Replace @Stable with @uncheckedStable

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

@@ -921,7 +921,6 @@ class Definitions {
   @tu lazy val ParamMetaAnnot: ClassSymbol = requiredClass("scala.annotation.meta.param")
   @tu lazy val SetterMetaAnnot: ClassSymbol = requiredClass("scala.annotation.meta.setter")
   @tu lazy val ShowAsInfixAnnot: ClassSymbol = requiredClass("scala.annotation.showAsInfix")
-  @tu lazy val StableAnnot: ClassSymbol = requiredClass("scala.annotation.internal.Stable")
   @tu lazy val FunctionalInterfaceAnnot: ClassSymbol = requiredClass("java.lang.FunctionalInterface")
   @tu lazy val TargetNameAnnot: ClassSymbol = requiredClass("scala.annotation.targetName")
   @tu lazy val VarargsAnnot: ClassSymbol = requiredClass("scala.annotation.varargs")

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

@@ -168,7 +168,11 @@ object Types {
       case _: SingletonType | NoPrefix => true
       case tp: RefinedOrRecType => tp.parent.isStable
       case tp: ExprType => tp.resultType.isStable
-      case tp: AnnotatedType => tp.annot.symbol == defn.StableAnnot || tp.parent.isStable
+      case tp: AnnotatedType =>
+        // NOTE UncheckedStableAnnot was originally meant to be put on fields,
+        // not on types. Allowing it on types is a Scala 3 extension. See:
+        // https://www.scala-lang.org/files/archive/spec/2.11/11-annotations.html#scala-compiler-annotations
+        tp.annot.symbol == defn.UncheckedStableAnnot || tp.parent.isStable
       case tp: AndType =>
         // TODO: fix And type check when tp contains type parames for explicit-nulls flow-typing
         // see: tests/explicit-nulls/pos/flow-stable.scala.disabled

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -3539,7 +3539,7 @@ class Typer extends Namer
                   // this is needed for -Ycheck. Without the annotation Ycheck will
                   // skolemize the result type which will lead to different types before
                   // and after checking. See i11955.scala.
-                  AnnotatedType(conj, Annotation(defn.StableAnnot))
+                  AnnotatedType(conj, Annotation(defn.UncheckedStableAnnot))
                 else conj
               else pt
             gadts.println(i"insert GADT cast from $tree to $target")