foo.scala

object Obj{
    type T = {
        /** */
        def f : Unit
    }
}

terminal.log

$ scala -version
Scala code runner version 2.7.7.final -- Copyright 2002-2009, LAMP/EPFL

$ scalac foo.scala 
foo.scala:6: error: only declarations allowed here
    def f : Unit
        ^
one error found