Treat non-open classes as effectively sealed.

This does not affect pattern matching exhaustivity (yet) since currently
only abstract sealed classes and sealed traits are checked for exhaustivity,
so defualt classes do not count. I wonder whether we should change that (?).

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

@@ -509,6 +509,7 @@ object Flags {
   /** Flags retained in export forwarders */
   val RetainedExportFlags = Given | Implicit | Extension
 
+  /** Flags that apply only to classes */
   val ClassOnlyFlags = Sealed | Open | Abstract.toTypeFlags
 
 // ------- Other flag sets -------------------------------------
@@ -517,6 +518,7 @@ object Flags {
   val AbstractOverride: FlagSet              = Abstract | Override
   val AbstractSealed: FlagSet                = Abstract | Sealed
   val AbstractOrTrait: FlagSet               = Abstract | Trait
+  val EffectivelyOpenFlags                   = Abstract | JavaDefined | Open | Scala2x | Trait
   val PrivateAccessor: FlagSet               = Accessor | Private
   val AccessorOrSynthetic: FlagSet           = Accessor | Synthetic
   val EnumCase: FlagSet                      = Case | Enum

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

@@ -1071,7 +1071,13 @@ object SymDenotations {
     final def isEffectivelyFinal(implicit ctx: Context): Boolean =
       isOneOf(EffectivelyFinalFlags) || !owner.isClass || owner.isOneOf(FinalOrModuleClass) || owner.isAnonymousClass
 
-    /** The class containing this denotation which has the given effective name. */
+    /** A class is effectively sealed if has the `final` or `sealed` modifier, or it
+     *  is defined in Scala 3 and is neither abstract nor open.
+     */
+    final def isEffectivelySealed(given Context): Boolean =
+      isOneOf(FinalOrSealed) || isClass && !isOneOf(EffectivelyOpenFlags)
+
+      /** The class containing this denotation which has the given effective name. */
     final def enclosingClassNamed(name: Name)(implicit ctx: Context): Symbol = {
       val cls = enclosingClass
       if (cls.effectiveName == name || !cls.exists) cls else cls.owner.enclosingClassNamed(name)
@@ -1369,16 +1375,16 @@ object SymDenotations {
      */
     def typeParamCreationFlags: FlagSet = TypeParam
 
-    override def toString: String = {
-      val kindString =
-        if (myFlags.is(ModuleClass)) "module class"
-        else if (isClass) "class"
-        else if (isType) "type"
-        else if (myFlags.is(Module)) "module"
-        else if (myFlags.is(Method)) "method"
-        else "val"
-      s"$kindString $name"
-    }
+    def kindString: String =
+      if myFlags.is(ModuleClass) then "module class"
+      else if myFlags.is(Trait)  then "trait"
+      else if isClass            then "class"
+      else if isType             then "type"
+      else if myFlags.is(Module) then "module"
+      else if myFlags.is(Method) then "method"
+      else                            "val"
+
+    override def toString: String = s"$kindString $name"
 
     // ----- Sanity checks and debugging */
 

compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala

@@ -1790,13 +1790,10 @@ object messages {
     extends Message(ClassAndCompanionNameClashID) {
     val kind: String = "Naming"
     val msg: String = em"Name clash: both ${cls.owner} and its companion object defines ${cls.name.stripModuleClassSuffix}"
-    val explanation: String = {
-      val kind = if (cls.owner.is(Flags.Trait)) "trait" else "class"
-
-      em"""|A $kind and its companion object cannot both define a ${hl("class")}, ${hl("trait")} or ${hl("object")} with the same name:
+    val explanation: String =
+      em"""|A ${cls.kindString} and its companion object cannot both define a ${hl("class")}, ${hl("trait")} or ${hl("object")} with the same name:
            |  - ${cls.owner} defines ${cls}
            |  - ${other.owner} defines ${other}"""
-      }
   }
 
   case class TailrecNotApplicable(symbol: Symbol)(implicit ctx: Context)

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

@@ -91,8 +91,10 @@ class SymUtils(val self: Symbol) extends AnyVal {
    *   - all of its children are generic products or singletons
    */
   def whyNotGenericSum(implicit ctx: Context): String =
-    if (!self.is(Sealed))
-      s"it is not a sealed ${if (self.is(Trait)) "trait" else "class"}"
+    if !self.isEffectivelySealed then
+      s"it is not a sealed ${self.kindString}"
+    else if self.is(Final) then
+      s"it is a final class"
     else {
       val children = self.children
       val companion = self.linkedClass
@@ -175,12 +177,16 @@ class SymUtils(val self: Symbol) extends AnyVal {
   def isEnumAnonymClass(implicit ctx: Context): Boolean =
     self.isAnonymousClass && (self.owner.name.eq(nme.DOLLAR_NEW) || self.owner.is(CaseVal))
 
-  /** Is this symbol defined locally (i.e. at some level owned by a term) and
-   *  defined in a different toplevel class than its supposed parent class `cls`?
-   *  Such children are not pickled, and have to be reconstituted manually.
+  /** Is this symbol defined locally (i.e. at some level owned by a term) so that
+   *  it cannot be seen from parent class `cls`?
    */
-  def isInaccessibleChildOf(cls: Symbol)(implicit ctx: Context): Boolean =
-    self.isLocal && !cls.topLevelClass.isLinkedWith(self.topLevelClass)
+  def isInaccessibleChildOf(cls: Symbol)(given Context): Boolean =
+    def isAccessible(sym: Symbol): Boolean =
+      sym == cls
+      || sym == cls.owner
+      || sym.owner.is(Package)
+      || sym.owner.isType && isAccessible(sym.owner)
+    !isAccessible(self)
 
   /** If this is a sealed class, its known children in the order of textual occurrence */
   def children(implicit ctx: Context): List[Symbol] = {

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

@@ -509,7 +509,7 @@ class SyntheticMembers(thisPhase: DenotTransformer) {
       if (clazz.is(Case)) makeSingletonMirror()
       else if (linked.isGenericProduct) makeProductMirror(linked)
       else if (linked.isGenericSum) makeSumMirror(linked)
-      else if (linked.is(Sealed))
+      else if linked.isEffectivelySealed then
         derive.println(i"$linked is not a sum because ${linked.whyNotGenericSum}")
     }
     else if (impl.removeAttachment(ExtendsSingletonMirror).isDefined)

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -550,7 +550,7 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
   def canDecompose(tp: Type): Boolean = {
     val dealiasedTp = tp.dealias
     val res =
-      (tp.classSymbol.is(Sealed) &&
+      (tp.classSymbol.isEffectivelySealed &&
         tp.classSymbol.isOneOf(AbstractOrTrait) &&
         !tp.classSymbol.hasAnonymousChild &&
         tp.classSymbol.children.nonEmpty ) ||
@@ -672,7 +672,7 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
           if (mergeList) "_: _*" else "_: List"
         else if (scalaConsType.isRef(sym))
           if (mergeList) "_, _: _*"  else "List(_, _: _*)"
-        else if (tp.classSymbol.is(Sealed) && tp.classSymbol.hasAnonymousChild)
+        else if (tp.classSymbol.isEffectivelySealed && tp.classSymbol.hasAnonymousChild)
           "_: " + showType(tp) + " (anonymous)"
         else if (tp.classSymbol.is(CaseClass) && !hasCustomUnapply(tp.classSymbol))
         // use constructor syntax for case class

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

@@ -876,10 +876,12 @@ class Namer { typer: Typer =>
 
       def register(child: Symbol, parent: Type) = {
         val cls = parent.classSymbol
-        if (cls.is(Sealed))
-          if ((child.isInaccessibleChildOf(cls) || child.isAnonymousClass) && !sym.hasAnonymousChild)
+        if cls.isEffectivelySealed
+           && child.associatedFile == cls.associatedFile // don't register ad-hoc extensions as children
+        then
+          if child.isInaccessibleChildOf(cls) && !sym.hasAnonymousChild then
             addChild(cls, cls)
-          else if (!cls.is(ChildrenQueried))
+          else if !cls.is(ChildrenQueried) then
             addChild(cls, child)
           else
             ctx.error(em"""children of $cls were already queried before $sym was discovered.