Ensure ExprType treat underlying annotations correctly

ExprTypes sometimes hold `AnnotatedType` underneath.
Make sure they are handled:
 - Before transformation, check under ExprTypes in `hasAnnotation`
 - During transformation, change `=> @annot T` to `@annot () ?=> T` instead of `() ?=> @annot T`
This makes `erased => T` parameters work, although it is of little value...

Author: natsukagami

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

@@ -1179,6 +1179,8 @@ class Definitions {
     def apply(tp: Type)(using Context): Type = tp match
       case tp @ EventuallyCapturingType(tp1, refs) if tp.annot.symbol == RetainsByNameAnnot =>
         CapturingType(apply(tp1), refs)
+      case AnnotatedType(parent, annot) =>
+        AnnotatedType(apply(parent), annot)
       case _ =>
         defn.ContextFunction0.typeRef.appliedTo(tp :: Nil)
     def unapply(tp: Type)(using Context): Option[Type] = tp match

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

@@ -374,6 +374,7 @@ object Types {
     /** Does the type carry an annotation that is an instance of `cls`? */
     @tailrec final def hasAnnotation(cls: ClassSymbol)(using Context): Boolean = stripTypeVar match {
       case AnnotatedType(tp, annot) => (annot matches cls) || (tp hasAnnotation cls)
+      case ExprType(tp) => tp.hasAnnotation(cls) // ExprType closes over the underlying annotated type. See `addAnnotation`.
       case _ => false
     }