Should work with 0.20.0-RC1

fmap.scala

// Compiles with https://github.com/lampepfl/dotty/pull/4672
// ./bin/dotc -classpath "$(coursier fetch -p org.typelevel:cats-core_2.13:2.0.0)" out/fmap.scala
import cats._
import cats.implicits._

object Test {
  val fmap: [A, B] -> (A => B) => [F[_]] -> F[A] => (given Functor[F]) => F[B] = [A, B] -> (f: A => B) => ([F[_]] -> (fa: F[A]) =>  (given ev: Functor[F]) => fa.map(f))

  val addOne = fmap((x: Int) => x + 1)

  val x = addOne(Option(42))
  val y = addOne(List(1,2,3))
}

This does not seem to work anymore with dotty 0.20.0-RC1:

val fmap: [A, B] => (A => B) => ([F[_]] => F[A] => given Functor[F] => F[B]) = [A, B] => (f: A => B) => ([F[_]] => (fa: F[A]) => given (ev: Functor[F]) => ev.map(fa)(f))

https://scastie.scala-lang.org/kwark/7sqzH8DqRnmUVKmFpOUdRw

@kwark Parens are required around given xxx, I've updated the gist.