Warn-unused uses type alias in nesting check

A reference from the body of a class is not a usage
of the class, but a reference to a local type alias of it
is a usage of the type alias.

Author: som-snytt

src/compiler/scala/tools/nsc/typechecker/TypeDiagnostics.scala

@@ -527,7 +527,7 @@ trait TypeDiagnostics {
           // Only record type references which don't originate within the
           // definition of the class being referenced.
           if (t.tpe ne null) {
-            for (tp <- t.tpe if !treeTypes(tp) && !currentOwner.ownerChain.contains(tp.typeSymbol)) {
+            for (tp <- t.tpe if !treeTypes(tp) && !currentOwner.hasTransOwner(tp.typeSymbolDirect)) {
               tp match {
                 case NoType | NoPrefix    =>
                 case NullaryMethodType(_) =>
@@ -550,7 +550,7 @@ trait TypeDiagnostics {
               m.isType
           && !m.isTypeParameterOrSkolem // would be nice to improve this
           && (m.isPrivate || m.isLocalToBlock)
-          && !(treeTypes.exists(tp => tp exists (t => t.typeSymbolDirect == m)))
+          && !(treeTypes.exists(_.exists(_.typeSymbolDirect == m)))
         )
         def isSyntheticWarnable(sym: Symbol) = (
           sym.isDefaultGetter 

test/files/pos/t10623.flags

@@ -0,0 +1 @@
+-Xfatal-warnings -Xlint:unused

test/files/pos/t10623.scala

@@ -0,0 +1,15 @@
+
+import language.higherKinds
+
+object `package` {
+  def refl[A]: A Is A = ???
+}
+
+sealed trait Is[A, B] { ab =>
+  def subst[F[_]](fa: F[A]): F[B]
+  final def flip: B Is A = {
+    type f[a] = a Is A
+    subst[f](refl)
+  }
+}
+