Fix assertion error message

Author: odersky

src/dotty/tools/dotc/ast/Desugar.scala

@@ -67,7 +67,7 @@ object desugar {
             val defctx = ctx.outersIterator.dropWhile(_.scope eq ctx.scope).next
             var local = defctx.denotNamed(tp.name).suchThat(_ is ParamOrAccessor).symbol
             if (local.exists) (defctx.owner.thisType select local).dealias
-            else throw new Error(s"no matching symbol for ${sym.showLocated} in ${defctx.owner} / ${defctx.effectiveScope}")
+            else throw new Error(s"no matching symbol for ${tp.symbol.showLocated} in ${defctx.owner} / ${defctx.effectiveScope}")
           case _ =>
             mapOver(tp)
         }