False-positive subtype checking for Scala 3 opaque types

[kyri-petrou]

For some reason, when 2 Scala 3 opaque types have the same underlying type, the subtype check <:< seems to falsely return true. AFAICT, this seems to be what's causing zio/zio#8882 (albeit there might be more to it, but first things first 😅)

Repro:

import izumi.reflect.Tag

type UserName = UserName.T
object UserName {
  opaque type T = String
}

type UserId = UserId.T
object UserId {
  opaque type T = String
}

val idTag   = Tag[UserId]
val nameTag = Tag[UserName]

@main def m = {
  assert(!(idTag =:= nameTag), "idTag =:= nameTag") // passes
  assert(!(idTag <:< nameTag), "idTag <:< nameTag") // fails
}

[neko-kai]

Addressed in #468

[jdegoes]

/bounty $250

[algora-pbc]

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Attempt	Started (GMT+0)	Solution
🟢 @neko-kai		#468

[algora-pbc]

💡 @neko-kai submitted a pull request that claims the bounty. You can visit your bounty board to reward.

[algora-pbc]

🎉🎈 @neko-kai has been awarded $250! 🎈🎊

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[neko-kai]

@jdegoes Maybe you made a mistake opening the bounty on this issue since the solution was already pending? I can refund the bounty back, although I don't mind either way.

[jdegoes]

@neko-kai Keep it, you earned it!

[neko-kai]

This still needs to be released