Underscores in type bounds on type constructors

问题

Can someone explain why the following doesn't compile? I want that BB[A] is also a List[A]. The method body only enforces this view.

scala> def x[A, BB[_] <: List[_]](p: BB[A]) {p: List[A]}
<console>:8: error: type mismatch;
 found   : BB[A]
 required: List[A]

       def x[A, BB[_] <: List[_]](p: BB[A]) {p: List[A]}
                                             ^

回答1:

I think you need to name the _ parameter.

scala> def x[A, BB[X] <: List[X]](p: BB[A]) {p: List[A]}

works.

来源：https://stackoverflow.com/questions/6492934/underscores-in-type-bounds-on-type-constructors