Eta expand according to expected type parameter variance

When eta expanding a type `C` to `[vX] => C[X]` the variance `v`
is now the variance of the expected type, not the variance of the
type constructor `C`, as long as the variance of the expected type
is compatible with the variance of `C`.

1 files changed, 47 insertions, 2 deletions

diff --git a/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala b/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala
index 321846de6..8f4b47058 100644
--- a/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala
+++ b/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala
@@ -151,6 +151,51 @@ object Scala2Unpickler {
     denot.info = ClassInfo( // final info
       denot.owner.thisType, denot.classSymbol, parentRefs, declsInRightOrder, ost)
   }
+
+  /** Adapt arguments to type parameters so that variance of type lambda arguments
+   *  agrees with variance of corresponding higherkinded type parameters. Example:
+   *
+   *     class Companion[+CC[X]]
+   *     Companion[List]
+   *
+   *  with adaptArgs, this will expand to
+   *
+   *     Companion[[X] => List[X]]
+   *
+   *  instead of
+   *
+   *      Companion[[+X] => List[X]]
+   *
+   *  even though `List` is covariant. This adaptation is necessary to ignore conflicting
+   *  variances in overriding members that have types of hk-type parameters such as `Companion[GenTraversable]`
+   *  or `Companion[ListBuffer]`. Without the adaptation we would end up with
+   *
+   *      Companion[[+X] => GenTraversable[X]]
+   *      Companion[[X] => List[X]]
+   *
+   *  and the second is not a subtype of the first. So if we have overridding memebrs of the two
+   *  types we get an error.
+   */
+  def adaptArgs(tparams: List[Symbol], args: List[Type])(implicit ctx: Context): List[Type] = tparams match {
+    case tparam :: tparams1 =>
+      val boundLambda = tparam.infoOrCompleter match {
+        case TypeBounds(_, hi) => hi.LambdaClass(forcing = false)
+        case _ => NoSymbol
+      }
+      def adaptArg(arg: Type): Type = arg match {
+        case arg: TypeRef if arg.symbol.isLambdaTrait =>
+          assert(arg.symbol.typeParams.length == boundLambda.typeParams.length)
+          arg.prefix.select(boundLambda)
+        case arg: RefinedType =>
+          arg.derivedRefinedType(adaptArg(arg.parent), arg.refinedName, arg.refinedInfo)
+        case _ =>
+          arg
+      }
+      val arg = args.head
+      val adapted = if (boundLambda.exists) adaptArg(arg) else arg
+      adapted :: adaptArgs(tparams1, args.tail)
+    case nil => args
+  }
 }
 
 /** Unpickle symbol table information descending from a class and/or module root
@@ -723,8 +768,8 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
           else TypeRef(pre, sym.name.asTypeName)
         val args = until(end, readTypeRef)
         if (sym == defn.ByNameParamClass2x) ExprType(args.head)
-        else if (args.isEmpty && sym.typeParams.nonEmpty) tycon.EtaExpand
-        else tycon.appliedTo(args)
+        else if (args.isEmpty && sym.typeParams.nonEmpty) tycon.EtaExpand(sym.typeParams)
+        else tycon.appliedTo(adaptArgs(sym.typeParams, args))
       case TYPEBOUNDStpe =>
         TypeBounds(readTypeRef(), readTypeRef())
       case REFINEDtpe =>