Scala Abstract Type Member

Question

I noticed that I can instantiate a scala trait with an abstract type member. The code below compiles. But what is the tB?

trait A {
    type B
}

val t = new A {}

Answer 1

The type is tB .

trait A {
  type B
  def f(b: B)
}

val t = new A { def f(b: B) = {} }

t.f(0)

has the error

error: type mismatch;
found   : Int(0)
required: t.B

Types don't have to be "overriden" like methods.

This type is its own thing. It's not very useful, but that's what it is.

Like all other types, it is a subtype of Any and a supertype of Nothing .

Seq[t.B](): Seq[Any]
Seq[Nothing](): Seq[t.b]

And that's about all that can be said about it.