Is it possible to find a common supertype on type-system level in Scala?

Question

Is it possible to make a type-alias (or something equivalent) in Scala that takes two parameters and returns their common supertype? In other words, I'm trying to find something with this signature:

type CommonSupertype[A, B] // can't be more specific

where these hold true: (pseudocode)

CommonSupertype[String, Int] = Any
CommonSupertype[JButton, JPanel] = JComponent

etc.

I haven't been able to find it myself, but I can't use alternatives like adding an extra parameter, since I have to follow a pre-specified interface.

Answer 1

(Not a complete solution, but might give some ideas)

One impressive feature of Scala is its ability to return a list of Fruits when an orange is appended to a list of apples. It's fine with values, precisely because you let the generic type be inferred.

import scala.reflect.Manifest

def CommonSuperType[A, B >: A : Manifest](a:A, b:B) = manifest[B]  

It works (kind of) :

scala> CommonSuperType(new JButton, new JPanel)
res42: Manifest[javax.swing.JComponent with javax.accessibility.Accessible] = javax.swing.JComponent with javax.accessibility.Accessible

Next step would be to lift this trick to higher kinded types (not tested).
An half baked solution consists in creating values from types (cf this answer) :

class CommonSuper[A:Manifest, B:Manifest] {
   def make[T:Manifest] = manifest[T].erasure.newInstance.asInstanceOf[T]
   val instanceA = make[A]
   val instanceB = make[B]
   def getType = CommonSuperType(instanceA, instanceB)
}   

But I'm stuck in this unintuitive inconsistency :

scala> val test = new CommonSuper[JButton, JPanel]

scala> test.getType
res66: Manifest[Any] = Any

scala> CommonSuperType(test.instanceA, test.instanceB)
res67: Manifest[javax.swing.JComponent with javax.accessibility.Accessible] = javax.swing.JComponent with javax.accessibility.Accessible

Anyway, whereas I'm fond of this type of questions (questions about types), here it smells like an XY Problem.