Fix rebase breakage

Author: odersky

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

@@ -84,9 +84,9 @@ class GadtConstraint private (
     }
 
   def externalize(tp: Type, theMap: TypeMap | Null = null)(using Context): Type = tp match
-    case param: TypeParamRef => reverseMapping(param) match
-      case sym: Symbol => sym.typeRef
-      case null        => param
+    case param: TypeParamRef =>
+      val sym = reverseMapping(param)
+      if sym != null then sym.typeRef else param
     case tp: TypeAlias       => tp.derivedAlias(externalize(tp.alias, theMap))
     case tp                  => (if theMap == null then ExternalizeMap() else theMap).mapOver(tp)