Foo[-T] extends Comparator[T @uncheckedVariance]    
defined trait Foo

This says that java.util.Comparator is naturally contra-variant, that is the type parameter T appears in parameters and never in a return type.

This raises the question: why is it also used in the Scala collections library which doesn’t extends from Java interfaces?

trait GenericTraversableTemplate[+A, +CC[X] <: Traversable[X]] extends HasNewBuilder[A, CC[A] @uncheckedVariance]

What are the valid uses for this annotation?

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1 Answer

  1. Editorial Team

    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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