Fix #4342: Handle Scala2 method calls with an outer param

When calling a Scala 2.x method with an outer parameter, adapt to the
old behavior in picking the outer parameter's type, i.e., prefer the
enclosing class's self type, rather than the enclosing class itself.

Author: gsps

compiler/src/dotty/tools/dotc/transform/ExplicitOuter.scala

@@ -140,6 +140,20 @@ object ExplicitOuter {
   private def newOuterAccessors(cls: ClassSymbol)(implicit ctx: Context) =
     newOuterAccessor(cls, cls) :: (if (cls is Trait) Nil else newOuterParamAccessor(cls) :: Nil)
 
+  /** Scala 2.x and Dotty don't always agree on what should be the type of the outer parameter,
+   *  so we replicate the old behavior when passing arguments to methods coming from Scala 2.x.
+   */
+  private def outerClass(cls: ClassSymbol)(implicit ctx: Context): Symbol = {
+    val encl = cls.owner.enclosingClass
+    if (cls.is(Scala2x))
+      encl.asClass.classInfo.selfInfo match {
+        case tp: TypeRef => tp.classSymbol
+        case self: Symbol => self
+        case _ => encl
+      }
+    else encl
+  }
+
   /** A new outer accessor or param accessor.
    *  @param  owner  The class where the outer accessor is located
    *  @param  cls    The class relative to which the outer is computed (can be a base class of owner)
@@ -155,7 +169,7 @@ object ExplicitOuter {
    */
   private def newOuterSym(owner: ClassSymbol, cls: ClassSymbol, name: TermName, flags: FlagSet)(implicit ctx: Context) = {
     val outerThis = owner.owner.enclosingClass.thisType
-    val outerCls = cls.owner.enclosingClass
+    val outerCls = outerClass(cls)
     val target =
       if (owner == cls)
         outerCls.appliedRef
@@ -340,7 +354,7 @@ object ExplicitOuter {
       if (hasOuterParam(cls)) {
         val mt @ MethodTpe(pnames, ptypes, restpe) = tp
         mt.derivedLambdaType(
-          nme.OUTER :: pnames, cls.owner.enclosingClass.typeRef :: ptypes, restpe)
+          nme.OUTER :: pnames, outerClass(cls).typeRef :: ptypes, restpe)
       } else tp
 
     /** If function in an apply node is a constructor that needs to be passed an