no type parameters for method handle: (x$1: java.util.function.BiFunction[_ >: T, Throwable, _ <: U])

[He-Pin]

Reproduction steps

Scala version: 2.12.17

Works in 2.13

https://github.com/akka/akka/pull/31918/files#diff-f9740bf92e4d83e90e297bde9ecbbb5a624b269455cc21a0c805fd7a0458785cR71-R72

  def afterCompletionStage[T](duration: FiniteDuration, using: Scheduler)(value: => CompletionStage[T])(
      implicit ec: ExecutionContext): CompletionStage[T] =
    if (duration.isFinite && duration.length < 1) {
      try value
      catch { case NonFatal(t) => Futures.failedCompletionStage(t) }
    } else {
      val p = new CompletableFuture[T]
      using.scheduleOnce(duration) {
        try {
          val future = value
          future.handle(new BiFunction[T, Throwable, Unit] {
            override def apply(t: T, ex: Throwable): Unit = {
              if (t != null) p.complete(t)
              if (ex != null) p.completeExceptionally(ex)
            }
          })
        } catch {
          case NonFatal(ex) => p.completeExceptionally(ex)
        }
      }
      p
    }

Problem

[error] C:\Users\hepin\IdeaProjects\akka\akka-actor\src\main\scala\akka\pattern\FutureTimeoutSupport.scala:71:18: no type parameters for method handle: (x$1: java.util.function.BiFunction[_ >: T, Throwable, _ <: U])java.util.concurrent.CompletionStage[U] exist so that it can be applied to arguments (java.util.function.BiFunction[T,Throwable,Unit])
[error]  --- because ---
[error] argument expression's type is not compatible with formal parameter type;
[error]  found   : java.util.function.BiFunction[T,Throwable,Unit]
[error]  required: java.util.function.BiFunction[_ >: T, Throwable, _ <: ?U]
[error] Note: T <: Any, but Java-defined trait BiFunction is invariant in type T.
[error] You may wish to investigate a wildcard type such as `_ <: Any`. (SLS 3.2.10)
[error]           future.handle(new BiFunction[T, Throwable, Unit] {
[error]                  ^
[error] C:\Users\hepin\IdeaProjects\akka\akka-actor\src\main\scala\akka\pattern\FutureTimeoutSupport.scala:71:25: type mismatch;
[error]  found   : java.util.function.BiFunction[T,Throwable,Unit]
[error]  required: java.util.function.BiFunction[_ >: T, Throwable, _ <: U]
[error]           future.handle(new BiFunction[T, Throwable, Unit] {
[error]                         ^
[error] two errors found
[error] (Compile / compileIncremental) Compilation failed
[error] Total time: 16 s, completed 2023-4-20 23:43:53
[IJ]

[SethTisue]

We don't keep a ticket open if the bug doesn't exist in 2.13, but the ticket can have value regardless as a place to share workarounds.

Whether a 2.12 backport of the fix would be possible, I don't know — someone would need to do a bisect to figure out what PR fixed this.

[SethTisue]

Note that I'm unable to easily try this code myself since it's missing a lot of needed imports.

[som-snytt]

Interestingly, 2.13.0 still says

[error]  found   : (String, Throwable) => Unit
[error]  required: java.util.function.BiFunction[_ >: String, Throwable, _]
[error]     val h = g.handle((s, t) => println(s"Unit handling $s on error $t"))
[error]                             ^
[error] one error found

but compiles in 2.13.1.

Minimize is trivialize:

new CompletableFuture[String].handle((s, t) => println(s"Unit handling $s on error $t"))

[He-Pin]

Hope this get back ported to 2.12.x，with the SAM lamda support too.

[He-Pin]

We don't keep a ticket open if the bug doesn't exist in 2.13, but the ticket can have value regardless as a place to share workarounds.

Whether a 2.12 backport of the fix would be possible, I don't know — someone would need to do a bisect to figure out what PR fixed this.

The problem is when handling the java Interface with use side varient, and another issue is lack of sam lamda in 2.12.x.

@som-snytt and interesting is the whenComplete compiles

@SethTisue sorry not minimize it as I was want to show which have a real usage

[som-snytt]

I'm looking at https://github.com/scala/bug/issues/11558 for a discrete backport. It is not obviously untenable.