Polymorphic conversions not being utilized

[TimWSpence]

Compiler version

3.0.1

Minimized code

import scala.language.implicitConversions

trait FunctionK[F[_], G[_]] {
  def apply[A](fa: F[A]): G[A]
}

val x: FunctionK[Option, Option] = new FunctionK[Option, Option] {
  def apply[A](fa: Option[A]): Option[A] = fa
}

given [F[_], G[_]]: Conversion[FunctionK[F, G], [A] => F[A] => G[A]] with
  def apply(f: FunctionK[F, G]): [A] => F[A] => G[A] = [A] => (fa: F[A]) => f(fa)

val y: [A] => Option[A] => Option[A] = x

Output

-- [E007] Type Mismatch Error: test.scala:14:39 --------------------------------
14 |val y: [A] => Option[A] => Option[A] = x
   |                                       ^
   |           Found:    (x : FunctionK[Option, Option])
   |           Required: PolyFunction{apply: [A](x$1: Option[A]): Option[A]}

longer explanation available when compiling with `-explain`
1 error found

Expectation

My expectation is that this conversion should be applied? I tried a monomorphic variation:

import scala.language.implicitConversions

case class Foo()
case class Bar()

given Conversion[Foo, Bar] with
  def apply(f: Foo): Bar = Bar()

val foo: Foo = Foo()
val bar: Bar = foo

and that compiled successfully

[odersky]

Polymorphic function types generally require more help for type inference, so this is not unexpected.

[TimWSpence]

Many thanks @odersky! Until we get a scala 3 native version of cats, I think this would be a big win to have conversions between polymorphic functions and FunctionK. But I appreciate that there are many, many priorities 😅

And many, many thanks for scala 3!!!