Type Inference Issues in Latest Version of Dotty

[adamgfraser]

We're seeing some type inference issues in trying to compile ZIO against the latest version of Dotty that don't entirely make sense to me. See compilation errors here. It appears that two of the issues can be minimized to the following:

trait ZIO[-R, +E, +A] { self =>

  final def +++[R1, B, E1 >: E](that: ZIO[R1, E1, B]): ZIO[Either[R, R1], E1, Either[A, B]] =
    for {
      e <- ZIO.environment[Either[R, R1]]
      r <- e.fold(self.map(Left(_)) provide _, that.map(Right(_)) provide _)
    } yield r
    // Found:    (Left[A, Any] | Right[Any, B])(r)
    // Required: Either[A, B]

  def flatMap[R1 <: R, E1 >: E, B](f: A => ZIO[R1, E1, B]): ZIO[R1, E1, B] = ???

  def map[B](f: A => B): ZIO[R, E, B] = ???

  def provide[R](R: R): ZIO[Any, E, A] = ???
}

object ZIO {
  def environment[R]: ZIO[R, Nothing, R] = ???
}

The other two also relate to types being inferred as Any and are here and here.

When you have the chance could you please advise whether there is an issue here or something we need to be doing differently? Thanks!

Copying @jdegoes.

[jdegoes]

We can ask @smarter and open a ticket in Dotty.

In this fold case we can probably solve workaround it by adding a type annotation to fold, e.g. fold[X], where X = IO[E1, Either[A, B]].

Ah, this is the Dotty repo. 😆 Nevermind, then.

[adamgfraser]

Yes confirmed that the fold case compiles using an explicit type annotation, though I don't think that should be necessary.

[odersky]

I think I might have a fix. Once #7161 is green, can you check whether it's sufficient to solve your problems? Thanks!

[adamgfraser]

Will do! Thanks for being so responsive on this! Really appreciate it!

[adamgfraser]

@odersky Just wanted to confirm that we are now able to compile against the latest nightly version of Dotty. Thanks again for your help with this and for everything you and your team are doing to build the next generation of Scala!