Stabilize boxedType cache

Make sure we cache the correct type.

Author: odersky

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

@@ -16,7 +16,7 @@ case class CaptureAnnotation(refs: CaptureSet, boxed: Boolean)(cls: Symbol) exte
   import CaptureAnnotation.*
   import tpd.*
 
-  var boxedType: Type = NoType
+  val boxedType = BoxedTypeCache()
 
   override def tree(using Context) =
     val elems = refs.elems.toList.map {
@@ -66,3 +66,16 @@ case class CaptureAnnotation(refs: CaptureSet, boxed: Boolean)(cls: Symbol) exte
     case _ => false
 
 end CaptureAnnotation
+
+/** A one-element cache for the boxed version of an unboxed capturing type */
+class BoxedTypeCache:
+  private var boxed: Type = compiletime.uninitialized
+  private var unboxed: Type = NoType
+
+  def apply(tp: AnnotatedType)(using Context): Type =
+    if tp ne unboxed then
+      unboxed = tp
+      val CapturingType(parent, refs) = tp: @unchecked
+      boxed = CapturingType(parent, refs, boxed = true)
+    boxed
+end BoxedTypeCache

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

@@ -11,7 +11,7 @@ import util.Property.Key
 import tpd.*
 
 private val Captures: Key[CaptureSet] = Key()
-private val Boxed: Key[Type] = Key()
+private val BoxedType: Key[BoxedTypeCache] = Key()
 
 def retainedElems(tree: Tree)(using Context): List[Tree] = tree match
   case Apply(_, Typed(SeqLiteral(elems, _), _) :: Nil) => elems
@@ -51,13 +51,12 @@ extension (tp: Type)
       if tp.isBoxed || refs.isAlwaysEmpty then tp
       else tp.annot match
         case ann: CaptureAnnotation =>
-          if !ann.boxedType.exists then ann.boxedType = boxedTp
-          ann.boxedType
+          ann.boxedType(tp)
         case ann =>
-          ann.tree.getAttachment(Boxed) match
-            case None => ann.tree.putAttachment(Boxed, boxedTp)
+          ann.tree.getAttachment(BoxedType) match
+            case None => ann.tree.putAttachment(BoxedType, BoxedTypeCache())
             case _ =>
-          ann.tree.attachment(Boxed)
+          ann.tree.attachment(BoxedType)(tp)
     case _ =>
       tp