Make checking state final

Overriding methods of State is erroneous and should be forbidden.

Author: liufengyun

compiler/src/dotty/tools/dotc/transform/init/Checking.scala

@@ -30,7 +30,7 @@ object Checking {
    *
    */
 
-  case class State(
+  final case class State(
     var visited: Set[Effect],                  // effects that have been checked or are being checked
     path: Vector[Tree],                        // the path that leads to the current effect
     thisClass: ClassSymbol,                    // the concrete class of `this`
@@ -47,7 +47,6 @@ object Checking {
       this.visited = state.visited
       res
 
-
     def isFieldInitialized(field: Symbol): Boolean =
       init || fieldsInited.contains(field)
 

compiler/src/dotty/tools/dotc/transform/init/CycleChecker.scala

@@ -216,9 +216,7 @@ class CycleChecker(cache: Cache) {
         dependencies = mutable.Set.empty,
         env = env,
         init = true
-      ) {
-        override def isFieldInitialized(field: Symbol): Boolean = true
-      }
+      )
 
       val pot = Hot(dep.cls)(dep.source)
       val effs = pot.potentialsOf(dep.symbol)(using env).promote(dep.source)