He-Pin opened a new issue, #3252:
URL: https://github.com/apache/pekko/issues/3252
### Motivation
Pekko's typed Receptionist uses `TypedMultiMap` with Scala 3 match types
(`Platform.scala`). This combination fails to compile on Scala 3.4 and 3.5 due
to SIP-56 match type reduction restrictions. Pekko currently pins
`scala3Version = "3.3.8"`, masking the issue. Upgrading to Scala 3.4+ or 3.5.x
would break the build.
### Current Behavior
**Three files form the breaking chain:**
1. `actor/src/main/scala/org/apache/pekko/util/TypedMultiMap.scala:51`:
```scala
def inserted(key: T)(value: K[key.type]): TypedMultiMap[T, K]
```
2. `actor-typed/src/main/scala-3/.../Platform.scala:26-32`:
```scala
type Service[K <: Aux[?]] = K match {
case Aux[t] => ActorRef[t]
}
```
3. `actor-typed/src/main/scala/.../LocalReceptionist.scala:78-79`:
```scala
val newServices = services.inserted(key)(serviceInstance)
```
### Compilation Error on Scala 3.4-3.5
```
Type Mismatch Error:
Found: ActorRef[key.Protocol]
Required: Platform.Service[(key : Key)]
Note: a match type could not be fully reduced:
trying to reduce Platform.Service[(key : Key)]
failed since selector (key : Key) does not uniquely determine parameter t
```
### Version Matrix
| Scala Version | Standalone TypedMultiMap | Full Usage (with match types) |
|---|---|---|
| 3.3.8 (current) | PASS | PASS |
| 3.4.x | PASS | **FAIL** |
| 3.5.x | PASS | **FAIL** |
| 3.6.0+ | PASS | PASS |
### Expected Behavior
The Receptionist types should compile on all supported Scala 3 versions.
Either restructure the match types to avoid the SIP-56 restriction, or provide
a `scala-3/` override for the affected code.
### Impact
Blocks upgrading `scala3Version` from 3.3.8 to 3.4.x or 3.5.x. Scala 3.6+
happens to work again.
### Environment
- Pekko Actor Typed (Scala 3 build)
- `TypedMultiMap.scala:51`, `Platform.scala:26-32`,
`LocalReceptionist.scala:78-79`
- Same pattern in
`cluster-typed/src/main/scala-3/.../ClusterReceptionistProtocol.scala:28-30`
### References
None
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