Fix scala#1401: Make sure all refs are forwarded

Faced with recursive dependencies through self types, we might have
to apply `normalizeToClassRefs` to a class P with a parent that is not
yet initialized (witnessed by P's parents being Nil). In that case
we should still execute forwardRefs on P, but we have to
wait in a suspension until P is initialized.

This avoids the problem raised in scala#1401. I am still not quite sure
why forwardRefs is needed, but it seems that asSeenFrom alone is not
enough to track the dependencies in this case.

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

@@ -361,13 +361,23 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
      *  to the current scope, provided (1) variances of both aliases are the same, and
      *  (2) X is not yet defined in current scope. This "short-circuiting" prevents
      *  long chains of aliases which would have to be traversed in type comparers.
+     *
+     *  Note: Test i1401.scala shows that `forwardRefs` is also necessary
+     *  for typechecking in the case where self types refer to type parameters
+     *  that are upper-bounded by subclass instances.
      */
     def forwardRefs(from: Symbol, to: Type, prefs: List[TypeRef]) = to match {
       case to @ TypeBounds(lo1, hi1) if lo1 eq hi1 =>
-        for (pref <- prefs)
-          for (argSym <- pref.decls)
-            if (argSym is BaseTypeArg)
-              forwardRef(argSym, from, to, cls, decls)
+        for (pref <- prefs) {
+          def forward(): Unit =
+            for (argSym <- pref.decls)
+              if (argSym is BaseTypeArg)
+                forwardRef(argSym, from, to, cls, decls)
+          pref.info match {
+            case info: TempClassInfo => info.suspensions = (() => forward()) :: info.suspensions // !!! dotty deviation `forward` alone does not eta expand
+            case _ => forward()
+          }
+        }
       case _ =>
     }
 
@@ -419,9 +429,9 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
         s"redefinition of ${decls.lookup(name).debugString} in ${cls.showLocated}")
       enterArgBinding(formals(name), refinedInfo, cls, decls)
     }
-    // Forward definitions in super classes that have one of the refined paramters
+    // Forward definitions in super classes that have one of the refined parameters
     // as aliases directly to the refined info.
-    // Note that this cannot be fused bwith the previous loop because we now
+    // Note that this cannot be fused with the previous loop because we now
     // assume that all arguments have been entered in `decls`.
     refinements foreachBinding { (name, refinedInfo) =>
       forwardRefs(formals(name), refinedInfo, parentRefs)

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

@@ -3043,9 +3043,20 @@ object Types {
     override def toString = s"ClassInfo($prefix, $cls)"
   }
 
-  final class CachedClassInfo(prefix: Type, cls: ClassSymbol, classParents: List[TypeRef], decls: Scope, selfInfo: DotClass)
+  class CachedClassInfo(prefix: Type, cls: ClassSymbol, classParents: List[TypeRef], decls: Scope, selfInfo: DotClass)
     extends ClassInfo(prefix, cls, classParents, decls, selfInfo)
 
+  /** A class for temporary class infos where `parents` are not yet known. */
+  final class TempClassInfo(prefix: Type, cls: ClassSymbol, decls: Scope, selfInfo: DotClass)
+  extends CachedClassInfo(prefix, cls, Nil, decls, selfInfo) {
+
+    /** A list of actions that were because they rely on the class info of `cls` to
+     *  be no longer temporary. These actions will be performed once `cls` gets a real
+     *  ClassInfo.
+     */
+    var suspensions: List[() => Unit] = Nil
+  }
+
   object ClassInfo {
     def apply(prefix: Type, cls: ClassSymbol, classParents: List[TypeRef], decls: Scope, selfInfo: DotClass = NoType)(implicit ctx: Context) =
       unique(new CachedClassInfo(prefix, cls, classParents, decls, selfInfo))

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

@@ -106,7 +106,7 @@ object Scala2Unpickler {
         // `denot.sourceModule.exists` provision i859.scala crashes in the backend.
         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
+    denot.info = new TempClassInfo(denot.owner.thisType, denot.classSymbol, 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) {

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

@@ -688,7 +688,8 @@ class Namer { typer: Typer =>
         else createSymbol(self)
 
       // pre-set info, so that parent types can refer to type params
-      denot.info = ClassInfo(cls.owner.thisType, cls, Nil, decls, selfInfo)
+      val tempClassInfo = new TempClassInfo(cls.owner.thisType, cls, decls, selfInfo)
+      denot.info = tempClassInfo
 
       // Ensure constructor is completed so that any parameter accessors
       // which have type trees deriving from its parameters can be
@@ -705,6 +706,8 @@ class Namer { typer: Typer =>
 
       index(rest)(inClassContext(selfInfo))
       denot.info = ClassInfo(cls.owner.thisType, cls, parentRefs, decls, selfInfo)
+      tempClassInfo.suspensions.foreach(_())
+
       Checking.checkWellFormed(cls)
       if (isDerivedValueClass(cls)) cls.setFlag(Final)
       cls.setApplicableFlags(

test/dotc/scala-collections.whitelist

@@ -83,7 +83,7 @@
 ./scala-scala/src/library/scala/collection/immutable/Seq.scala
 ./scala-scala/src/library/scala/collection/mutable/IndexedSeq.scala
 ./scala-scala/src/library/scala/collection/mutable/ListBuffer.scala
-#./scala-scala/src/library/scala/collection/mutable/BufferLike.scala // works under junit, fails under partest, but can't see more info on the cause
+./scala-scala/src/library/scala/collection/mutable/BufferLike.scala
 
 ./scala-scala/src/library/scala/collection/mutable/ArrayBuilder.scala
 

tests/pos/i1401.scala

@@ -0,0 +1,25 @@
+package i1401
+
+trait Subtractable[A, +Repr <: Subtractable[A, Repr]] {
+  def -(elem: A): Repr
+}
+
+trait BufferLike[BA, +This <: BufferLike[BA, This] with Buffer[BA]]
+                extends Subtractable[BA, This]
+{ self : This =>
+
+  /* Without fix-#1401:
+   *
+     error: overriding method - in trait Subtractable of type (elem: A)This & i1401.Buffer[A];
+     method - of type (elem: BA)This has incompatible type
+     def -(elem: BA): This
+         ^
+     one error found
+  */
+  def -(elem: BA): This
+}
+
+trait Buffer[A] extends BufferLike[A, Buffer[A]]
+
+
+