Failed implicit resolution for Nothing with <:<

问题

The following code does not compile on Scala 2.12 / 2.13. Why?

class X[U, T]

object X {
  implicit def genericX[U, T](implicit ev: T <:< U): X[U, T] = new X[U, T]
}

implicitly[X[AnyRef, String]]  // compiles
implicitly[X[String, Nothing]] // does not compile

回答1:

Long story short, compiler doesn't like to infer Nothing in implicits.

Why doesn't Scala's implicit class work when one of the type parameters should be Nothing?

Implicit error when trying to implement the `Absurd` typeclass

https://www.reddit.com/r/scala/comments/73791p/nothings_twin_brother_the_better_one/

http://guillaume.martres.me/talks/typelevel-summit-oslo/?fbclid=IwAR1yDSz-MetOgBh0uWMeuBuuL6wlD79fN_4NrxAtl3c46JB0fYCYeeGgp1Y#/9 (slide 10 "The fear of Nothing")

https://www.youtube.com/watch?v=YIQjfCKDR5A?t=459 (7:39)

https://www.youtube.com/watch?v=lMvOykNQ4zs

The fear of Nothing

• scalac instantiates eagerly, even when it isn't safe

• Counterbalanced by never inferring Nothing

  class Foo[T] { def put(x: T) = {} }
  (new Foo).put("") // T? = String
  

• Fails if the lower bound isn't Nothing

  class Foo[T >: Null] { def put(x: T) = {} }
  (new Foo).put("") // T? = Null
  // type mismatch: Null does not match String
  

• Sometimes you really want to infer Nothing!

  class Foo[T]
  def foo[T](x: Foo[T]) = x
  foo(new Foo[Nothing]) // error
  

Workaround is to introduce type Bottom

type Bottom <: Nothing

implicitly[Bottom =:= Nothing]
implicitly[Nothing =:= Bottom]

implicitly[X[AnyRef, String]]  // compiles
// implicitly[X[String, Nothing]] // does not compile
implicitly[X[String, Bottom]] // compiles

来源：https://stackoverflow.com/questions/61951621/failed-implicit-resolution-for-nothing-with