Small optimization

Author: odersky

compiler/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

@@ -848,7 +848,7 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
           isImplicit = tag == IMPLICITMETHODtpe || params.nonEmpty && params.head.is(Implicit))
         val result = maker.fromSymbols(params, restpe)
         result.resType match
-          case restpe1: MethodType =>
+          case restpe1: MethodType if restpe1 ne restpe =>
             val prevResParams = paramsOfMethodType.remove(restpe)
             if prevResParams != null then
               paramsOfMethodType.put(restpe1, prevResParams)