Cannot override a type with non-volatile upper bound

Question

I have a compiler error in scala and I don\'t know what does it refer to:
Assume these declarations:

trait Abstract {
  type MyType
}
trait AInner
trait         


        

Answer 1

You can rewrite trait B with (more later about your goal, which, I think, is a bit different)

trait B extends A {
  type MyType <: BInner with AInner
}

And this makes total sense. A value of type B#MyType can be seen as either a BInner or a AInner.

You don't need to repeat Abstract because A is already a subclass of Abstract. You don't have to write override as this is implicit for a type declaration. So the question is why A#MyType is not working as AInner?

Here is what the scala language spec says about volatile types.

3.6 Volatile Types

Type volatility approximates the possibility that a type parameter or abstract type instance of a type does not have any non-null values. As explained in (§3.1), a value member of a volatile type cannot appear in a path. A type is volatile if it falls into one of four categories: A compound type T1 with ... with Tn {R } is volatile if one of the following two conditions hold. 1. One of T2, ..., Tn is a type parameter or abstract type, or 2. T1 is an abstract type and and either the refinement R or a type Tj for j > 1 contributes an abstract member to the compound type, or 3. one of T1, ..., Tn is a singleton type. Here, a type S contributes an abstract member to a type T if S contains an abstract member that is also a member of T . A refinement R contributes an abstract member to a type T if R contains an abstract declaration which is also a member of T . A type designator is volatile if it is an alias of a volatile type, or if it designates a type parameter or abstract type that has a volatile type as its upper bound. A singleton type p.type is volatile, if the underlying type of path p is volatile. An existential type T forSome {Q } is volatile if T is volatile.

Other important item mentioned by the spec is about abstract type overriding:

Another restriction applies to abstract type members: An abstract type member with a volatile type (§3.6) as its upper bound may not override an abstract type member which does not have a volatile upper bound.

The compiler error is:

error: overriding type MyType in trait A with bounds <: AInner;
type MyType is a volatile type; cannot override a type with non-volatile upper bound

This is consistent with the spec. BInner with A#MyType is volatile. Before that MyType had a non-volatile as Any.

The matter is that a type in the scala type system must have a unique meaning. An abstract type can be thought as a type which declaration is deferred to a subclass. Therefore there is no problem for declaring values of an abstract type when it is still abstract. On the other hand if we have a type like BInner with A#MyType, this type may have several meaning. It is called volatile and it does not makes sense to have a non null value of this type, as it could have as many types as subclasses instantiating the MyType abstract type. To simplify things, we could think of a volatile type as a type not being a subtype of Any (and volatile as being a subtype Any). We therefore have a contradiction that the compiler mentions.

Coming back to your goal, which you stated as

What I'm trying to achieve here(in trait B) is to further restrict the type MyType declared > in Abstract, so any value of type MyType must extend all the MyTypes in the mixin tree.

You can achieve this thanks to inner traits like this.

trait Abstract {
  type MyType
}
trait B extends Abstract {
  trait MyType {
    def bMethod : Int
  }
}
trait A extends B {
  trait MyType extends super.MyType {
  }
}

Well I hope this somewhat what you're looking for.