Address review comments

Author: Linyxus

compiler/src/dotty/tools/dotc/cc/CaptureOps.scala

@@ -445,15 +445,12 @@ extension (tp: AnnotatedType)
     case ann: CaptureAnnotation => ann.boxed
     case _ => false
 
-class CleanupRetains(using Context) extends TypeMap:
-  def apply(tp: Type): Type = cleanupRetains(tp, this)
-
 /** Drop retains annotations in the type. */
-def cleanupRetains(tp: Type, theMap: CleanupRetains | Null = null)(using Context): Type =
-  def mapOver = (if theMap != null then theMap else new CleanupRetains).mapOver(tp)
-  tp match
-    case RetainingType(tp, _) => tp
-    case _ => mapOver
+class CleanupRetains(using Context) extends TypeMap:
+  def apply(tp: Type): Type =
+    tp match
+      case RetainingType(tp, _) => tp
+      case _ => mapOver(tp)
 
 /** An extractor for `caps.reachCapability(ref)`, which is used to express a reach
  *  capability as a tree in a @retains annotation.

compiler/src/dotty/tools/dotc/transform/PostTyper.scala

@@ -286,8 +286,12 @@ class PostTyper extends MacroTransform with InfoTransformer { thisPhase =>
     // See #20035.
     private def cleanupRetainsAnnot(symbol: Symbol, tpt: Tree)(using Context): Tree =
       tpt match
-        case tpt: InferredTypeTree if !symbol.allOverriddenSymbols.hasNext =>
-          val tpe1 = cleanupRetains(tpt.tpe) 
+        case tpt: InferredTypeTree
+        if !symbol.allOverriddenSymbols.hasNext =>
+          // if there are overridden symbols, the annotation comes from an explicit type of the overridden symbol
+          // and should be retained.
+          val tm = new CleanupRetains
+          val tpe1 = tm(tpt.tpe)
           tpt.withType(tpe1)
         case _ => tpt
 

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -48,7 +48,7 @@ import staging.StagingLevel
 import reporting.*
 import Nullables.*
 import NullOpsDecorator.*
-import cc.{CheckCaptures, isRetainsLike, cleanupRetains}
+import cc.{CheckCaptures, isRetainsLike}
 import config.Config
 import config.MigrationVersion