Drop convention on classes inheriting from universal capturing types

Drop convention that classes inheriting from universal capturing types are capability classes. Capture sets of parents
are instead ignored.

The convention led to algebraic anomalies. For instance if

    class C extends A => B, Serializable

then C <: (A => B) & Serializable, which has an empty capture set. Yet we treat every occurrence of C as
implicitly carrying `cap`.

Author: odersky

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

@@ -23,7 +23,6 @@ trait SetupAPI:
   def setupUnit(tree: Tree, recheckDef: DefRecheck)(using Context): Unit
   def isPreCC(sym: Symbol)(using Context): Boolean
   def postCheck()(using Context): Unit
-  def isCapabilityClassRef(tp: Type)(using Context): Boolean
 
 object Setup:
 
@@ -68,29 +67,6 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
     && !sym.owner.is(CaptureChecked)
     && !defn.isFunctionSymbol(sym.owner)
 
-  private val capabilityClassMap = new util.HashMap[Symbol, Boolean]
-
-  /** Check if the class is capability, which means:
-   *  1. the class has a capability annotation,
-   *  2. or at least one of its parent type has universal capability.
-   */
-  def isCapabilityClassRef(tp: Type)(using Context): Boolean = tp.dealiasKeepAnnots match
-    case _: TypeRef | _: AppliedType =>
-      val sym = tp.classSymbol
-      def checkSym: Boolean = sym.info.parents.exists(hasUniversalCapability)
-      sym.isClass && capabilityClassMap.getOrElseUpdate(sym, checkSym)
-    case _ => false
-
-  private def hasUniversalCapability(tp: Type)(using Context): Boolean = tp.dealiasKeepAnnots match
-    case CapturingType(parent, refs) =>
-      refs.isUniversal || hasUniversalCapability(parent)
-    case AnnotatedType(parent, ann) =>
-      if ann.symbol.isRetains then
-        try ann.tree.toCaptureSet.isUniversal || hasUniversalCapability(parent)
-        catch case ex: IllegalCaptureRef => false
-      else hasUniversalCapability(parent)
-    case tp => isCapabilityClassRef(tp)
-
   private def fluidify(using Context) = new TypeMap with IdempotentCaptRefMap:
     def apply(t: Type): Type = t match
       case t: MethodType =>
@@ -317,10 +293,7 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
           case t: TypeVar =>
             this(t.underlying)
           case t =>
-            // Map references to capability classes C to C^
-            if isCapabilityClassRef(t)
-            then CapturingType(t, defn.expandedUniversalSet, boxed = false)
-            else recur(t)
+            recur(t)
     end expandAliases
 
     val tp1 = expandAliases(tp) // TODO: Do we still need to follow aliases?

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

@@ -33,7 +33,6 @@ object Recheck:
    *  Scala2ModuleVar cannot be also ParamAccessors.
    */
   val ResetPrivate = Scala2ModuleVar
-  val ResetPrivateParamAccessor = ResetPrivate | ParamAccessor
 
   /** Attachment key for rechecked types of TypeTrees */
   val RecheckedType = Property.Key[Type]