Fix problems arising when hierarchies of classes are under completion

Fix problems arising when hierarchies of classes are under completion at the same time.
In this case it can happen that we see a subclass (e.g. Arrays.scala) which depends
on a superclass (e.g. GenTraversableLike.scala) that itself does not have its parents
defined yet. Previously, several things went wrong here

 - One of the base classes would be marked as frozen, even though it dod not have all
   members entered yet. This led to an error in finger printing.
 - The baseclasses and super class bits of the subclass would be computed before the parents
   of the middle class were known. The baseclasses would then be chached, leading to
   false results for isDerivedFrom.

We need to refine the logic for computing base classes, super class bits, and fingerprints
to account for that issue.

Author: odersky

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

@@ -348,6 +348,9 @@ object Flags {
   /** A bridge method. Set by Erasure */
   final val Bridge = termFlag(34, "<bridge>")
 
+  /** All class attributes are fully defined */
+  final val FullyCompleted = typeFlag(34, "<fully-completed>")
+
   /** Symbol is a Java varargs bridge */ // (needed?)
   final val VBridge = termFlag(35, "<vbridge>") // TODO remove
 

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

@@ -1213,8 +1213,17 @@ object SymDenotations {
      *  ClassDenotations compiled from source are first completed, then fully completed.
      *  @see Namer#ClassCompleter
      */
-    private def isFullyCompleted(implicit ctx: Context): Boolean =
-      isCompleted && classParents.nonEmpty
+    private def isFullyCompleted(implicit ctx: Context): Boolean = {
+      def isFullyCompletedRef(tp: TypeRef) = tp.denot match {
+        case d: ClassDenotation => d.isFullyCompleted
+        case _ => false
+      }
+      def isLocallyFullyCompleted =
+        if (classParents.isEmpty) is(Package) || symbol.eq(defn.AnyClass)
+        else classParents.forall(isFullyCompletedRef)
+      flagsUNSAFE.is(FullyCompleted) ||
+        isCompleted && isLocallyFullyCompleted && { setFlag(FullyCompleted); true }
+    }
 
     // ------ syncing inheritance-related info -----------------------------
 
@@ -1300,7 +1309,7 @@ object SymDenotations {
         baseTypeRefValid = ctx.runId
       }
 
-    private def computeBases(implicit ctx: Context): Unit = {
+    private def computeBases(implicit ctx: Context): (List[ClassSymbol], BitSet) = {
       if (myBaseClasses eq Nil) throw CyclicReference(this)
       myBaseClasses = Nil
       val seen = new mutable.BitSet
@@ -1324,17 +1333,22 @@ object SymDenotations {
         case nil =>
           to
       }
-      myBaseClasses = classSymbol :: addParentBaseClasses(classParents, Nil)
-      mySuperClassBits = seen.toImmutable
+      val bcs = classSymbol :: addParentBaseClasses(classParents, Nil)
+      val scbits = seen.toImmutable
+      if (isFullyCompleted) {
+        myBaseClasses = bcs
+        mySuperClassBits = scbits
+      }
+      else myBaseClasses = null
+      (bcs, scbits)
     }
 
     /** A bitset that contains the superId's of all base classes */
     private def superClassBits(implicit ctx: Context): BitSet =
       if (classParents.isEmpty) BitSet() // can happen when called too early in Namers
       else {
         checkBasesUpToDate()
-        if (mySuperClassBits == null) computeBases
-        mySuperClassBits
+        if (mySuperClassBits != null) mySuperClassBits else computeBases._2
       }
 
     /** The base classes of this class in linearization order,
@@ -1344,8 +1358,7 @@ object SymDenotations {
       if (classParents.isEmpty) classSymbol :: Nil // can happen when called too early in Namers
       else {
         checkBasesUpToDate()
-        if (myBaseClasses == null) computeBases
-        myBaseClasses
+        if (myBaseClasses != null) myBaseClasses else computeBases._1
       }
 
     final override def derivesFrom(base: Symbol)(implicit ctx: Context): Boolean =
@@ -1378,9 +1391,9 @@ object SymDenotations {
       while (ps.nonEmpty) {
         val parent = ps.head.typeSymbol
         parent.denot match {
-          case classd: ClassDenotation =>
-            fp.include(classd.memberFingerPrint)
-            parent.denot.setFlag(Frozen)
+          case parentDenot: ClassDenotation =>
+            fp.include(parentDenot.memberFingerPrint)
+            if (parentDenot.isFullyCompleted) parentDenot.setFlag(Frozen)
           case _ =>
         }
         ps = ps.tail
@@ -1393,10 +1406,13 @@ object SymDenotations {
      *  not be used for package classes because cache never
      *  gets invalidated.
      */
-    def memberFingerPrint(implicit ctx: Context): FingerPrint = {
-      if (myMemberFingerPrint == FingerPrint.unknown) myMemberFingerPrint = computeMemberFingerPrint
-      myMemberFingerPrint
-    }
+    def memberFingerPrint(implicit ctx: Context): FingerPrint =
+      if (myMemberFingerPrint != FingerPrint.unknown) myMemberFingerPrint
+      else {
+        val fp = computeMemberFingerPrint
+        if (isFullyCompleted) myMemberFingerPrint = fp
+        fp
+      }
 
     private[this] var myMemberCache: LRUCache[Name, PreDenotation] = null
     private[this] var myMemberCachePeriod: Period = Nowhere

test/dotc/scala-collections.whitelist

@@ -140,7 +140,7 @@
 
 # breaks genMapLike https://github.com/lampepfl/dotty/issues/937
 #./scala-scala/src/library/scala/collection/GenTraversable.scala
-#./scala-scala/src/library/scala/collection/GenTraversableLike.scala
+./scala-scala/src/library/scala/collection/GenTraversableLike.scala
 
 ./scala-scala/src/library/scala/collection/GenTraversableOnce.scala
 ./scala-scala/src/library/scala/collection/IndexedSeq.scala