App + cats-effect hangs on 3.1.0 but works fine in 2.13.7

This code

import cats.effect._
import cats.implicits._
import cats.effect.unsafe.implicits.global

object IOComposition extends App {
  val hello = IO(println(s"[${Thread.currentThread.getName}] Hello"))
  val world = IO(println(s"[${Thread.currentThread.getName}] World"))
  val hw1: IO[Unit] = for {
    _ <- hello
    _ <- world
  } yield ()
  val hw2: IO[Unit] =
    (hello, world).mapN((_, _) => ())
  hw1.unsafeRunSync()
  hw2.unsafeRunSync()
}

hangs with Scala 3.1.0 after printing only “Hello”

Compiling project (Scala 3.1.0, JVM)
Compiled project (Scala 3.1.0, JVM)
[io-compute-2] Hello

but works perfecty with Scala 2.13.7

Compiling project (Scala 2.13.7, JVM)
Compiled project (Scala 2.13.7, JVM)
[io-compute-2] Hello
[io-compute-4] World

[SethTisue]

I’d also be interested in knowing if the problem goes away if you avoid using App. App is not recommended in Scala 3, as per https://docs.scala-lang.org/scala3/reference/changed-features/main-functions.html

[djspiewak]

Sweet! So this is definitely the MethodHandle concurrent initialization issue then. It’s actually part of the JVM itself and not really anything that either Scala or Cats Effect can address. In fact, you can reproduce this issue using bare Java.