Fix problems handling annotations in dependent function types

Fix problems handling function types that are dependent through their result annotations.

Author: odersky

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

@@ -67,6 +67,19 @@ object Annotations {
         else if diff.exists then derivedAnnotation(tm.mapOver(tree))
         else this
 
+    /** Does this annotation refer to a parameter of `tl`?
+     *  Overridden in ConcreteAnnotation
+     */
+    def refersToParamOf(tl: TermLambda)(using Context): Boolean =
+      val args = arguments
+      if args.isEmpty then false
+      else tree.existsSubTree {
+        case id: Ident => id.tpe match
+          case TermParamRef(tl1, _) => tl eq tl1
+          case _ => false
+        case _ => false
+      }
+
     /** A string representation of the annotation. Overridden in BodyAnnotation.
      */
     def toText(printer: Printer): Text = printer.annotText(this)

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

@@ -3604,6 +3604,9 @@ object Types {
           case tp: AppliedType => tp.fold(status, compute(_, _, theAcc))
           case tp: TypeVar if !tp.isInstantiated => combine(status, Provisional)
           case tp: TermParamRef if tp.binder eq thisLambdaType => TrueDeps
+          case AnnotatedType(parent, ann) =>
+            if ann.refersToParamOf(thisLambdaType) then TrueDeps
+            else compute(status, parent, theAcc)
           case _: ThisType | _: BoundType | NoPrefix => status
           case _ =>
             (if theAcc != null then theAcc else DepAcc()).foldOver(status, tp)
@@ -3656,8 +3659,10 @@ object Types {
       if (isResultDependent) {
         val dropDependencies = new ApproximatingTypeMap {
           def apply(tp: Type) = tp match {
-            case tp @ TermParamRef(thisLambdaType, _) =>
+            case tp @ TermParamRef(`thisLambdaType`, _) =>
               range(defn.NothingType, atVariance(1)(apply(tp.underlying)))
+            case AnnotatedType(parent, ann) if ann.refersToParamOf(thisLambdaType) =>
+              mapOver(parent)
             case _ => mapOver(tp)
           }
         }

tests/pos/dependent-annot.scala

@@ -0,0 +1,7 @@
+class C
+class ann(x: Any*) extends annotation.Annotation
+
+def f(y: C, z: C) =
+  def g(): C @ann(y, z) = ???
+  val ac: ((x: C) => Array[String @ann(x)]) = ???
+  val dc = ac(g())