scala - 混合隐式、类型参数和Nothing时编译错误

Question

为什么这不能用类型参数编译:

error: value explode is not a member of Test.A[Nothing]

如果我删除它们，那么它会编译。我不明白什么，更重要的是，我能做些什么来解决它。

object Test extends App {

  implicit class B[E](set: A[E]) {
    def explode() = println("boom")
  }

  case class A[E](name: String)

  A[Nothing]("coldplay").explode()
}

(这个例子中的类型参数不做任何事情，但在现实世界的用例中我有多个类型参数，有些可以是 Nothing ，有些不是)。

Answer 1

它真的不喜欢推断 Nothing:

scala> implicit def a2b(a: A[Nothing]): B[Nothing] = new B(a)
<console>:17: error: type mismatch;
 found   : A[Nothing]
 required: A[T]
Note: Nothing <: T, but class A is invariant in type E.
You may wish to define E as +E instead. (SLS 4.5)
       implicit def a2b(a: A[Nothing]): B[Nothing] = new B(a)
                                                           ^

scala> implicit def a2b(a: A[Nothing]): B[Nothing] = new B[Nothing](a)
warning: there were 1 feature warning(s); re-run with -feature for details
a2b: (a: A[Nothing])B[Nothing]

scala> A[Nothing]("coldplay").explode()
boom

-Ytyper-debug :

|    |    |    |    |    |    |    solving for (T: ?T)
|    |    |    |    |    |    |    |-- $iw.this.X.B BYVALmode-EXPRmode-FUNmode-POLYmode (silent solving: type T: method f in X) implicits disabled
|    |    |    |    |    |    |    |    [adapt] [T](set: $line3.$read.$iw.$iw.X.A[T])$line3.$read.$iw.$iw... adapted to [T](set: $line3.$read.$iw.$iw.X.A[T])$line3.$read.$iw.$iw...
|    |    |    |    |    |    |    |    \-> (set: X.A[T])X.B[T]
|    |    |    |    |    |    |    solving for (T: ?T, T: ?T)
|    |    |    |    |    |    |    \-> <error>
<console>:10: error: value explode is not a member of X.A[Nothing]
       def f() = A[Nothing]("coldplay").explode() }