Fix #21402: Always allow type member extraction for stable scrutinees in match types.

Previously, through the various code paths, we basically allowed
type member extraction for stable scrutinees if the type member
was an alias or a class member. In the alias case, we took the
alias, whereas in the class case, we recreated a selection on the
stable scrutinee. We did not allow that on abstract type members.

We now uniformly do it for all kinds of type members. If the
scrutinee is a (non-skolem) stable type, we do not even look at
the info of the type member. We directly create a selection to it,
which corresponds to what we did before for class members.

We only try to dealias type members if the scrutinee type is not
a stable type.

Author: sjrd

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -3681,19 +3681,37 @@ class MatchReducer(initctx: Context) extends TypeComparer(initctx) {
 
             stableScrut.member(typeMemberName) match
               case denot: SingleDenotation if denot.exists =>
-                val info = denot.info match
-                  case alias: AliasingBounds           => alias.alias        // Extract the alias
-                  case ClassInfo(prefix, cls, _, _, _) => prefix.select(cls) // Re-select the class from the prefix
-                  case info => info // Notably, RealTypeBounds, which will eventually give a MatchResult.NoInstances
-                val info1 = stableScrut match
+                val info = stableScrut match
                   case skolem: SkolemType =>
-                    dropSkolem(info, skolem).orElse:
-                      info match
-                        case info: TypeBounds  => info                       // Will already trigger a MatchResult.NoInstances
-                        case _                 => RealTypeBounds(info, info) // Explicitly trigger a MatchResult.NoInstances
-                  case _ => info
-                rec(capture, info1, variance = 0, scrutIsWidenedAbstract)
+                    /* If it is a skolem type, we cannot have class selections nor
+                     * abstract type selections. If it is an alias, we try to remove
+                     * any reference to the skolem from the right-hand-side. If that
+                     * succeeds, we take the result, otherwise we fail as not-specific.
+                     */
+
+                    def adaptToTriggerNotSpecific(info: Type): Type = info match
+                      case info: TypeBounds => info
+                      case _                => RealTypeBounds(info, info)
+
+                    denot.info match
+                      case denotInfo: AliasingBounds =>
+                        val alias = denotInfo.alias
+                        dropSkolem(alias, skolem).orElse(adaptToTriggerNotSpecific(alias))
+                      case ClassInfo(prefix, cls, _, _, _) =>
+                        // for clean error messages
+                        adaptToTriggerNotSpecific(prefix.select(cls))
+                      case denotInfo =>
+                        adaptToTriggerNotSpecific(denotInfo)
+
+                  case _ =>
+                    // The scrutinee type is truly stable. We select the type member directly on it.
+                    stableScrut.select(typeMemberName)
+                end info
+
+                rec(capture, info, variance = 0, scrutIsWidenedAbstract)
+
               case _ =>
+                // The type member was not found; no match
                 false
       end rec
 

tests/pos/i21402.scala

@@ -0,0 +1,41 @@
+abstract class AbstractServiceKey:
+  type Protocol
+
+abstract class ServiceKey[T] extends AbstractServiceKey:
+  type Protocol = T
+
+type Aux[P] = AbstractServiceKey { type Protocol = P }
+type Service[K <: Aux[?]] = K match
+  case Aux[t] => ActorRef[t]
+type Subscriber[K <: Aux[?]] = K match
+  case Aux[t] => ActorRef[ReceptionistMessages.Listing[t]]
+
+trait ActorRef[-T]
+
+object ReceptionistMessages:
+  final case class Listing[T](key: ServiceKey[T])
+
+class TypedMultiMap[T <: AnyRef, K[_ <: T]]:
+  def get(key: T): Set[K[key.type]] = ???
+  transparent inline def getInlined(key: T): Set[K[key.type]] = ???
+  inline def inserted(key: T, value: K[key.type]): TypedMultiMap[T, K] = ???
+
+object LocalReceptionist {
+  final case class State(
+      services: TypedMultiMap[AbstractServiceKey, Service],
+      subscriptions: TypedMultiMap[AbstractServiceKey, Subscriber]
+  ):
+    def testInsert(key: AbstractServiceKey)(serviceInstance: ActorRef[key.Protocol]): State = {
+      val fails = services.inserted(key, serviceInstance) // error
+      ???
+    }
+
+  def testGet[T](key: AbstractServiceKey): Unit = {
+    val newState: State = ???
+    val fails: Set[ActorRef[key.Protocol]] = newState.services.get(key) // error
+    val works: Set[ActorRef[key.Protocol]] = newState.services.getInlined(key) // workaround
+
+    val fails2: Set[ActorRef[ReceptionistMessages.Listing[key.Protocol]]] = newState.subscriptions.get(key) // error
+    val works2: Set[ActorRef[ReceptionistMessages.Listing[key.Protocol]]] = newState.subscriptions.getInlined(key) // workaround
+  }
+}

tests/pos/match-type-extract-path-dependent.scala

@@ -0,0 +1,27 @@
+// Test that match types can extract path-dependent abstract types out of singleton types
+
+trait Base:
+  type Value
+
+  def getValue(): Value
+  def setValue(v: Value): Unit
+end Base
+
+object Extractor:
+  type Helper[X] = Base { type Value = X }
+
+  type Extract[B <: Base] = B match
+    case Helper[x] => x
+end Extractor
+
+object Test:
+  import Extractor.Extract
+
+  /* As is, this is a bit silly, since we could use `b.Value` instead. However,
+   * in larger examples with more indirections, it is not always possible to
+   * directly use the path-dependent version. See i21402 for a real-world use
+   * case.
+   */
+  def foo(b: Base): Extract[b.type] = b.getValue()
+  def bar(b: Base, v: Extract[b.type]): Unit = b.setValue(v)
+end Test