Nala的Scala类型参数推断失败

Question

scala> class A[T]
defined class A

scala> class B[T](a: A[T])
defined class B

scala> val b = new B(new A[Int])
b: B[Int] = B@1ff8982d

Great! I can create instances of B from instances of A . With one exception. For instances of A[Nothing] the type inference fails.

scala> val b = new B(new A[Nothing])
<console>:9: error: type mismatch;
 found   : A[Nothing]
 required: A[T]
Note: Nothing <: T, but class A is invariant in type T.
You may wish to define T as +T instead. (SLS 4.5)
       val b = new B(new A[Nothing])

Specifying the type manually works.

scala> val b = new B[Nothing](new A[Nothing])
b: B[Nothing] = B@3aad5958

Should I file a bug for it, or is it an intentional trap to scare away programmers that lack sufficient resolve for Scala?

Answer 1

The answer is in the compilation feedback. You need to declare A as being covariant in T, so that when you don't specify the type parameter during the construction of a new B, the compiler can infer that the A[Nothing] may be treated as type T (or treat the A[T] parameter as a T). That's be basic idea behind covariance.

Answer 2

自2008 年以来，这已被人们所熟知并被忽视为问题SI-1570 。假设没有人将Scala用于任何严肃的事情。