Why?

gistfile1.txt

scala> case class Pair[A,B](head: A, tail: B)

defined class Pair



scala> type HeadlessPair[B] = Pair[Any, B] // Setting it to any, so the compiler wont care when type view is accessed

defined type alias HeadlessPair

scala> def f[A[_], B](ab: A[B]) {}

warning: there were 1 feature warning(s); re-run with -feature for details


f: [A[_], B](ab: A[B])Unit



scala> val headless: HeadlessPair[String] = Pair("begone", "tail")

headless: HeadlessPair[String] = Pair(begone,tail)

scala> f(headless)
<console>:13: error: no type parameters for method f: (ab: A[B])Unit exist so that it can be applied to arguments (Pair[_$1,String])
 --- because ---
argument expression's type is not compatible with formal parameter type;
 found   : Pair[_$1,String] where type _$1
 required: ?A
              f(headless)
              ^
<console>:13: error: type mismatch;
 found   : Pair[_$1,String] where type _$1
 required: A[B]
              f(headless)
                ^