Fix #5831: types in parent visible in child under Scala2

Author: liufengyun

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

@@ -1680,9 +1680,12 @@ object SymDenotations {
           val denots1 = collect(denots, ps)
           p.classSymbol.denot match {
             case parentd: ClassDenotation =>
+              val parentDenots =
+                // Scala2 compatibility: private types are visible in children, see i5831.scala
+                if (name.isTypeName && ctx.scala2Setting) parentd.membersNamed(name)
+                else parentd.nonPrivateMembersNamed(name)
               denots1 union
-                parentd.nonPrivateMembersNamed(name)
-                .mapInherited(ownDenots, denots1, thisType)
+                parentDenots.mapInherited(ownDenots, denots1, thisType)
             case _ =>
               denots1
           }

tests/pos-scala2/i5831.scala

@@ -0,0 +1,5 @@
+trait Matchers { matchers =>
+  private sealed trait Collected
+  def foo(collected: Collected): Int  = 3
+}
+object Matchers extends Matchers