notuples.scala

scala> import scala.language.existentials
import scala.language.existentials

scala> class Outer { class Inner }
defined class Outer

scala> def f[I <: o.Inner forSome { val o: Outer }](i1: I, i2: I) = ()
f: [I <: o.Inner forSome { val o: Outer }](i1: I, i2: I)Unit

scala> val o1, o2 = new Outer
o1: Outer = Outer@52d455b8
o2: Outer = Outer@4f4a7090

scala>   val i1, i2 = new o1.Inner
i1: o1.Inner = Outer$Inner@3ab39c39
i2: o1.Inner = Outer$Inner@2eee9593

scala>   val i3 = new o2.Inner
i3: o2.Inner = Outer$Inner@3ac3fd8b

scala> f(i1, i2)

scala> f(i1, i3)
<console>:18: error: inferred type arguments [Outer#Inner] do not conform to method f's type parameter bounds [I <: o.Inner forSome { val o: Outer }]
       f(i1, i3)
       ^
<console>:18: error: type mismatch;
 found   : o1.Inner
 required: I
       f(i1, i3)
         ^
<console>:18: error: type mismatch;
 found   : o2.Inner
 required: I
       f(i1, i3)
             ^