Avoid cyclic reference errors when unpickling classes

Set info early in order to avoid cyclic reference errors.
Errors were observed when compiling

    scala/Predef.scala scala/package.scala scala/collection/GenSeqLike.scala

Author: odersky

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

@@ -1883,5 +1883,5 @@ object SymDenotations {
 
   private val AccessorOrLabel = Accessor | Label
 
-  private var indent = 0 // for completions printing
+  @sharable private var indent = 0 // for completions printing
 }

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

@@ -99,17 +99,21 @@ object Scala2Unpickler {
       case TempPolyType(tps, cinfo) => (tps, cinfo)
       case cinfo => (Nil, cinfo)
     }
+    val ost =
+      if ((selfInfo eq NoType) && (denot is ModuleClass))
+        denot.owner.thisType select denot.sourceModule
+      else selfInfo
+
+    denot.info = ClassInfo(denot.owner.thisType, denot.classSymbol, Nil, decls, ost) // first rough info to avoid CyclicReferences
     var parentRefs = ctx.normalizeToClassRefs(parents, cls, decls)
     if (parentRefs.isEmpty) parentRefs = defn.ObjectType :: Nil
     for (tparam <- tparams) {
       val tsym = decls.lookup(tparam.name)
       if (tsym.exists) tsym.setFlag(TypeParam)
       else denot.enter(tparam, decls)
     }
-    val ost =
-      if ((selfInfo eq NoType) && (denot is ModuleClass))
-        denot.owner.thisType select denot.sourceModule
-      else selfInfo
+    denot.info = ClassInfo(
+      denot.owner.thisType, denot.classSymbol, parentRefs, decls, ost) // more refined infowith parents
     if (!(denot.flagsUNSAFE is JavaModule)) ensureConstructor(denot.symbol.asClass, decls)
 
     val scalacCompanion = denot.classSymbol.scalacLinkedClass
@@ -140,7 +144,7 @@ object Scala2Unpickler {
         decls1
       }
 
-    denot.info = ClassInfo(
+    denot.info = ClassInfo( // final info
       denot.owner.thisType, denot.classSymbol, parentRefs, declsInRightOrder, ost)
   }
 }

test/dotc/scala-collections.whitelist

@@ -64,7 +64,6 @@
 ./scala-scala/src/library/scala/Product8.scala
 ./scala-scala/src/library/scala/Product9.scala
 
-
 ./scala-scala/src/library/scala/Proxy.scala
 ./scala-scala/src/library/scala/SerialVersionUID.scala
 ./scala-scala/src/library/scala/Serializable.scala
@@ -136,3 +135,9 @@
 ./scala-scala/src/library/scala/transient.scala
 ./scala-scala/src/library/scala/unchecked.scala
 ./scala-scala/src/library/scala/volatile.scala
+
+# cyclic reference
+./scala-scala/src/library/scala/collection/GenSeq.scala
+./scala-scala/src/library/scala/collection/GenSeqLike.scala
+
+