SAMtype extractor should not use avoidance in same class

When extracting a SAMtype we use avoidance of the class parameters since these
are not visible outside the class. But we should not do that when the SAMType
is extracted with an owner within the class.

Fixes a problem with sammy_scope.scala that was left dormant until now since
RefChecks did not check methods generated by ExpandSAMs.

Author: odersky

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

@@ -5006,7 +5006,9 @@ object Types {
                     mapOver(tp)
                 }
               }
-              val approx = approxParams(mt).asInstanceOf[MethodType]
+              val approx =
+                if ctx.owner.isContainedIn(cls) then mt
+                else approxParams(mt).asInstanceOf[MethodType]
               Some(approx)
             case _ =>
               None