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

Author: odersky

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

@@ -312,7 +312,7 @@ final class ProperGadtConstraint private(
 
   override def fresh = new ProperGadtConstraint
   override def restore(other: GadtConstraint): Unit =
-    assert(!other.isNarrowing, "cannot restore a non-empty GADTMap")
+    assert(other eq EmptyGadtConstraint, "cannot restore a non-empty GADTMap")
 
   override def debugBoundsDescription(using Context): String = "EmptyGadtConstraint"