Scala: expression type incompatible with formal parameter type

trait Ingredient{}

case class Papperoni() extends Ingredient{}
case class Mushroom() extends Ingredient{}

trait ToppingDef[T] {
}

object PepperoniDef extends Serializable with ToppingDef[Papperoni] {
}

object MushroomDef extends Serializable with ToppingDef[Mushroom] {
}

class Oven[T <: Ingredient](val topping:ToppingDef[T]) {
}

class Pizza {
  def cook = {
    val topping =
      if(someCondition()) { PepperoniDef }
      else { MushroomDef}

    new Oven(topping) // <-- build error here
  }
}

      

        
        
        
      

    

I am using Scala 2.11. This example is somewhat contrived, but I've separated all unrelated ones to give a concise example.

Error on the last line:

Error:(26, 5) no type parameters for constructor Oven: (topping: ToppingDef[T])Oven[T] exist so that it can be applied to arguments (Serializable with ToppingDef[_ >: Papperoni with Mushroom <: Product with Serializable with Ingredient])
 --- because ---
argument expression type is not compatible with formal parameter type;
 found   : Serializable with ToppingDef[_ >: Papperoni with Mushroom <: Product with Serializable with Ingredient]
 required: ToppingDef[?T]
    new Oven(topping)

      

        
        
        
      

    

However, changing the last line to this, for example:

new Oven(PepperoniDef)

      

        
        
        
      

    

builds perfectly. Therefore, the compiler has no problem finding the type when the parameter is passed explicitly like this.

Also, remove the trait Serializable

from PepperoniDef

and MushroomDef

as follows:

object PepperoniDef extends ToppingDef[Papperoni] {
}

object MushroomDef extends ToppingDef[Mushroom] {
}

      

        
        
        
      

    

also builds. However, in my case, I need Serializable.

I think that maybe I can rebuild the code to work around this if needed, but I would like to understand what is going on, I do not know why this type is ambiguous in the first case or why the presence of the Serializable symbol has any effect ... Thanks in advance for any ideas.

EDIT: Thanks for the answers, very helpful. I think the most concise solution is to change this:

val topping =

      

        
        
        
      

    

:

val topping:ToppingDef[_ <: Ingredient] =

      

        
        
        
      

    

Which cures the build error and doesn't need to change for the generic classes, which I would like to keep as simple as possible, without constraints, so that Scala will output as much type information as possible.

This does not answer the question of why presence Serializable

influences this.