Unminimized OOM

Working on minimizing this. Still a ways to go but I’m getting much closer. Please see this branch: https://github.com/djspiewak/cats-effect/tree/bug/dotty-compiler-oom You can reproduce the OOM by running sbt ++3.0.0-M3 kernelJVM/clean kernelJVM/compile (ignore the errors; that code could just be deleted but I haven’t done it yet).

The offending call site appears to be this one: https://github.com/djspiewak/cats-effect/blob/bug/dotty-compiler-oom/kernel/shared/src/main/scala/cats/effect/kernel/Resource.scala#L33 If you uncomment the explicit type parameters, it compiles reasonably quickly. Forcing the compiler to make this inference for us (which 2.13 does just fine) causes it to spin its wheels for about a minute (on my laptop) before running out of memory.

Note that the import Resource._ and the implicit in the companion object are also necessary. I haven’t minimized the other imports, but I suspect most of them are unneeded.

In case it’s helpful, I coaxed this stack trace out of it:

java.lang.OutOfMemoryError: GC overhead limit exceeded
  at scala.collection.immutable.List.$colon$colon(List.scala:97)
  at dotty.tools.dotc.core.Types$TypeMap.mapArgs(Types.scala:5070)
  at dotty.tools.dotc.core.Types$TypeMap.mapOver(Types.scala:5102)
  at dotty.tools.dotc.core.Substituters$.substParam(Substituters.scala:145)
  at dotty.tools.dotc.core.Substituters$SubstParamMap.apply(Substituters.scala:193)
  at dotty.tools.dotc.core.Types$TypeMap.mapOverLambda(Types.scala:5080)
  at dotty.tools.dotc.core.Types$TypeMap.mapOver(Types.scala:5105)
  at dotty.tools.dotc.core.Substituters$.substParam(Substituters.scala:145)
  at dotty.tools.dotc.core.Substituters$SubstParamMap.apply(Substituters.scala:193)
  at dotty.tools.dotc.core.Types$TypeMap.op$proxy13$1(Types.scala:5067)
  at dotty.tools.dotc.core.Types$TypeMap.mapArgs(Types.scala:5067)
  at dotty.tools.dotc.core.Types$TypeMap.mapOver(Types.scala:5102)
  at dotty.tools.dotc.core.Substituters$.substParam(Substituters.scala:145)
  at dotty.tools.dotc.core.Substituters$SubstParamMap.apply(Substituters.scala:193)
  at dotty.tools.dotc.core.Types$TypeMap.mapOverLambda(Types.scala:5080)
  at dotty.tools.dotc.core.Types$TypeMap.mapOver(Types.scala:5105)
  at dotty.tools.dotc.core.Substituters$.substParam(Substituters.scala:145)
  at dotty.tools.dotc.core.Substituters$SubstParamMap.apply(Substituters.scala:193)
  at dotty.tools.dotc.core.Types$TypeMap.op$proxy13$1(Types.scala:5067)
  at dotty.tools.dotc.core.Types$TypeMap.mapArgs(Types.scala:5067)
  at dotty.tools.dotc.core.Types$TypeMap.mapOver(Types.scala:5102)
  at dotty.tools.dotc.core.Substituters$.substParam(Substituters.scala:145)
  at dotty.tools.dotc.core.Substituters$SubstParamMap.apply(Substituters.scala:193)
  at dotty.tools.dotc.core.Types$TypeMap.mapOverLambda(Types.scala:5080)
  at dotty.tools.dotc.core.Types$TypeMap.mapOver(Types.scala:5105)
  at dotty.tools.dotc.core.Substituters$.substParam(Substituters.scala:145)
  at dotty.tools.dotc.core.Substituters$SubstParamMap.apply(Substituters.scala:193)
  at dotty.tools.dotc.core.Types$TypeMap.op$proxy13$1(Types.scala:5067)
  at dotty.tools.dotc.core.Types$TypeMap.mapArgs(Types.scala:5067)
  at dotty.tools.dotc.core.Types$TypeMap.mapOver(Types.scala:5102)
  at dotty.tools.dotc.core.Substituters$.substParam(Substituters.scala:145)
  at dotty.tools.dotc.core.Substituters$SubstParamMap.apply(Substituters.scala:193)

This reproduces against both M2 and M3.

[odersky]

I found the cause. It was a rather stupid mistake where we compared the wrong symbols. But all the real work was done by @griggt here.

[griggt]

It’s probably not fully minimized, but this self-contained example seems to trigger the issue:

trait Monoid[A]
trait Semigroup[A]
trait Applicative[F[_]]

trait OptionT[F[_], A]
trait EitherT[F[_], A, B]
trait IorT[F[_], A, B]
trait WriterT[F[_], L, V]
trait Kleisli[F[_], A, B]

final class ApplicativeIdOps[A](private val a: A) extends AnyVal {
  def pure[F[_]](implicit F: Applicative[F]): F[A] = ???
}

object ApplicativeSyntax {
  implicit final def syntaxApplicativeId[A](a: A): ApplicativeIdOps[A] = new ApplicativeIdOps[A](a)
}

trait Sync[F[_]]

object Sync {
  implicit def syncForOptionT[F[_]](implicit F0: Sync[F]): Sync[OptionT[F, *]] = ???
  implicit def syncForEitherT[F[_], E](implicit F0: Sync[F]): Sync[EitherT[F, E, *]] = ???
  implicit def syncForIorT[F[_], L](implicit F0: Sync[F], L0: Semigroup[L]): Sync[IorT[F, L, *]] = ???
  implicit def syncForWriterT[F[_], L](implicit F0: Sync[F], L0: Monoid[L]): Sync[WriterT[F, L, *]] = ???
  implicit def syncForKleisli[F[_], R](implicit F0: Sync[F]): Sync[Kleisli[F, R, *]] = ???
}

trait Async[F[_]] extends Sync[F]

object Async {
  implicit def asyncForOptionT[F[_]](implicit F0: Async[F]): Async[OptionT[F, *]] = ???
  implicit def asyncForEitherT[F[_], E](implicit F0: Async[F]): Async[EitherT[F, E, *]] = ???
  implicit def asyncForIorT[F[_], L](implicit F0: Async[F], L0: Semigroup[L]): Async[IorT[F, L, *]] = ???
  implicit def asyncForWriterT[F[_], L](implicit F0: Async[F], L0: Monoid[L]): Async[WriterT[F, L, *]] = ???
  implicit def asyncForKleisli[F[_], R](implicit F0: Async[F]): Async[Kleisli[F, R, *]] = ???
}

trait Concurrent[F[_], E] extends Applicative[F]

trait Ref[F[_], A]

object Ref {
  trait Make[F[_]]
  object Make extends MakeInstances

  trait MakeInstances extends MakeLowPriorityInstances {
    implicit def concurrentInstance[F[_]](implicit F: Concurrent[F, _]): Make[F] = ???
  }

  trait MakeLowPriorityInstances {
    implicit def syncInstance[F[_]](implicit F: Sync[F]): Make[F] = ???
  }

  def of[F[_], A](a: A)(implicit mk: Make[F]): F[Ref[F, A]] = ???
}


class Resource[F[_], A] {
  import ApplicativeSyntax._

  implicit def asyncForResource[F[_]](implicit F0: Async[F]): Async[Resource[F, *]] = ???

  def parZip(implicit F: Concurrent[F, Throwable]) = {
    Ref.of /*[F, (F[Unit], F[Unit])]*/ (().pure[F] -> ().pure[F])
    ()
  }
}