WojciechMazur commented on PR #1474:
URL: https://github.com/apache/pekko/pull/1474#issuecomment-2343276003
Here's the reproducer:
```scala
import scala.annotation.unchecked.uncheckedVariance
object scaladsl{
trait FlowOpsMat[+Out, +Mat] {
type Repr[+O] <: ReprMat[O, Mat] {
type Repr[+OO] = FlowOpsMat.this.Repr[OO]
type ReprMat[+OO, +MM] = FlowOpsMat.this.ReprMat[OO, MM]
}
type ReprMat[+O, +M] <: FlowOpsMat[O, M] {
type Repr[+OO] = FlowOpsMat.this.ReprMat[OO, M @uncheckedVariance]
type ReprMat[+OO, +MM] = FlowOpsMat.this.ReprMat[OO, MM]
}
}
trait FlowOps[+Out, +Mat] {
type Repr[+O] <: FlowOps[O, Mat] {
type Repr[+OO] = FlowOps.this.Repr[OO]
}
}
trait SubFlow[+Out, +Mat, +F[+_], C] extends FlowOps[Out, Mat] {
override type Repr[+T] = SubFlow[T, Mat @uncheckedVariance, F
@uncheckedVariance, C @uncheckedVariance]
}
final class Flow[-In, +Out, +Mat] extends FlowOpsMat[Out, Mat]{
override type Repr[+O] = Flow[In @uncheckedVariance, O, Mat
@uncheckedVariance]
override type ReprMat[+O, +M] = Flow[In @uncheckedVariance, O, M]
}
final class Sink[-In, +Mat]
}
class SubFlow[In, Out, Mat](
delegate: scaladsl.SubFlow[Out, Mat, scaladsl.Flow[In, Out, Mat]#Repr,
scaladsl.Sink[In, Mat]]) {
def asScala: scaladsl.SubFlow[Out, Mat, scaladsl.Flow[In, Out, Mat]#Repr,
scaladsl.Sink[In, Mat]] @uncheckedVariance =
delegate
}
```
Compilation with both Scala 2.13 and 3.3.4-RC2 would produce the following
signatures:
```scala
private final scaladsl$SubFlow<Out, Mat, ?, scaladsl$Sink<In, Mat>>
delegate;
descriptor: Lscaladsl$SubFlow;
Signature: #19 //
Lscaladsl$SubFlow<TOut;TMat;*Lscaladsl$Sink<TIn;TMat;>;>;
public scaladsl$SubFlow<Out, Mat, ?, scaladsl$Sink<In, Mat>> asScala();
descriptor: ()Lscaladsl$SubFlow;
Signature: #22 //
()Lscaladsl$SubFlow<TOut;TMat;*Lscaladsl$Sink<TIn;TMat;>;>;
```
In Scala 3.3.3 we were generating
```scala
private final scaladsl$SubFlow<Out, Mat, scaladsl$Flow<In,
java.lang.Object, Mat>, scaladsl$Sink<In, Mat>> delegate;
descriptor: Lscaladsl$SubFlow;
Signature: #19 //
Lscaladsl$SubFlow<TOut;TMat;Lscaladsl$Flow<TIn;Ljava/lang/Object;TMat;>;Lscaladsl$Sink<TIn;TMat;>;>;
public SubFlow(scaladsl$SubFlow<Out, Mat, scaladsl$Flow<In,
java.lang.Object, Mat>, scaladsl$Sink<In, Mat>>);
descriptor: (Lscaladsl$SubFlow;)V
Signature: #22 //
(Lscaladsl$SubFlow<TOut;TMat;Lscaladsl$Flow<TIn;Ljava/lang/Object;TMat;>;Lscaladsl$Sink<TIn;TMat;>;>;)V
```
tl;dr
In 3.3.3 `Flow[In, Out, Repr]#Repr` was dealiased, but in Scala 2.13 and
3.3.4-RC1/RC2 it is a wildcard param type.
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