type aliases for opaque types break

[drdozer]

SO ...

opaque type T = String
object T {
  def unwrap(t: T): String = t
}

opaque type U = String
type W = U
object U {
  def unwrap(w: W): String = w
}

The unwrap in T works fine, as advertised. However, unwrap for U fails because it's defined in terms of the type alias W for U.

error]      |  def unwrap(w: W): String = w
[error]     |                             ^
[error]     |                             Found:    W(w)
[error]     |                             Required: String

I would expect W to de-alias to V, and then work.

[odersky]

That's a no fix, unfortunately. The logic of opaque types is as follows:

• There are two types U and U.U
• Inside object U, the compiler knows that U.U = U and that U.U <: String
• But that's not a global knowledge, and subtyping is not always transitive.

To change this would be a major upheaval which would probably break more things than it fixes. On the other hand, the following modification of the code with a type ascription compiles because it spells out the two steps one by one, without needing transitive reasoning.

opaque type U = String
type W = U
object U {
  def unwrap(w: W): String = w: U
}

[drdozer]

Thanks. Can this be added to the docs for opaque types as a gotcha+fix? I hit it quite quickly, so it's possible other people will stumble over it.

[odersky]

Yes, we should make a more detailed spec for opaque types.

[nafg]

That's interesting, I didn't realize that it worked by locally shadowing the opaque type with an inner type that conforms to both the opaque type and its right hand side. Why does it use subtype though, couldn't it be U.U = U & String ?

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On Wed, Mar 6, 2019, 10:17 AM odersky ***@***.***> wrote: Yes, we should make a more detailed spec for opaque types. — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#6003 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAGAUPv9YN4-cJqnK-IK0u9eq_s0cDQXks5vT9wCgaJpZM4bZcRh> .