Covariant type parameter mentioned as a parameter in type member

Asked Oct 09 '18 at 14:34

Active Oct 10 '18 at 05:08

Viewed 77 times

trait Element[+A] {
  type T = A
}
// Error: covariant type A occurs in invariant 
// position in type A of type T type T = A

Why it's not allowed to use covariant type parameter in type member?

There is an answer for this question, but the explanation doesn't cover question "why" enough good. Would be nice to have some examples where it break type system in runtime.

Edit:

As I found there is a long history of this restriction:

SI-6566 (and this) - non-private type aliases must be invariant

SI-7093 - the protected[this] variance escape hatch is unsound

edited Oct 10 '18 at 05:08

asked Oct 09 '18 at 14:34

Bogdan Vakulenko

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Maybe this can help you: https://stackoverflow.com/questions/10523199/variance-annotations-in-type-aliases – Markus Appel Oct 09 '18 at 14:49
Seems like the only way to go is `type T[A] = Seq[A]` – Markus Appel Oct 09 '18 at 14:49
Possible duplicate of [Scala type members variance](https://stackoverflow.com/questions/33458782/scala-type-members-variance) – Yaneeve Oct 09 '18 at 15:27

Covariant type parameter mentioned as a parameter in type member

0 Answers0