Scala：为什么不能从复合类型成员创建匿名类？

Question

trait Trait1
trait Trait2

object O {
  type Type1 = Trait1
  type Type2 = Trait1 with Trait2

  trait TraitMember1 extends Type1
  // this line fails to compile
  trait TraitMember2 extends Type2

  val anon1 = new Trait1 {}
  val anon2 = new Type1 {}

  val anon3 = new Trait1 with Trait2 {}
  // this line fails as well
  val anon4 = new Type2 {}
}

When you try to compile the code above, the commented lines fail with error class type required but Trait1 with Trait2 found . Why is that? I can manually mixin all traits in the compound member and create an anonymous class just fine.

Answer 1

In http://www.scala-lang.org/old/node/6817.html , Martin explained:

I agree it might be useful. It's not quite clear what it should mean, though. Expand all aliases, drop parens and then do linearization? Or do the parents affect linearization, and, if, yes, how? we took the easy way out and outlawed these idioms.

While that sort of complexity could well be sorted out in the spec and the compiler, the end result might lead to suprises when the spec doesn't align with the user's mental model.