[Scala 3] tighten HKT wildcards (_ → ?)

MIGRATION.md

@@ -54,6 +54,10 @@ the bottom of this file. Restoration ships incrementally per feature area.
   compiles).
 - `enum` was renamed to `e` at one call site in `GenKeyCodec` (`enum` is reserved in Scala 3).
 - `@targetName` annotation added to `CloseableIterator` overloaded methods.
+- HKT wildcard `_` → `?` in applied positions (slice 3.2). Type-argument-position syntax only;
+  kind-parameter declarations (`class Foo[F[_]]`, `def baz[M[_], A]`, etc.) preserved per Scala 3.
+  Pure type-level rename — no source-compat impact for downstream callers; type-argument site syntax
+  only.
 
 ### mongo
 
@@ -63,6 +67,8 @@ the bottom of this file. Restoration ships incrementally per feature area.
   Scala 3 forbids type projections on non-concrete prefixes). Public-API signature change.
 - `BsonValueOutput.write` / `BsonValueInput.read` call sites require explicit `using` keyword.
 - `MongoPolyDataCompanion` / `TypedMapFormat` / `TypedMapRefOps` widened from `K[_]` / `D[_]` to `K[Any]` / `D[Any]`.
+- HKT wildcard `_` → `?` in applied positions (slice 3.2). Same as core entry — pure
+  type-argument-position syntax change with no source-compat impact.
 
 ### hocon
 

core/src/main/scala/com/avsystem/commons/di/Component.scala

@@ -8,10 +8,10 @@ import java.util.concurrent.atomic.AtomicReference
 import scala.annotation.compileTimeOnly
 import scala.annotation.unchecked.uncheckedVariance
 
-case class ComponentInitializationException(component: Component[_], cause: Throwable)
+case class ComponentInitializationException(component: Component[?], cause: Throwable)
   extends Exception(s"failed to initialize component ${component.info}", cause)
 
-case class DependencyCycleException(cyclePath: List[Component[_]])
+case class DependencyCycleException(cyclePath: List[Component[?]])
   extends Exception(
     s"component dependency cycle detected:\n${cyclePath.iterator.map(_.info).map("  " + _).mkString(" ->\n")}"
   )
@@ -46,7 +46,7 @@ object ComponentInfo {
   */
 final class Component[+T](
   val info: ComponentInfo,
-  deps: => IndexedSeq[Component[_]],
+  deps: => IndexedSeq[Component[?]],
   creator: IndexedSeq[Any] => ExecutionContext => Future[T],
   destroyer: DestroyFunction[T] = Component.emptyDestroy,
   cachedStorage: Opt[AtomicReference[Future[T]]] = Opt.Empty,
@@ -61,18 +61,18 @@ final class Component[+T](
   /** Returns dependencies of this component extracted from the component definition. You can use this to inspect the
     * dependency graph without initializing any components.
     */
-  lazy val dependencies: IndexedSeq[Component[_]] = deps
+  lazy val dependencies: IndexedSeq[Component[?]] = deps
 
   private[this] val storage: AtomicReference[Future[T]] =
     cachedStorage.getOrElse(new AtomicReference)
 
-  private def sameStorage(otherStorage: AtomicReference[_]): Boolean =
+  private def sameStorage(otherStorage: AtomicReference[?]): Boolean =
     storage eq otherStorage
 
   // equality based on storage identity is important for cycle detection with cached components
   override def hashCode(): Int = storage.hashCode()
   override def equals(obj: Any): Boolean = obj match {
-    case c: Component[_] => c.sameStorage(storage)
+    case c: Component[?] => c.sameStorage(storage)
     case _ => false
   }
 
@@ -106,7 +106,7 @@ final class Component[+T](
 
   /** Forces a dependency on another component or components.
     */
-  def dependsOn(moreDeps: Component[_]*): Component[T] =
+  def dependsOn(moreDeps: Component[?]*): Component[T] =
     new Component(info, deps ++ moreDeps, creator, destroyer, cachedStorage)
 
   /** Specifies an asynchronous function that will be used to destroy this component, i.e. free up any resources that
@@ -189,7 +189,7 @@ object Component {
   def async[T](definition: => T): ExecutionContext => Future[T] =
     implicit ctx => Future(definition)
 
-  def validateAll(components: Seq[Component[_]]): Unit =
+  def validateAll(components: Seq[Component[?]]): Unit =
     GraphUtils.dfs(components)(
       _.dependencies.toList,
       onCycle = (node, stack) => {
@@ -203,9 +203,9 @@ object Component {
     * ensured that a component is only destroyed after all components that depend on it are destroyed (reverse
     * initialization order). Independent components are destroyed in parallel, using given `ExecutionContext`.
     */
-  def destroyAll(components: Seq[Component[_]])(implicit ec: ExecutionContext): Future[Unit] = {
-    val reverseGraph = new MHashMap[Component[_], MListBuffer[Component[_]]]
-    val terminals = new MHashSet[Component[_]]
+  def destroyAll(components: Seq[Component[?]])(implicit ec: ExecutionContext): Future[Unit] = {
+    val reverseGraph = new MHashMap[Component[?], MListBuffer[Component[?]]]
+    val terminals = new MHashSet[Component[?]]
     GraphUtils.dfs(components)(
       _.dependencies.toList,
       onEnter = { (c, _) =>
@@ -218,9 +218,9 @@ object Component {
           terminals += c
       },
     )
-    val destroyFutures = new MHashMap[Component[_], Future[Unit]]
+    val destroyFutures = new MHashMap[Component[?], Future[Unit]]
 
-    def doDestroy(c: Component[_]): Future[Unit] =
+    def doDestroy(c: Component[?]): Future[Unit] =
       destroyFutures.getOrElseUpdate(c, Future.traverse(reverseGraph(c))(doDestroy).flatMap(_ => c.doDestroy))
 
     Future.traverse(reverseGraph.keys)(doDestroy).toUnit

core/src/main/scala/com/avsystem/commons/di/Components.scala

@@ -29,7 +29,7 @@ trait Components extends ComponentsLowPrio {
   protected def asyncSingleton[T](definition: ExecutionContext => Future[T])(implicit sourceInfo: SourceInfo)
     : Component[T] = ???
 
-  private lazy val singletonsCache = new ConcurrentHashMap[ComponentInfo, AtomicReference[Future[_]]]
+  private lazy val singletonsCache = new ConcurrentHashMap[ComponentInfo, AtomicReference[Future[?]]]
 
   protected def cached[T](component: Component[T], freshInfo: ComponentInfo): Component[T] = {
     val cacheStorage =

core/src/main/scala/com/avsystem/commons/misc/TypeString.scala

@@ -31,11 +31,11 @@ object TypeString {
   // TODO[scala3-port]: TypeString.materialize (Scala 2 macro def) (L)
   implicit def materialize[T]: TypeString[T] = ???
 
-  implicit val keyCodec: GenKeyCodec[TypeString[_]] =
-    GenKeyCodec.create[TypeString[_]](new TypeString(_), _.value)
+  implicit val keyCodec: GenKeyCodec[TypeString[?]] =
+    GenKeyCodec.create[TypeString[?]](new TypeString(_), _.value)
 
-  implicit val codec: GenCodec[TypeString[_]] =
-    GenCodec.nonNullSimple[TypeString[_]](i => new TypeString(i.readString()), (o, ts) => o.writeString(ts.value))
+  implicit val codec: GenCodec[TypeString[?]] =
+    GenCodec.nonNullSimple[TypeString[?]](i => new TypeString(i.readString()), (o, ts) => o.writeString(ts.value))
 }
 
 /** Typeclass that contains JVM fully qualified class name corresponding to given type. `JavaClassName.of[T]` is always
@@ -80,11 +80,11 @@ object JavaClassName extends JavaClassNameLowPrio {
     new JavaClassName("[" + elementName)
   }
 
-  implicit val keyCodec: GenKeyCodec[JavaClassName[_]] =
-    GenKeyCodec.create[JavaClassName[_]](new JavaClassName(_), _.value)
+  implicit val keyCodec: GenKeyCodec[JavaClassName[?]] =
+    GenKeyCodec.create[JavaClassName[?]](new JavaClassName(_), _.value)
 
-  implicit val codec: GenCodec[JavaClassName[_]] =
-    GenCodec.nonNullSimple[JavaClassName[_]](i => new JavaClassName(i.readString()), (o, ts) => o.writeString(ts.value))
+  implicit val codec: GenCodec[JavaClassName[?]] =
+    GenCodec.nonNullSimple[JavaClassName[?]](i => new JavaClassName(i.readString()), (o, ts) => o.writeString(ts.value))
 }
 trait JavaClassNameLowPrio { this: JavaClassName.type =>
   // TODO[scala3-port]: JavaClassName.materialize (Scala 2 macro def) (L)

core/src/main/scala/com/avsystem/commons/misc/TypedMap.scala

@@ -78,7 +78,7 @@ object TypedMap {
   def empty[K[_]]: TypedMap[K] =
     new TypedMap[K](Map.empty)
 
-  def apply[K[_]](entries: Entry[K, _]*): TypedMap[K] = {
+  def apply[K[_]](entries: Entry[K, ?]*): TypedMap[K] = {
     val raw = Map.newBuilder[K[Any], Any]
     entries.foreach(e => raw += e.pair.asInstanceOf[(K[Any], Any)])
     new TypedMap[K](raw.result())

core/src/main/scala/com/avsystem/commons/rpc/AsRawReal.scala

@@ -119,7 +119,7 @@ object RpcMetadata {
   // TODO[scala3-port]: RpcMetadata.auto (Scala 2 macro def) (L)
   def auto[T]: T = ???
 
-  def nextInstance[T](it: Iterator[_], description: String): T =
+  def nextInstance[T](it: Iterator[?], description: String): T =
     if (it.hasNext) it.next().asInstanceOf[T]
     else throw new NoSuchElementException(s"typeclass instance for $description was not provided")
 }

core/src/main/scala/com/avsystem/commons/rpc/RPCFramework.scala

@@ -58,7 +58,7 @@ trait RPCFramework {
   trait Signature {
     @reifyName def name: String
     @multi
-    @rpcParamMetadata def paramMetadata: List[ParamMetadata[_]]
+    @rpcParamMetadata def paramMetadata: List[ParamMetadata[?]]
     @reifyAnnot
     @multi def annotations: List[MetadataAnnotation]
   }

core/src/main/scala/com/avsystem/commons/rpc/StandardRPCFramework.scala

@@ -16,7 +16,7 @@ trait ProcedureRPCFramework extends RPCFramework {
 
   case class ProcedureSignature(
     name: String,
-    paramMetadata: List[ParamMetadata[_]],
+    paramMetadata: List[ParamMetadata[?]],
     annotations: List[MetadataAnnotation],
   ) extends Signature
       with TypedMetadata[Unit]
@@ -34,7 +34,7 @@ trait FunctionRPCFramework extends RPCFramework {
 
   case class FunctionSignature[T](
     name: String,
-    paramMetadata: List[ParamMetadata[_]],
+    paramMetadata: List[ParamMetadata[?]],
     annotations: List[MetadataAnnotation],
     @infer resultTypeMetadata: ResultTypeMetadata[T],
   ) extends Signature
@@ -58,7 +58,7 @@ trait GetterRPCFramework extends RPCFramework {
 
   case class GetterSignature[T](
     name: String,
-    paramMetadata: List[ParamMetadata[_]],
+    paramMetadata: List[ParamMetadata[?]],
     annotations: List[MetadataAnnotation],
     @infer @checked resultMetadata: RPCMetadata.Lazy[T],
   ) extends Signature
@@ -75,8 +75,8 @@ trait StandardRPCFramework extends GetterRPCFramework with FunctionRPCFramework
     @reifyName name: String,
     @reifyAnnot @multi annotations: List[MetadataAnnotation],
     @multi @verbatim @rpcMethodMetadata procedureSignatures: Map[String, ProcedureSignature],
-    @multi @rpcMethodMetadata functionSignatures: Map[String, FunctionSignature[_]],
-    @multi @rpcMethodMetadata getterSignatures: Map[String, GetterSignature[_]],
+    @multi @rpcMethodMetadata functionSignatures: Map[String, FunctionSignature[?]],
+    @multi @rpcMethodMetadata getterSignatures: Map[String, GetterSignature[?]],
   )
   object RPCMetadata extends RpcMetadataCompanion[RPCMetadata]
 }
@@ -91,7 +91,7 @@ trait OneWayRPCFramework extends GetterRPCFramework with ProcedureRPCFramework {
     @reifyName name: String,
     @reifyAnnot @multi annotations: List[MetadataAnnotation],
     @multi @verbatim @rpcMethodMetadata procedureSignatures: Map[String, ProcedureSignature],
-    @multi @rpcMethodMetadata getterSignatures: Map[String, GetterSignature[_]],
+    @multi @rpcMethodMetadata getterSignatures: Map[String, GetterSignature[?]],
   )
   object RPCMetadata extends RpcMetadataCompanion[RPCMetadata]
 }

core/src/main/scala/com/avsystem/commons/serialization/FieldValues.scala

@@ -12,7 +12,7 @@ object FieldValues {
 }
 final class FieldValues(
   private val fieldNames: Array[String],
-  codecs: Array[GenCodec[_]],
+  codecs: Array[GenCodec[?]],
   ofWhat: OptArg[String] = OptArg.Empty,
 ) {
 

core/src/main/scala/com/avsystem/commons/serialization/GenCodec.scala

@@ -331,8 +331,8 @@ object GenCodec extends RecursiveAutoCodecs with TupleGenCodecs {
     }
   }
 
-  def underlyingCodec(codec: GenCodec[_]): GenCodec[_] = codec match {
-    case tc: Transformed[_, _] => underlyingCodec(tc.wrapped)
+  def underlyingCodec(codec: GenCodec[?]): GenCodec[?] = codec match {
+    case tc: Transformed[?, ?] => underlyingCodec(tc.wrapped)
     case _ => codec
   }
 

core/src/main/scala/com/avsystem/commons/serialization/InputOutput.scala

@@ -74,7 +74,7 @@ trait Output extends Any with AcceptsCustomEvents {
     * [[com.avsystem.commons.serialization.InputMetadata InputMetadata]] identifier. If this method returns `false` then
     * this `Output` does not support this medatata type and codec should fall back to some other serialization strategy.
     */
-  def keepsMetadata(metadata: InputMetadata[_]): Boolean = false
+  def keepsMetadata(metadata: InputMetadata[?]): Boolean = false
 
   /** This ugly workaround has been introduced when standard `Option` encoding changed from zero-or-one element list
     * encoding to unwrapped-or-null encoding which effectively disallowed serializing `null` and `Some(null)`. If some
@@ -168,7 +168,7 @@ trait SequentialOutput extends Any with AcceptsCustomEvents {
   /** Based on given collection's `knownSize` and [[sizePolicy]], declares the size of this output as size of this
     * collection if it is either cheap or required to do so.
     */
-  final def declareSizeOf(coll: BIterable[_]): Unit = sizePolicy match {
+  final def declareSizeOf(coll: BIterable[?]): Unit = sizePolicy match {
     case SizePolicy.Ignored =>
     case SizePolicy.Optional =>
       coll.knownSize match {

core/src/main/scala/com/avsystem/commons/serialization/cbor/CborAdtMetadata.scala

@@ -77,7 +77,7 @@ object CborAdtMetadata extends AdtMetadataCompanion[CborAdtMetadata] {
   final class CborKeyInfo[T](
     @reifyName val sourceName: String,
     @optional @reifyAnnot val nameAnnot: Opt[name],
-    @optional @reifyAnnot val cborKey: Opt[cborKey[_]],
+    @optional @reifyAnnot val cborKey: Opt[cborKey[?]],
   ) {
     val stringKey: String = nameAnnot.fold(sourceName)(_.name)
     val rawKey: RawCbor = cborKey.fold(CborOutput.writeRawCbor(stringKey))(_.rawKey)
@@ -86,9 +86,9 @@ object CborAdtMetadata extends AdtMetadataCompanion[CborAdtMetadata] {
   @positioned(positioned.here)
   final class Union[T](
     @reifyName val sourceName: String,
-    @optional @reifyAnnot val discriminator: Opt[cborDiscriminator[_]],
+    @optional @reifyAnnot val discriminator: Opt[cborDiscriminator[?]],
     @optional @reifyAnnot val flattenAnnot: Opt[flatten],
-    @multi @adtCaseMetadata val cases: List[Case[_]],
+    @multi @adtCaseMetadata val cases: List[Case[?]],
   ) extends CborAdtMetadata[T] { union =>
     private val caseNamesKeyCodec =
       new MappingCborKeyCodec(cases.map(_.keyInfo))
@@ -106,7 +106,7 @@ object CborAdtMetadata extends AdtMetadataCompanion[CborAdtMetadata] {
             nestedCodec.caseNames,
             nestedCodec.cases,
           ) {
-            def caseDependencies: Array[GenCodec[_]] =
+            def caseDependencies: Array[GenCodec[?]] =
               (nestedCodec.caseDependencies.iterator zip union.cases.iterator).map {
                 case (caseCodec: ApplyUnapplyCodec[Any @unchecked], theCase: CborAdtMetadata.Record[Any @unchecked]) =>
                   theCase.adjustCodec(caseCodec)
@@ -137,9 +137,9 @@ object CborAdtMetadata extends AdtMetadataCompanion[CborAdtMetadata] {
             override protected def doReadCaseName(input: Input): String =
               input.readCustom(RawCbor).map(caseNamesKeyCodec.strKeys).getOrElse(super.doReadCaseName(input))
 
-            def oooDependencies: Array[GenCodec[_]] = flatCodec.oooDependencies
+            def oooDependencies: Array[GenCodec[?]] = flatCodec.oooDependencies
 
-            def caseDependencies: Array[GenCodec.OOOFieldsObjectCodec[_]] =
+            def caseDependencies: Array[GenCodec.OOOFieldsObjectCodec[?]] =
               (flatCodec.caseDependencies.iterator zip union.cases.iterator).map {
                 case (caseCodec: ApplyUnapplyCodec[Any @unchecked], theCase: CborAdtMetadata.Record[Any @unchecked]) =>
                   theCase.adjustFlatCaseCodec(caseCodec)
@@ -175,7 +175,7 @@ object CborAdtMetadata extends AdtMetadataCompanion[CborAdtMetadata] {
   @positioned(positioned.here)
   final class Record[T](
     @composite val keyInfo: CborKeyInfo[T],
-    @multi @adtParamMetadata val fields: List[Field[_]],
+    @multi @adtParamMetadata val fields: List[Field[?]],
   ) extends Case[T] {
     private val keyCodec = new MappingCborKeyCodec(fields.map(_.keyInfo))
 
@@ -223,7 +223,7 @@ object CborAdtMetadata extends AdtMetadataCompanion[CborAdtMetadata] {
     def validate(): Unit = ()
   }
 
-  private class MappingCborKeyCodec(keyInfos: List[CborKeyInfo[_]]) extends CborKeyCodec {
+  private class MappingCborKeyCodec(keyInfos: List[CborKeyInfo[?]]) extends CborKeyCodec {
     val rawKeys: Map[String, RawCbor] =
       keyInfos.mkMap(_.stringKey, _.rawKey)
     val strKeys: Map[RawCbor, String] =

core/src/main/scala/com/avsystem/commons/serialization/cbor/CborOutput.scala

@@ -172,7 +172,7 @@ class CborOutput(out: DataOutput, keyCodec: CborKeyCodec, sizePolicy: SizePolicy
         super.writeCustom(typeMarker, value)
     }
 
-  override def keepsMetadata(metadata: InputMetadata[_]): Boolean = metadata match {
+  override def keepsMetadata(metadata: InputMetadata[?]): Boolean = metadata match {
     case InitialByte | Tags => true
     case _ => super.keepsMetadata(metadata)
   }

core/src/main/scala/com/avsystem/commons/serialization/json/JsonStringOutput.scala

@@ -123,7 +123,7 @@ final class JsonStringOutput(builder: JStringBuilder, options: JsonOptions = Jso
 
   def writeRawJson(json: String): Unit = builder.append(json)
 
-  override def keepsMetadata(metadata: InputMetadata[_]): Boolean =
+  override def keepsMetadata(metadata: InputMetadata[?]): Boolean =
     metadata == JsonType
 
   override def writeCustom[T](typeMarker: TypeMarker[T], value: T): Boolean =

core/src/main/scala/com/avsystem/commons/serialization/macroCodecs.scala

@@ -24,7 +24,7 @@ abstract class ApplyUnapplyCodec[T](
 ) extends ErrorReportingCodec[T]
     with OOOFieldsObjectCodec[T] {
 
-  protected def dependencies: Array[GenCodec[_]]
+  protected def dependencies: Array[GenCodec[?]]
   protected def instantiate(fieldValues: FieldValues): T
 
   private[this] lazy val deps = dependencies
@@ -130,7 +130,7 @@ abstract class SealedHierarchyCodec[T](
   val typeRepr: String,
   val nullable: Boolean,
   val caseNames: Array[String],
-  val cases: Array[Class[_]],
+  val cases: Array[Class[?]],
 ) extends ErrorReportingCodec[T]
     with ObjectCodec[T] {
 
@@ -149,10 +149,10 @@ abstract class NestedSealedHierarchyCodec[T](
   typeRepr: String,
   nullable: Boolean,
   caseNames: Array[String],
-  cases: Array[Class[_]],
+  cases: Array[Class[?]],
 ) extends SealedHierarchyCodec[T](typeRepr, nullable, caseNames, cases) {
 
-  def caseDependencies: Array[GenCodec[_]]
+  def caseDependencies: Array[GenCodec[?]]
 
   private[this] lazy val caseDeps = caseDependencies
 
@@ -177,16 +177,16 @@ abstract class FlatSealedHierarchyCodec[T](
   typeRepr: String,
   nullable: Boolean,
   caseNames: Array[String],
-  cases: Array[Class[_]],
+  cases: Array[Class[?]],
   val oooFieldNames: Array[String],
   val caseDependentFieldNames: Set[String],
   override val caseFieldName: String,
   val defaultCaseIdx: Int,
   val defaultCaseTransient: Boolean,
 ) extends SealedHierarchyCodec[T](typeRepr, nullable, caseNames, cases) {
 
-  def oooDependencies: Array[GenCodec[_]]
-  def caseDependencies: Array[OOOFieldsObjectCodec[_]]
+  def oooDependencies: Array[GenCodec[?]]
+  def caseDependencies: Array[OOOFieldsObjectCodec[?]]
 
   private[this] lazy val oooDeps = oooDependencies
   private[this] lazy val caseDeps = caseDependencies
@@ -386,7 +386,7 @@ abstract class JavaBuilderBasedCodec[T, B](
 ) extends ErrorReportingCodec[T]
     with GenCodec.ObjectCodec[T] {
 
-  protected def dependencies: Array[GenCodec[_]]
+  protected def dependencies: Array[GenCodec[?]]
 
   private lazy val deps = dependencies