Simple application of opaque types, extension methods hangs in runtime

Compiler version

3.0.0-M3

Minimized code

as scastie: https://scastie.scala-lang.org/3YbjmeaUTYWzwBOQOdRt8g

trait Eq[A]:
  def eqv(self: A, another: A): Boolean
  extension (self: A) def ===(another: A): Boolean = eqv(self, another)

object Eq:
  def apply[T: Eq]: Eq[T] = summon[Eq[T]]
  given Eq[String] = (self, another) => self == another

opaque type UserId = String
object UserId {
  def apply(value: String): UserId = value

  given Eq[UserId] =
    println("bar")
    Eq[String]
}

object Main:
  def main(args: Array[String]): Unit =
    println(UserId("foo") === UserId("foo"))

Output

bar
<hangs now>

Expectation

outputs bar, then true, then exits.

More context

This doesn’t happen when Eq[UserId].eqv(..., ...) is used instead.

[smarter]

Yep! (but before adding complexity to the language, it’s also worth asking first if case class UserId(value: String) derives Eq isn’t good enough, sure you get extra allocations but I’m not convinced that is a big deal: https://contributors.scala-lang.org/t/synthesize-constructor-for-opaque-types/4616/76?u=smarter)

[Swoorup]

@kubukoz duplicate of https://github.com/lampepfl/dotty/issues/10947