Fix Scala 3.8.4 compilation compatibility while preserving 2.13 and 3.3.x cross-build

[He-Pin]

Motivation

The codebase needs to compile with both Scala 3.3.x (current production) and Scala 3.8.4 (forward compatibility for the upcoming 3.9 LTS). Several patterns in the codebase break under Scala 3.8 due to changes in how context bounds are desugared (they become using clauses), stricter deprecation warnings, and syntax changes.

Modification

171 files changed across the codebase:

• ClassTag context bound fixes: In Scala 3.8, [T: ClassTag] desugars to a using clause, making explicit ClassTag passing positional-incompatible. Fixed by using implicit val ct: ClassTag[T] = ClassTag(clazz) in scope and relying on implicit resolution instead of explicit passing. Key files: PersistencePlugin.scala, EventSourcedBehaviorTestKit.scala, ActorContextSpec.scala, BehaviorTestKitSpec.scala, Behavior.scala, EventStream.scala, Topic.scala, ShardingProducerController.scala, ShardedDaemonProcessImpl.scala, StreamTestKit.scala.

• PersistencePlugin implicit parameter merge: Merged [T: ClassTag] context bound and (implicit ev: PluginProvider[...]) into a single implicit parameter list (implicit ct: ClassTag[T], ev: PluginProvider[T, ScalaDsl, JavaDsl]) to avoid context bound + separate implicit parameter mismatch in Scala 3.8.

• .apply() trick: Used decoration[T].apply(behavior) instead of decoration[T](behavior) to separate implicit resolution from function application (Scala 2.13 interprets (behavior) as the implicit ClassTag argument).

• private[this] → private: Scala 3.8 deprecates private[this] in favor of private.

• = _ initializers → explicit defaults: Replaced = _ with = null, = None, = 0, etc. as Scala 3.8 deprecates uninitialized vars.

• SortedSet.empty(ordering) fix: Bound ordering as implicit val ord before calling .empty[Member] to avoid implicit resolution issues.

• @unchecked annotations: Added for type-test patterns that can't be checked at runtime under Scala 3.

• @nowarn filters: Used message-based @nowarn("msg=never used") instead of category-based filters for cross-version compatibility.

• PekkoDisciplinePlugin.scala: Added -Wconf suppression rules for Scala 3.8 deprecation/syntax warnings in both main and test scopes.

• No using keyword in shared sources: All changes use cross-compatible patterns (implicit val, .apply() trick). The using keyword only appears in scala-3/ directories.

Result

All modules compile with both Scala 2.13.18 and Scala 3.8.4. Production Scala 3 version remains at 3.3.8 — this change only ensures forward compatibility. No behavioral changes to the runtime.

Tests

• sbt "++3.8.4" "Test / compile" — success across all modules
• sbt "Test / compile" (Scala 2.13.18) — success across all modules

References

None - proactive Scala 3.8 forward compatibility

[pjfanning]

it's ok to make this change but the casting seems weird and could be a bug in scala 3.8

[He-Pin]

Agreed, the asInstanceOf cast was a workaround for Scala 3.8 context bound desugaring. It has been reverted to the original context.messageAdapter(f)(ClassTag(messageClass)) pattern in commit 683f874 — the explicit ClassTag passing still works correctly in Scala 3.8 since using parameters can be passed positionally. The refactored ClassTag[U] version did not compile on Scala 2.13 because U is erased in pattern matching.

[He-Pin]

The 3.9.0-RC1 has just been released, hope this will make the code compiles with 3.9.0-RC1

[He-Pin]

Closing to reopen with a rebased branch that resolves merge conflicts with main (Hub.scala private[this] changes from #3063).