Avoid spurious warnings about forward references in refinements

The warning was triggered by cases like:
class A
type F = A { type T = Int; def f: T }
Which is treated differently from the following which did not produce a warning:
type F = A { type T = Int } { def f: T }

1 files changed, 2 insertions, 2 deletions

diff --git a/src/dotty/tools/dotc/typer/Checking.scala b/src/dotty/tools/dotc/typer/Checking.scala
index 9e13ec13b..56b5100b1 100644
--- a/src/dotty/tools/dotc/typer/Checking.scala
+++ b/src/dotty/tools/dotc/typer/Checking.scala
@@ -162,13 +162,13 @@ object Checking {
    *  This depends also on firming up the DOT calculus. For the moment we only issue
    *  deprecated warnings, not errors.
    */
-  def checkRefinementNonCyclic(refinement: Tree, refineCls: ClassSymbol)(implicit ctx: Context): Unit = {
+  def checkRefinementNonCyclic(refinement: Tree, refineCls: ClassSymbol, seen: mutable.Set[Symbol])
+    (implicit ctx: Context): Unit = {
     def flag(what: String, tree: Tree) =
       ctx.deprecationWarning(i"$what reference in refinement is deprecated", tree.pos)
     def forwardRef(tree: Tree) = flag("forward", tree)
     def selfRef(tree: Tree) = flag("self", tree)
     val checkTree = new TreeAccumulator[Unit] {
-      private var seen = Set[Symbol]()
       def checkRef(tree: Tree, sym: Symbol) =
         if (sym.maybeOwner == refineCls && !seen(sym)) forwardRef(tree)
       def apply(x: Unit, tree: Tree) = tree match {