t8736-c.scala:4: warning: higher-kinded type should be enabled
by making the implicit value scala.language.higherKinds visible.
This can be achieved by adding the import clause 'import scala.language.higherKinds'
or by setting the compiler option -language:higherKinds.
See the Scaladoc for value scala.language.higherKinds for a discussion
why the feature should be explicitly enabled.
  def hk[M[_]] = ???
         ^
error: No warnings can be incurred under -Xfatal-warnings.
one warning found
one error found