fix nested @unchecked

Author: liufengyun

compiler/src/dotty/tools/dotc/ast/Trees.scala

@@ -805,7 +805,7 @@ object Trees {
     def unforced: AnyRef
     protected def force(x: AnyRef): Unit
     def forceIfLazy(implicit ctx: Context): T = unforced match {
-      case lzy: Lazy[T] =>
+      case lzy: Lazy[T @unchecked] =>
         val x = lzy.complete
         force(x)
         x

compiler/src/dotty/tools/dotc/transform/IsInstanceOfChecker.scala

@@ -64,7 +64,10 @@ object Checkable {
 
     def replaceBinderMap(implicit ctx: Context) = new TypeMap {
       def apply(tp: Type) = tp match {
-        case tref: TypeRef if !tref.typeSymbol.isClass && tref.symbol.is(Case) => WildcardType
+        case tref: TypeRef
+        if !tref.typeSymbol.isClass && tref.symbol.is(Case) => WildcardType
+        case AnnotatedType(_, annot)
+        if annot.symbol == defn.UncheckedAnnot => WildcardType
         case _ => mapOver(tp)
       }
     }

tests/neg-custom-args/isInstanceOf/t2755.scala

@@ -55,4 +55,7 @@ object Test {
     println(f3(Array(1L)))
     println(f3(null))
   }
+
+  def foo(x: Any): Boolean =
+    x.isInstanceOf[List[String]]  // error
 }