Typer regression in otavia-projects/otavia under -Yexplicit-nulls

[WojciechMazur]

Code no longer compiles when -Yexplicit-nulls is used.
Based on Open CB failure of otavia-projects/otavia - build logs

Compiler version

3.4.0

Last good release: 3.3.2-RC1-bin-20230605-348729e-NIGHTLY
First bad release: 3.3.2-RC1-bin-20230606-5d2812a-NIGHTLY
Bisect points to 3e1fbd6

Minimized code

//> using options -Yexplicit-nulls

sealed trait Message extends Serializable
type MessageOf[A <: Actor[?]] <: Call = A match {
    case Actor[m] => m
}

sealed trait Call extends Message
trait Ask[R <: Reply] extends Call

type ReplyOf[A <: Ask[? <: Reply]] <: Reply = A match
    case Ask[r] => r

trait Reply extends Message
sealed trait UnitReply private () extends Reply

trait Future[+V]
sealed trait MessageFuture[+R <: Reply] extends Future[R]

trait Actor[+M <: Call]

trait Address[-M <: Call]: // extends EventableAddress {
  def send[A <: M & Ask[? <: Reply]](send: A): Unit
  def ask[A <: M & Ask[? <: Reply]](ask: A, future: MessageFuture[ReplyOf[A]]): MessageFuture[ReplyOf[A]]

final case class AcceptedChannel() extends Ask[UnitReply]
abstract class AcceptedWorkerActor[M <: Call] extends Actor[M | AcceptedChannel]
abstract class AcceptorActor[W <: AcceptedWorkerActor[? <: Call]]{
    private var workers: Address[MessageOf[W]] = ???

    def future: MessageFuture[UnitReply] = ???
    def case1 = workers.send(AcceptedChannel()) // error
    def case2 = workers.ask(AcceptedChannel(), future) // error
}

Output

Compiling project (Scala 3.4.0, JVM (17))
[error] ./bisect/main.scala:32:30
[error] Found:    AcceptedChannel
[error] Required: MessageOf[W] & Ask[? <: Reply]
[error]     def case1 = workers.send(AcceptedChannel()) // error
[error]                              ^^^^^^^^^^^^^^^^^
[error] ./bisect/main.scala:33:29
[error] Found:    AcceptedChannel
[error] Required: MessageOf[W] & Ask[? <: Reply]
[error]     def case2 = workers.ask(AcceptedChannel(), future) // error
[error]                             ^^^^^^^^^^^^^^^^^
[error] ./bisect/main.scala:33:48
[error] Found:    MessageFuture[UnitReply]
[error] Required: MessageFuture[ReplyOf[A]]
[error] 
[error] where:    A is a type variable with constraint <: MessageOf[W] & Ask[? <: Reply]
[error] 
[error] 
[error] Note: a match type could not be fully reduced:
[error] 
[error]   trying to reduce  ReplyOf[A]
[error]   failed since selector W
[error]   does not uniquely determine parameter m in
[error]     case Actor[m] => m
[error]   The computed bounds for the parameter are:
[error]     m <: Call | AcceptedChannel
[error]     def case2 = workers.ask(AcceptedChannel(), future) // error
[error]                                                ^^^^^^
Error compiling project (Scala 3.4.0, JVM (17))

Expectation

Should probably compile

[odersky]

-Yexplicit-nulls is unstable, so code might break. @noti0na1 should take a look.

[noti0na1]

The minimized code produces the same errors with and without explicit nulls, so I don't think it relates to null safety.

After looking at the tests in #17180, I guess the errors are expected?

Could you confirm the expected behaviour? @odersky @sjrd

[sjrd]

Oh yes that's expected. In terms of SIP-56 this was made more precise. See occurrences of "concrete type" and in particular the step

If scrutIsWidenedAbstract is true, fail as not specific.

at https://docs.scala-lang.org/sips/match-types-spec.html#matching