问题描述
@uncheckedVariance 可用于弥补Scala的声明，网站方差注释和Java的泛型不变之间的差距。
@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
这说了java.util.Comparator自然是禁忌的变体，即 T 出现在参数和从来没有在返回类型的类型参数。
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.
这引出了一个问题：为什么它也以不从Java接口扩展了Scala集合库使用
This begs 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]
什么是有效的用途这个注释？
What are the valid uses for this annotation?
推荐答案
问题在于GenericTraversableTemplate使用了两次：一次是针对可变集合（其中，其类型参数应该是不变的），并且一旦为不可改变的集合（其中，协方差是不可避免王）。
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的typechecks假设无论是协方差或不变性为A类型参数。然而，当我们在一个可变的特点继承它，我们必须选择不变性。相反，我们希望协方差一个不变的子类。
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.
由于我们可以通过在GenericTraversableTemplate方差注释（尚未;-)）不是抽象的，这样我们就可以在实例它任何一个依赖于子类，我们不得不诉诸铸造（@uncheckVariance本质上是一种-投）。为进一步阅读，我建议我的论文（对不起;-)）或我们最近的<一个href=\"http://scholar.google.com/scholar?q=%22Fighting+bit+Rot+with+Types+%28Experience+Report+Scala+Collections%29.%22\">bitrot纸
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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