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

[jmgimeno]

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

Also opened in cats-effect as typelevel/cats-effect#2748

[SethTisue]

This is the issue tracker of the Scala website, not of the Scala language. If you want to report a bug in Scala 3, that's https://github.com/lampepfl/dotty/issues

But actually, I think you should hold off on opening a ticket there. You should wait until you or the cats-effect folks have solid evidence that a bug in Scala itself is actually the root cause of the hang you are experiencing.