`Exception: java.lang.OutOfMemoryError` (diverging implicits)

[armanbilge]

Compiler version

3.1.1-RC1

Minimized code

armanbilge/schrodinger@ed9a8ba

Clone project in clean workspace and run sbt compile.

Apologies that this isn't further minimized, not entirely sure how to approach this one. I'd appreciate any pointers.

FWIW it's not a big project, and the module that seems to have the problem is 4 files with < 1k LOC total.

Output

[warn] In the last 8 seconds, 6.706 (86.2%) were spent in GC. [Heap: 0.10GB free of 4.00GB, max 4.00GB] Consider increasing the JVM heap using `-Xmx` or try a different collector, e.g. `-XX:+UseG1GC`, for better performance.

  | => monteCarlo / Compile / compileIncremental 312s

...

[warn] In the last 6 seconds, 5.215 (99.8%) were spent in GC. [Heap: 0.00GB free of 4.00GB, max 4.00GB] Consider increasing the JVM heap using `-Xmx` or try a different collector, e.g. `-XX:+UseG1GC`, for better performance.

Exception: java.lang.OutOfMemoryError thrown from the UncaughtExceptionHandler in thread "classloader-cache-cleanup-0

Note this occurs frequently, but not always. Sometimes it compiles just fine.

Expectation

My code compiles the first time and everything is hunky-dory :)

[SethTisue]

Is it a regression, was some previous Scala 3 version able to compile the code?

[armanbilge]

Thanks for the response. No, not a regression to my knowledge but can't know for sure. I've been on 3.1.1 nightlies until the RC1 because I needed this fix in #13650.

Looking through commit history/ci the earliest I saw this was actually when I bumped to 3.1.1-RC1 in armanbilge/schrodinger@48e93d2, but that's also around the commits where I started fleshing out the monteCarlo module that seems to have the problem.

Update: tried armanbilge/schrodinger@1f34ede which is using 3.1.1-RC1-bin-20211014-af9594d-NIGHTLY and same issue.

Update 2: I tried bisecting but the non-determinism is confounding things. I couldn't get anything before armanbilge/schrodinger@1f34ede to have this problem, but then I just tried that commit again and this time it compiled fine 🙃

[dwijnand]

Other than minimising the code, you could also approach it by "minimising" it by removing sbt (and thus zinc) usage by switching to cli scala. Also, removing dependencies (which makes removing sbt/zinc easier).

[armanbilge]

Thanks for the advice. I did some trivial minimizing in https://github.com/armanbilge/schrodinger/tree/dotty/issue/13838, I'll keep chipping away at it.

I also captured a heapdump, not sure if there's anything useful in there?

[armanbilge]

@dwijnand's advice was spot on: I removed sbt/zinc from the equation and started using the cli scala3-compiler. This made results deterministic, and much easier to minimize.

Current progress is in: https://github.com/armanbilge/schrodinger/tree/dotty/issue/13838

This is starting to get really weird: for example, there is an empty file UInt128.scala in there. If I delete it or rename it everything compiles fine. But as long as it's there, the problem persists. I suspect there are other files like that as well.

[armanbilge]

Ok, minimized down to 14 SLOC w/ a cats dependency, I hope this is sufficient 😅

import cats.kernel.Eq
import cats.kernel.Order
import cats.syntax.all.*

sealed abstract class Foo[A]
object Foo:
  given [A: Eq]: Eq[Foo[A]] = ???

opaque type FooT[F[_], A] = F[Foo[A]]
object FooT:
  def liftF[F[_], A](fa: F[A]): FooT[F, A] =
    fa.map(???)

  extension [F[_], A](ffa: FooT[F, A])
    def map[B](f: A => B): FooT[F, B] =
      ???
  
  given [F[_], A](using Ord: Order[F[Foo[A]]]): Order[FooT[F, A]] = ???  

[nicolasstucki]

Nice minimization. But we will need to have a version that does not have dependencies. A good way to start is to just copy the parts of the cats library that are involved and then trim everything that is not used.

[armanbilge]

Thanks. I got it down to this, unfortunately still with a cats-kernel dependency. I don't know if I have what it takes to minimize this any further: bringing in Eq and Order from cats-kernel also needs PartialOrder, Hash, Band, and Semigroup, as well as several auto-generated boilerplate sources for tuples. This is complicated by circular dependencies (Order extends Eq, but Eq's companion object defines implicit Orders). Untangling all of that is going be a challenge.

Edit: yup, untangling this is very messy. Actually you pretty much need everything to start—Eq depends on the instances package, which itself depends on all of kernel. It's very difficult to delete stuff without working surgically.

import cats.kernel.Eq
import cats.kernel.Order

trait EqSyntax {
  implicit def catsSyntaxEq[A: Eq](a: A): EqOps[A] = ???
}

final class EqOps[A]

object eq extends EqSyntax

import eq.*

sealed abstract class Foo[A]
object Foo:
  given [A: Eq]: Eq[Foo[A]] = ???

opaque type FooT[F[_], A] = F[Foo[A]]
object FooT:
  def liftF[F[_], A](fa: F[A]): FooT[F, A] =
    fa.map(???)

  extension [F[_], A](ffa: FooT[F, A])
    def map[B](f: A => B): FooT[F, B] =
      ???
  
  given [F[_], A](using Ord: Order[F[Foo[A]]]): Order[FooT[F, A]] = ???  

[armanbilge]

@nicolasstucki ok, dependency free-minimization 😅

This no longer crashes with java.lang.OutOfMemoryError or even leaks memory AFAICT. I think it's just taking a very long time to compile (minutes??). Removing some of those tuple instances speeds things up.

trait EqSyntax {
  implicit def catsSyntaxEq[A: Eq](a: A): EqOps[A] = ???
}

final class EqOps[A]

object eq extends EqSyntax

import eq.*

sealed abstract class Foo[A]
object Foo:
  given [A: Eq]: Eq[Foo[A]] = ???

opaque type FooT[F[_], A] = F[Foo[A]]
object FooT:
  def liftF[F[_], A](fa: F[A]): FooT[F, A] =
    fa.map(???)

  extension [F[_], A](ffa: FooT[F, A])
    def map[B](f: A => B): FooT[F, B] =
      ???
  
  given [F[_], A](using Ord: Order[F[Foo[A]]]): Order[FooT[F, A]] = ???  

trait Order[A] extends Eq[A]

trait Eq[A]

object Eq {
  implicit def catsKernelOrderForTuple1[A0](implicit A0: Order[A0]): Order[Tuple1[A0]] = ???
  implicit def catsKernelOrderForTuple2[A0, A1](implicit A0: Order[A0], A1: Order[A1]): Order[(A0, A1)] = ???
  implicit def catsKernelOrderForTuple3[A0, A1, A2](implicit A0: Order[A0], A1: Order[A1], A2: Order[A2]): Order[(A0, A1, A2)] = ???
  implicit def catsKernelOrderForTuple4[A0, A1, A2, A3](implicit A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3]): Order[(A0, A1, A2, A3)] = ???
  implicit def catsKernelOrderForTuple5[A0, A1, A2, A3, A4](implicit A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4]): Order[(A0, A1, A2, A3, A4)] = ???
  implicit def catsKernelOrderForTuple6[A0, A1, A2, A3, A4, A5](implicit A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5]): Order[(A0, A1, A2, A3, A4, A5)] = ???
  implicit def catsKernelOrderForTuple7[A0, A1, A2, A3, A4, A5, A6](implicit A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6]): Order[(A0, A1, A2, A3, A4, A5, A6)] = ???
  implicit def catsKernelOrderForTuple8[A0, A1, A2, A3, A4, A5, A6, A7](implicit A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7]): Order[(A0, A1, A2, A3, A4, A5, A6, A7)] = ???
  implicit def catsKernelOrderForTuple9[A0, A1, A2, A3, A4, A5, A6, A7, A8](implicit A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7], A8: Order[A8]): Order[(A0, A1, A2, A3, A4, A5, A6, A7, A8)] = ???
  implicit def catsKernelOrderForTuple10[A0, A1, A2, A3, A4, A5, A6, A7, A8, A9](implicit A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7], A8: Order[A8], A9: Order[A9]): Order[(A0, A1, A2, A3, A4, A5, A6, A7, A8, A9)] = ???
  implicit def catsKernelOrderForTuple11[A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10](implicit A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7], A8: Order[A8], A9: Order[A9], A10: Order[A10]): Order[(A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10)] = ???
  implicit def catsKernelOrderForTuple12[A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11](implicit A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7], A8: Order[A8], A9: Order[A9], A10: Order[A10], A11: Order[A11]): Order[(A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11)] = ???
  implicit def catsKernelOrderForTuple13[A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12](implicit A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7], A8: Order[A8], A9: Order[A9], A10: Order[A10], A11: Order[A11], A12: Order[A12]): Order[(A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12)] = ???
  implicit def catsKernelOrderForTuple14[A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13](implicit A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7], A8: Order[A8], A9: Order[A9], A10: Order[A10], A11: Order[A11], A12: Order[A12], A13: Order[A13]): Order[(A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13)] = ???
  implicit def catsKernelOrderForTuple15[A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14](implicit A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7], A8: Order[A8], A9: Order[A9], A10: Order[A10], A11: Order[A11], A12: Order[A12], A13: Order[A13], A14: Order[A14]): Order[(A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14)] = ???
  implicit def catsKernelOrderForTuple16[A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15](implicit A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7], A8: Order[A8], A9: Order[A9], A10: Order[A10], A11: Order[A11], A12: Order[A12], A13: Order[A13], A14: Order[A14], A15: Order[A15]): Order[(A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15)] = ???
  implicit def catsKernelOrderForTuple17[A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16](implicit A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7], A8: Order[A8], A9: Order[A9], A10: Order[A10], A11: Order[A11], A12: Order[A12], A13: Order[A13], A14: Order[A14], A15: Order[A15], A16: Order[A16]): Order[(A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16)] = ???
  implicit def catsKernelOrderForTuple18[A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17](implicit A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7], A8: Order[A8], A9: Order[A9], A10: Order[A10], A11: Order[A11], A12: Order[A12], A13: Order[A13], A14: Order[A14], A15: Order[A15], A16: Order[A16], A17: Order[A17]): Order[(A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17)] = ???
  implicit def catsKernelOrderForTuple19[A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18](implicit A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7], A8: Order[A8], A9: Order[A9], A10: Order[A10], A11: Order[A11], A12: Order[A12], A13: Order[A13], A14: Order[A14], A15: Order[A15], A16: Order[A16], A17: Order[A17], A18: Order[A18]): Order[(A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18)] = ???
  implicit def catsKernelOrderForTuple20[A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18, A19](implicit A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7], A8: Order[A8], A9: Order[A9], A10: Order[A10], A11: Order[A11], A12: Order[A12], A13: Order[A13], A14: Order[A14], A15: Order[A15], A16: Order[A16], A17: Order[A17], A18: Order[A18], A19: Order[A19]): Order[(A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18, A19)] = ???
  implicit def catsKernelOrderForTuple21[A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18, A19, A20](implicit A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7], A8: Order[A8], A9: Order[A9], A10: Order[A10], A11: Order[A11], A12: Order[A12], A13: Order[A13], A14: Order[A14], A15: Order[A15], A16: Order[A16], A17: Order[A17], A18: Order[A18], A19: Order[A19], A20: Order[A20]): Order[(A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18, A19, A20)] = ???
  implicit def catsKernelOrderForTuple22[A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18, A19, A20, A21](implicit A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7], A8: Order[A8], A9: Order[A9], A10: Order[A10], A11: Order[A11], A12: Order[A12], A13: Order[A13], A14: Order[A14], A15: Order[A15], A16: Order[A16], A17: Order[A17], A18: Order[A18], A19: Order[A19], A20: Order[A20], A21: Order[A21]): Order[(A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18, A19, A20, A21)] = ???
}

[nicolasstucki]

If we only have implicit instances from catsKernelOrderForTuple1 to catsKernelOrderForTupleN the compilation time is as follows

N=0:   5 seconds
N=1:   6 seconds
N=2:   7 seconds
N=3:   8 seconds
N=4:  10 seconds
N=5:  32 seconds
N=6: 256 seconds

Seems to grow exponentially

[nicolasstucki]

Minimized

implicit def catsSyntaxEq[A: Eq](a: A): Foo[A] = ???

class Foo[A]
object Foo:
  given [A: Eq]: Eq[Foo[A]] = ???

object FooT:
  def liftF[F[_], A](fa: F[A]): F[Foo[A]] = map(fa)(???)

  def map[F[_], A](ffa: F[Foo[A]])(f: A): Nothing = ???

  given OrderFFooA[F[_], A](using Ord: Order[F[Foo[A]]]): Order[F[Foo[A]]] = ???

trait Eq[A]
trait Order[A] extends Eq[A]

object Eq {
  given catsKernelOrderForTuple1[A0](using A0: Order[A0]): Order[Tuple1[A0]] = ???
  given catsKernelOrderForTuple2[A0, A1](using A0: Order[A0], A1: Order[A1]): Order[(A0, A1)] = ???
  given catsKernelOrderForTuple3[A0, A1, A2](using A0: Order[A0], A1: Order[A1], A2: Order[A2]): Order[(A0, A1, A2)] = ???
  given catsKernelOrderForTuple4[A0, A1, A2, A3](using A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3]): Order[(A0, A1, A2, A3)] = ???
  given catsKernelOrderForTuple5[A0, A1, A2, A3, A4](using A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4]): Order[(A0, A1, A2, A3, A4)] = ???
  given catsKernelOrderForTuple6[A0, A1, A2, A3, A4, A5](using A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5]): Order[(A0, A1, A2, A3, A4, A5)] = ???
  given catsKernelOrderForTuple7[A0, A1, A2, A3, A4, A5, A6](using A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6]): Order[(A0, A1, A2, A3, A4, A5, A6)] = ???
  given catsKernelOrderForTuple8[A0, A1, A2, A3, A4, A5, A6, A7](using A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7]): Order[(A0, A1, A2, A3, A4, A5, A6, A7)] = ???
  given catsKernelOrderForTuple9[A0, A1, A2, A3, A4, A5, A6, A7, A8](using A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7], A8: Order[A8]): Order[(A0, A1, A2, A3, A4, A5, A6, A7, A8)] = ???
  given catsKernelOrderForTuple10[A0, A1, A2, A3, A4, A5, A6, A7, A8, A9](using A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7], A8: Order[A8], A9: Order[A9]): Order[(A0, A1, A2, A3, A4, A5, A6, A7, A8, A9)] = ???
  given catsKernelOrderForTuple11[A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10](using A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7], A8: Order[A8], A9: Order[A9], A10: Order[A10]): Order[(A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10)] = ???
  given catsKernelOrderForTuple12[A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11](using A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7], A8: Order[A8], A9: Order[A9], A10: Order[A10], A11: Order[A11]): Order[(A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11)] = ???
  given catsKernelOrderForTuple13[A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12](using A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7], A8: Order[A8], A9: Order[A9], A10: Order[A10], A11: Order[A11], A12: Order[A12]): Order[(A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12)] = ???
  given catsKernelOrderForTuple14[A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13](using A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7], A8: Order[A8], A9: Order[A9], A10: Order[A10], A11: Order[A11], A12: Order[A12], A13: Order[A13]): Order[(A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13)] = ???
  given catsKernelOrderForTuple15[A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14](using A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7], A8: Order[A8], A9: Order[A9], A10: Order[A10], A11: Order[A11], A12: Order[A12], A13: Order[A13], A14: Order[A14]): Order[(A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14)] = ???
  given catsKernelOrderForTuple16[A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15](using A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7], A8: Order[A8], A9: Order[A9], A10: Order[A10], A11: Order[A11], A12: Order[A12], A13: Order[A13], A14: Order[A14], A15: Order[A15]): Order[(A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15)] = ???
  given catsKernelOrderForTuple17[A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16](using A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7], A8: Order[A8], A9: Order[A9], A10: Order[A10], A11: Order[A11], A12: Order[A12], A13: Order[A13], A14: Order[A14], A15: Order[A15], A16: Order[A16]): Order[(A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16)] = ???
  given catsKernelOrderForTuple18[A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17](using A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7], A8: Order[A8], A9: Order[A9], A10: Order[A10], A11: Order[A11], A12: Order[A12], A13: Order[A13], A14: Order[A14], A15: Order[A15], A16: Order[A16], A17: Order[A17]): Order[(A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17)] = ???
  given catsKernelOrderForTuple19[A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18](using A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7], A8: Order[A8], A9: Order[A9], A10: Order[A10], A11: Order[A11], A12: Order[A12], A13: Order[A13], A14: Order[A14], A15: Order[A15], A16: Order[A16], A17: Order[A17], A18: Order[A18]): Order[(A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18)] = ???
  given catsKernelOrderForTuple20[A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18, A19](using A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7], A8: Order[A8], A9: Order[A9], A10: Order[A10], A11: Order[A11], A12: Order[A12], A13: Order[A13], A14: Order[A14], A15: Order[A15], A16: Order[A16], A17: Order[A17], A18: Order[A18], A19: Order[A19]): Order[(A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18, A19)] = ???
  given catsKernelOrderForTuple21[A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18, A19, A20](using A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7], A8: Order[A8], A9: Order[A9], A10: Order[A10], A11: Order[A11], A12: Order[A12], A13: Order[A13], A14: Order[A14], A15: Order[A15], A16: Order[A16], A17: Order[A17], A18: Order[A18], A19: Order[A19], A20: Order[A20]): Order[(A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18, A19, A20)] = ???
  given catsKernelOrderForTuple22[A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18, A19, A20, A21](using A0: Order[A0], A1: Order[A1], A2: Order[A2], A3: Order[A3], A4: Order[A4], A5: Order[A5], A6: Order[A6], A7: Order[A7], A8: Order[A8], A9: Order[A9], A10: Order[A10], A11: Order[A11], A12: Order[A12], A13: Order[A13], A14: Order[A14], A15: Order[A15], A16: Order[A16], A17: Order[A17], A18: Order[A18], A19: Order[A19], A20: Order[A20], A21: Order[A21]): Order[(A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18, A19, A20, A21)] = ???
}

The following change fixes the performance issue

- implicit def catsSyntaxEq[A: Eq](a: A): Foo[A] = ???
+ given catsSyntaxEq[A: Eq]: Conversion[A, Foo[A]] = ???

Not sure if this is a compiler but or a fundamental limitation of implicit conversions that we fixed with the introduction of the Conversion type class.

Note that the program fails to compile and tries the implicit conversion with all combinations of Eq it can find.