Typer regressions in `getkyo/kyo`

[WojciechMazur]

Based on the OpenCB failures in getkyo/kyo - build logs

2 issues found in the same codebase related to the same change

Compiler version

Last good release: 3.8.0-RC1-bin-20250915-afbb66b-NIGHTLY
First bad release: 3.8.0-RC1-bin-20250916-eb1bb73-NIGHTLY

For both bisect points to b1067d9 / #23923

Issue 1

Minimized code

trait <[+A, -S]
object `<`:
  extension [A, S](v: A < S) def map[B, S2](f: A => B < S2): B < (S & S2) = ???

def lift[A, S](v: A): A < S = ???

abstract class ArrowEffect[-Input[_], +Output[_]]
object ArrowEffect:
  inline def handleCatching[I[_], O[_], E <: ArrowEffect[I, O], A, B, S, S2, S3](
      v: A < (E & S)
  ): B < (S & S2 & S3) = ???

trait Result[+E, +A]
object Result:
  def succeed[E, A](value: A): Result[E, A] = ???

abstract class Error[+E]

type Const[A] = [B] =>> A
sealed trait Abort[-E] extends ArrowEffect[Const[Error[E]], Const[Unit]]
object Abort:
  inline def runWith[E, A, S, ER, B, S2](v: A < (Abort[E | ER] & S)) =
    ArrowEffect.handleCatching[
      Const[Error[E]],
      Const[Unit],
      Abort[E],
      Result[E, A],
      B,
      Abort[ER] & S,
      Abort[ER] & S,
      S2
    ](
      v.map: 
        value => lift:
          Result.succeed[E, A](value)
    )

Output

Compiling project (Scala 3.8.0-RC1-bin-20250928-b67fcf2-NIGHTLY, JVM (21))
[error] ./test.scala:33:7
[error] Found:    Result[E, A] < (Abort[E | ER] & S)
[error] Required: Result[E, A] < (Abort[E] & (Abort[ER] & S))

Issue 2

Minimized code

trait <[+A, -S]

trait TypeMap[+A]:
  def union[B](that: TypeMap[B]): TypeMap[A & B] = ???

trait Tag[E]
trait ContextEffect[+A]
object ContextEffect:
  def handle[A, E <: ContextEffect[A], B, S](effectTag: Tag[E], ifDefined: A => A)(
      v: B < (E & S)
  ): B < S = ???

sealed trait Env[+R] extends ContextEffect[TypeMap[R]]
object Env:
  def runAll[R >: Nothing, A, S, VR](
      env: TypeMap[R],
      tag: Tag[Env[R]]
  )(
      v: A < (Env[R & VR] & S)
  ) =
    ContextEffect.handle(
      tag,
      _.union(env)
    )(v): A < (Env[VR] & S)
    ???

Output

Compiling project (Scala 3.8.0-RC1-bin-20250928-b67fcf2-NIGHTLY, JVM (21))
[error] ./test2.scala:24:7
[error] Found:    (v : A < (Env[R & VR] & S))
[error] Required: A < (Env[R] & (Env[VR] & S))
[error] 
[error] where:    R is a type in method runAll
[error]     )(v): A < (Env[VR] & S)
[error]       ^
Error compiling project (Scala 3.8.0-RC1-bin-20250928-b6

Expectation

Both should compile

[Gedochao]

cc @hamzaremmal @EugeneFlesselle

[EugeneFlesselle]

Both of these seem to precisely be instances of the unsound subtyping rule disabled in #23441.

[WojciechMazur]

I've tried to adjust the kyo codebase and open a PR, however this change might have a significant impact on this codebase
While the intersections in explicit parameters is easy to fix (although not source compatible, which is unfortunete since there is large amount of now prohibited intersection types in the code base), there are cases that are raising new concerns.
Here's extension to second example adding Env.useAll and Layer, this one is more difficult to adjust for the new rules

trait <[+A, -S]

trait TypeMap[+A]:
  def union[B](that: TypeMap[B]): TypeMap[A & B] = ???

trait Tag[E]
trait ContextEffect[+A]
object ContextEffect:
  def handle[A, E <: ContextEffect[A], B, S](effectTag: Tag[E], ifDefined: A => A)(
      v: B < (E & S)
  ): B < S = ???

sealed trait Env[+R] extends ContextEffect[TypeMap[R]]
object Env:
  inline def useAll[R](using DummyImplicit)[A, S](f: TypeMap[R] => A < S): A < (Env[R] & S) = ???

abstract class Layer[+Out, -S]
object Layer:
  def from[A, B, C, S](f: (A, B) => C < S): Layer[C, Env[A] & Env[B] & S] = apply(
      Env.useAll[A & B](???) //.asInstanceOf[C < (Env[A] & Env[B] & S)] //workaround
    )
  def apply[A, S](kyo: => A < S): Layer[A, S] = ???

[error] ./test2.scala:33:7
[error] Found:    Nothing < Env[A & B]
[error] Required: C < (Env[A] & Env[B] & S)
[error]       Env.useAll[A & B](???) //.asInstanceOf[C < (Env[A] & Env[B] & S)] //workaround
[error]       ^^^^^^^^^^^^^^^^^^^^^^
Error compiling project (Scala 3.8.0-RC1-bin-20251001-28a4921-NIGHTLY, JVM (21))

In this case it would require explicit unsafe cast all over the code, especially for the downstream users of the kyo ecosystem.
AFAIK match types would not be able to adjust the results as well, since they won't work well in case of intersection/union types.

It does also lead to other compilation errors, that I have not yet well analyzed

[hamzaremmal]

@WojciechMazur we are reverting the PR that drops the rule until we understand better the interactions it has and don't break things.