Scala类型别名破坏了类型兼容性 [英] Scala type aliasing breaks the type compatibility

问题描述

我一直认为，如果需要，类型别名总是会扩展到其原始类型.但是，这是一个麻烦制造者

I always believed that type aliases always expand to their original type if necessary. But, here is a troublemaker

def a[P](a: Option[P]) = {
    type Res = List[P] // result type alias
    Nil: Res // Replace this line with Nil: List[P] to clear the error
}
def b[V](v: V) = a(Some(v)): List[V] 

它失败，并显示( scastie )

error: type mismatch;
found   : Res (which expands to)  List[P]
required: List[V]

您看到a转换了Option[P] => List[P]，并且由于b提供了Some[V]，因此a在b调用时转换了Option[V] => List[V].但是，编译器说结果与List[V]不兼容.这怎么可能?如果将Nil: Res替换为a中的Nil: List[P]，则错误消失( scastie ).您需要消除类型别名以消除该错误.这意味着类型别名是罪魁祸首.

You see that a converts Option[P] => List[P] and, since b supplies Some[V], a converts Option[V] => List[V] when b calls it. But, compiler says that result is incompatible with List[V]. How is this possible? The error goes away (scastie) if you replace Nil: Res with Nil: List[P] in the a. You need to eliminate the type alias to get rid of the error. This means that the type alias is the culprit.

推荐答案

我几乎可以肯定这是一个编译器错误. Scala中的类型别名应该自动扩展，在这种情况下，看起来a的类型被推断为[P](Option[P]) => Res而不是[P](Option[P]) => List[P].而且由于Res在内部范围内，因此编译器无法找到它来正确推断b的类型.

I'm almost certain this is a compiler bug. Type aliases in Scala are supposed to automatically expand, and it looks like in this case the type of a is inferred as [P](Option[P]) => Res, instead of [P](Option[P]) => List[P]. And since Res is in an inner scope, the compiler can't find it to infer the type of b correctly.

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