Requiring a Type Class in Enum Definition

[ashwinbhaskar]

For the code below, the compiler complains [error] |No implicit Ordering defined for A..

import scala.math.Ordering

enum BinaryTree[+A: Ordering]:
    case Leaf
    case Node(left: BinaryTree[A], value: A, right: BinaryTree[A])

The reason for this error was pointed out in this post.

But for someone who doesn't know what enums desugar to, the error might be puzzling because they have already said A has Ordering in the enum declaration.

is this a valid complaint?

[bishabosha]

i think it would be useful to also copy over any context bounds in the desugaring, which would have to be a change in a future minor version.

It is worth noting though that by adding an Ordering context bound - your BinaryTree now has hidden state that makes it harder to program generically. e.g. there will be no scala.deriving.Mirror generated

[nicolasstucki]

Workarround

import scala.math.Ordering
enum BinaryTree[+A: Ordering]:
    case Leaf
-    case Node(left: BinaryTree[A], value: A, right: BinaryTree[A])
+    case Node(left: BinaryTree[A], value: A, right: BinaryTree[A])(using Ordering[A])