Remove macro in FunctionK.lift implementation

Instead, use the trick proposed by Alex in
typelevel#2553 (comment),
this is perhaps not ideal but it seems simpler than maintaining two
separate source files for Scala 2 and Dotty, just for this one function.
Additionally, since Dotty is able to call into non-macro methods in
Scala 2 dependencies, this makes FunctionK.lift usable from
Dotty without cats havin to publish Dotty artifacts.

Co-authored-by: Alexander Konovalov <alex.knvl@gmail.com>

core/src/main/scala/cats/arrow/FunctionK.scala

@@ -1,7 +1,6 @@
 package cats
 package arrow
 
-import scala.reflect.macros.blackbox.Context
 
 import cats.data.{EitherK, Tuple2K}
 
@@ -78,85 +77,15 @@ object FunctionK {
    *   val lifted: FunctionK[List, Option] = FunctionK.lift(headOption)
    * }}}
    *
-   * Note: This method has a macro implementation that returns a new
-   * `FunctionK` instance as follows:
-   *
-   * {{{
-   *   new FunctionK[F, G] {
-   *     def apply[A](fa: F[A]): G[A] = f(fa)
-   *   }
-   * }}}
-   *
-   * Additionally, the type parameters on `f` must not be specified.
+   * Note: The weird `τ[F, G]` parameter is there to compensate for
+   * the lack of polymorphic function types in Scala 2.
    */
-  def lift[F[_], G[_]](f: (F[α] => G[α]) forSome { type α }): FunctionK[F, G] =
-    macro FunctionKMacros.lift[F, G]
-
-}
-
-private[arrow] object FunctionKMacros {
-
-  def lift[F[_], G[_]](c: Context)(
-    f: c.Expr[(F[α] => G[α]) forSome { type α }]
-  )(
-    implicit evF: c.WeakTypeTag[F[_]],
-    evG: c.WeakTypeTag[G[_]]
-  ): c.Expr[FunctionK[F, G]] =
-    c.Expr[FunctionK[F, G]](new Lifter[c.type](c).lift[F, G](f.tree))
-  // ^^note: extra space after c.type to appease scalastyle
-
-  private[this] class Lifter[C <: Context](val c: C) {
-    import c.universe._
-
-    def lift[F[_], G[_]](tree: Tree)(
-      implicit evF: c.WeakTypeTag[F[_]],
-      evG: c.WeakTypeTag[G[_]]
-    ): Tree = unblock(tree) match {
-      case q"($param) => $trans[..$typeArgs](${arg: Ident})" if param.name == arg.name =>
-        typeArgs
-          .collect { case tt: TypeTree => tt }
-          .find(tt => Option(tt.original).isDefined)
-          .foreach { param =>
-            c.abort(param.pos, s"type parameter $param must not be supplied when lifting function $trans to FunctionK")
-          }
-
-        val F = punchHole(evF.tpe)
-        val G = punchHole(evG.tpe)
-
-        q"""
-        new _root_.cats.arrow.FunctionK[$F, $G] {
-          def apply[A](fa: $F[A]): $G[A] = $trans(fa)
-        }
-       """
-      case other =>
-        c.abort(other.pos, s"Unexpected tree $other when lifting to FunctionK")
-    }
-
-    private[this] def unblock(tree: Tree): Tree = tree match {
-      case Block(Nil, expr) => expr
-      case _                => tree
-    }
-
-    private[this] def punchHole(tpe: Type): Tree = tpe match {
-      case PolyType(undet :: Nil, underlying: TypeRef) =>
-        val α = TypeName("α")
-        def rebind(typeRef: TypeRef): Tree =
-          if (typeRef.sym == undet) tq"$α"
-          else {
-            val args = typeRef.args.map {
-              case ref: TypeRef => rebind(ref)
-              case arg          => tq"$arg"
-            }
-            tq"${typeRef.sym}[..$args]"
-          }
-        val rebound = rebind(underlying)
-        tq"""({type λ[$α] = $rebound})#λ"""
-      case TypeRef(pre, sym, Nil) =>
-        tq"$sym"
-      case _ =>
-        c.abort(c.enclosingPosition, s"Unexpected type $tpe when lifting to FunctionK")
+  def lift[F[_], G[_]](f: F[τ[F, G]] => G[τ[F, G]]): FunctionK[F, G] =
+    new FunctionK[F, G] {
+      def apply[A](fa: F[A]): G[A] =
+        f.asInstanceOf[F[A] => G[A]](fa)
     }
 
-  }
-
+  /** Used in the signature of `lift` to emulate a polymorphic function type */
+  private[this] sealed trait τ[F[_], G[_]]
 }