scala> class W[C[_], A](c: C[A])
defined class W

scala> def f[C[_], A](c: C[A]) = new W(c)
<console>:6: error: kinds of the type arguments (C[_],A) do not conform to the expected kinds of the type parameters (type C,type A) in class W.
C[_]'s type parameters do not match type C's expected parameters: type C has one type parameter, but type C (in class W) has one
       def f[C[_], A](c: C[A]) = new W(c)
           ^
<console>:6: error: kinds of the type arguments (C[_],A) do not conform to the expected kinds of the type parameters (type C,type A) in class W.
C[_]'s type parameters do not match type C's expected parameters: type C has one type parameter, but type C (in class W) has one
       def f[C[_], A](c: C[A]) = new W(c)
                                     ^

scala> def f[C[_], A](c: C[A]) = new W[C, A](c)
f: [C[_],A](c: C[A])W[C,A]

scala> case class W[C[_], A](c: C[A])          
defined class W

scala> def f[C[_], A](c: C[A]) = W(c)    
f: [C[_],A](c: C[A])W[C,A]

scala> def g[C[_], A](c: C[A]) = f(c)
g: [C[_],A](c: C[A])W[C,A]