object MTrie {
  type Trie[K, V] = Map[K, Node[K, V]]

  sealed abstract class Node[K, V] {
    def fold[A](b: Branch[K, V] => A, t: Value[K, V] => A): A = ???
  }

  final case class Branch[K, V](value: Option[Value[K, V]], next: Trie[K, V]) extends Node[K, V]
  final case class Value[K, V](value: V) extends Node[K, V]

  implicit class Ops[K, V](val trie: MTrie.Trie[K, V]) extends AnyVal {
    type Node = MTrie.Node[K, V]

    def foldl[A](z: A)(f: (A, K, Node) => A): A =
      ???

    def cataN[A](z: A)(f: (A, Node) => A): A =
      foldl(z)((q, _, n) =>
        n.fold(
          b => b.next.cataN(f(q, n))(f),
          _ => f(q, n)))
  }
}

[info] Compiling 1 Scala source to /tmp/shipreq.sbt/base-util/jvm/target/scala-2.13/classes ...
[error] ## Exception when compiling 66 sources to /tmp/shipreq.sbt/base-util/jvm/target/scala-2.13/classes
[error] scala.reflect.internal.Types$TypeError: type mismatch;
[error]  found   : (A, $this.Node) => A
[error]     (which expands to)  (A, MTrie.Node[K,V]) => A
[error]  required: (A, MTrie.Node[K,V]) => A