编译器似乎在类型优化中忽略了类型绑定

Question

The type refinement in the code below seems to say that the path-dependent type vt.ValueT includes this.type :

trait ValueType {
  type ValueT <: Value

  type ConstrainedT <: ConstrainedValue

  def makeConstrainedValue(v: ValueT): ConstrainedT = ???
}

trait Value {
  type ValueTypeT <: ValueType { type ValueT >: this.type } // <--- HEY, COMPILER, READ THIS

  val vt: ValueTypeT

  def asConstrainedValue = vt.makeConstrainedValue(this) // <--- Compiler complains here
}

trait ConstrainedValue { /* details omitted */ }

but the Scala compiler (version 2.11.2) says:

error: type mismatch;
found   : Value.this.type (with underlying type Test.Value)
required: Value.this.vt.ValueT
   override def asConstrainedValue = vt.makeConstrainedValue(this)
                                                             ^

Is there some reason why deducing that this.type <: vt.ValueT is illegitimate here? Is there another way to tell the compiler what it needs to know?

I've tried putting the type refinement on the declaration of vt . The compiler objects that the resulting type is volatile . Perhaps that's a clue to the problem.

The refinement { type ValueT = this.type } generates the same error message.

Answer 1

I think the problem is that in the bound >: this.type , the binding for this gets muddled somehow by the compiler. If I make the following changes (& remove override from asConstrainedValue ), compilation succeeds for me:

trait Value { self =>
  type ValueTypeT <: ValueType { type ValueT >: self.type }
  …