Split Abstract from Open

The Abstract flag no longer implies Open; the two flags are
handled separately.

Author: odersky

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

@@ -287,7 +287,7 @@ object Flags {
    *  Note: You should never see Abstract on any symbol except a class.
    *  Note: the flag counts as common, because it can be combined with OVERRIDE in a term.
    */
-  val (Abstract @ _, _, _) = newFlags(23, "abstract")
+  val (Abstract @ _, _, AbstractClass @ _) = newFlags(23, "abstract")
 
   /** Lazy val or method is known or assumed to be stable and realizable */
   val (_, StableRealizable @ _, _) = newFlags(24, "<stable>")
@@ -518,7 +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 EffectivelyOpenFlags                   = 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/classfile/ClassfileConstants.scala

@@ -343,7 +343,7 @@ object ClassfileConstants {
       case JAVA_ACC_FINAL      => Final
       case JAVA_ACC_SYNTHETIC  => Synthetic
       case JAVA_ACC_STATIC     => JavaStatic
-      case JAVA_ACC_ABSTRACT   => if (isClass) Abstract else Deferred
+      case JAVA_ACC_ABSTRACT   => if (isClass) Abstract | Open else Deferred
       case JAVA_ACC_INTERFACE  => PureInterfaceCreationFlags | JavaDefined
       case _                   => EmptyFlags
     }

compiler/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

@@ -469,6 +469,8 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
       name = name.asTermName.unmangle(SuperAccessorName)
       flags = flags &~ Scala2SuperAccessor
     }
+    if flags.is(Abstract, butNot = FinalOrSealed) then
+      flags = flags | Open
     name = name.mapLast(_.decode)
 
     def nameMatches(rootName: Name) = name == rootName

compiler/src/dotty/tools/dotc/tastyreflect/ReflectionCompilerInterface.scala

@@ -1544,6 +1544,7 @@ class ReflectionCompilerInterface(val rootContext: core.Contexts.Context) extend
   def Flags_Private: Flags = core.Flags.Private
   def Flags_Protected: Flags = core.Flags.Protected
   def Flags_Abstract: Flags = core.Flags.Abstract
+  def Flags_Open: Flags = core.Flags.Open
   def Flags_Final: Flags = core.Flags.Final
   def Flags_Sealed: Flags = core.Flags.Sealed
   def Flags_Case: Flags = core.Flags.Case

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

@@ -292,7 +292,9 @@ class Namer { typer: Typer =>
           case _ => false
         if !ok then
           ctx.error(i"modifier(s) `${flags.flagsString}` incompatible with $kind definition", tree.sourcePos)
-        if adapted.is(Private) && canBeLocal then adapted | Local else adapted
+        if adapted.is(Private) && canBeLocal then adapted | Local
+        else if adapted.is(AbstractClass, butNot = FinalOrSealed) then adapted | Open
+        else adapted
       }
 
     /** Add moduleClass/sourceModule to completer if it is for a module val or class */

tests/pos-special/adhoc-extension/A.scala

@@ -1,2 +1,3 @@
 package adhoc
-class A
+class A
+abstract class Abs

tests/pos-special/adhoc-extension/B.scala

@@ -1,3 +1,4 @@
 package adhoc
 import language.adhocExtensions
 class B extends A
+class B2 extends Abs