Can't define case class field of PolyFunction type

[Odomontois]

Compiler version

3.0.0-RC1

Minimized code

case class Baz[F[_], G[_]](f: [B] => F[B] => G[B])

Output

Found:    (Baz.this.f : PolyFunction{apply: [B](x$1: F[B]): G[B]})
Required: PolyFunction{apply: [B](x$1: <?>): <?>}