fix type signature to pass null check

Author: Linyxus

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

@@ -157,7 +157,7 @@ final class ProperGadtConstraint private(
     */
   def subsumes(left: GadtConstraint, right: GadtConstraint, pre: GadtConstraint)(using Context): Boolean =
     def checkSubsumes(left: ProperGadtConstraint, right: ProperGadtConstraint, pre: ProperGadtConstraint): Boolean = {
-      def rightToLeft: TypeParamRef => TypeParamRef = {
+      def rightToLeft: TypeParamRef => TypeParamRef | Null = {
         val preParams = pre.constraint.domainParams.toSet
         val mapping = {
           var res: SimpleIdentityMap[TypeParamRef, TypeParamRef] = SimpleIdentityMap.empty