infinite loop when evaluating a recursive given definition

When running a program which consumes opaque types as follows, it appears to be stuck on an infinite loop. Not sure what is going on here.

Minimized code

object Prices {
  opaque type Price = BigDecimal

  object Price{
    given Ordering[Price] = summon[Ordering[BigDecimal]]
  }

}

extension[K,V] (m1: Map[K,V]) def computeDiff(m2: Map[K,V])(using Ordering[K])(using orderAsc: Boolean = true): Unit = println("Ok....")

import Prices._
@main def mainz = {
    println("begin")
    val result = Map.empty[Price,Int].computeDiff(Map.empty[Price,Int])
    println(s"end $result")
}

Scastie link: https://scastie.scala-lang.org/ajEagvzvTbitmzbqFt0Oxw

Output

begin

Expectation

Should successfully print the following:

begin
Ok....
end

This works if the Price was swapped with BigDecimal in the line

  val result = Map.empty[Price,Int].computeDiff(Map.empty[Price,Int])

This is happening on 3.0.0-M3

[kubukoz]

What’s interesting about this one is that one common use case is derivation (a la Haskell’s generalize newtype deriving). Would it be possible to hook something into the derivation mechanism?

@tel I really wish this was possible. I don’t see people switching from @newtype to opaque types anytime soon if this isn’t made easier - I just bumped on the issue at question myself and the boilerplate needed to do it “properly” is very discouraging.

[liufengyun]

We could enhance the initialization check to check for possible non-termination of lazy fields. A related issue #9668.