inner.scala

scala> class Outer { class Inner; val i = new Inner }
defined class Outer

scala> class Outer { class Inner; val i = new Inner; type I2 = (Inner, Inner) }
defined class Outer

scala> def foo(i: Outer#I2) = ()
foo: (i: (_7.Inner, _7.Inner) forSome { val _7: Outer })Unit

scala> val o1, o2 = new Outer
o1: Outer = Outer@3f6f6701
o2: Outer = Outer@3527942a

scala> foo((o1.i, o2.i))
<console>:15: error: type mismatch;
 found   : (o1.Inner, o2.Inner)
 required: (_7.Inner, _7.Inner) forSome { val _7: Outer }

       foo((o1.i, o2.i))
           ^

scala> foo((o1.i, o1.i))