more flexible reified check for final classes

Summary:
## What

loosen a reified check, see test case

## Why

The reified check (among other things) prevents trying to reify an unknown type. So it is understandably careful with late static bindings, since there are cases where the type cannot be known statically.

However, in final classes, `this` isn't really late static binding: we know that it just refers to the lexically enclosing class and so the type is known.

Reviewed By: andrewjkennedy

Differential Revision: D75142486

fbshipit-source-id: 2541472b4a9d9431989f9dea41477c612bb7e26b

Author: mheiber

hphp/hack/src/typing/tast_check/reified_check.ml

@@ -10,7 +10,6 @@
 open Hh_prelude
 open Aast
 open Typing_defs
-open Typing_reified_check (* validator *)
 module Env = Tast_env
 module SN = Naming_special_names
 module UA = SN.UserAttributes
@@ -118,7 +117,7 @@ let verify_targ_valid env reification tparam targ =
           err
       in
 
-      validator#validate_hint
+      Typing_reified_check.validator#validate_hint
         (Tast_env.tast_env_as_typing_env env)
         (snd targ)
         ~reification

hphp/hack/src/typing/type_validator.ml

@@ -28,6 +28,7 @@ type validation_state = {
   inside_reified_class_generic_position: bool;
   reification: reification;
   expanded_typedefs: SSet.t;
+  class_from_taccess_lhs: Folded_class.t option;
 }
 
 type error_emitter = Pos.t -> (Pos_or_decl.t * string) list Lazy.t -> unit
@@ -94,8 +95,15 @@ class virtual type_validator =
                      already reported an error. *)
                   None
                 | Ok ety_env ->
-                  Some (this#on_typeconst { acc with ety_env } class_ typeconst)
-              )
+                  (* stash and restore `class_from_taccess_lhs` *)
+                  let { class_from_taccess_lhs; _ } = acc in
+                  let acc =
+                    this#on_typeconst
+                      { acc with ety_env; class_from_taccess_lhs = Some class_ }
+                      class_
+                      typeconst
+                  in
+                  Some { acc with class_from_taccess_lhs } )
           | _ -> acc)
 
     method! on_tapply acc r (pos, name) tyl =
@@ -178,6 +186,7 @@ class virtual type_validator =
             validity = Valid;
             inside_reified_class_generic_position = false;
             reification;
+            class_from_taccess_lhs = None;
           }
           root_ty
       in

hphp/hack/src/typing/type_validator.mli

@@ -22,6 +22,10 @@ type validation_state = {
   inside_reified_class_generic_position: bool;
   reification: reification;
   expanded_typedefs: SSet.t;
+      (**
+  `Some Klass` if we're traversing a type and have gone through `Klass::TheTy`.
+   *)
+  class_from_taccess_lhs: Folded_class.t option;
 }
 
 type error_emitter = Pos.t -> (Pos_or_decl.t * string) list Lazy.t -> unit

hphp/hack/src/typing/typing_reified_check.ml

@@ -79,7 +79,16 @@ let validator =
       super#on_taccess acc r (root, ids)
 
     method! on_tthis acc r =
-      this#invalid acc r "the late static bound `this` type"
+      let is_known_to_be_final =
+        match acc.class_from_taccess_lhs with
+        | Some class_ -> Folded_class.final class_
+        | None -> false
+      in
+      if is_known_to_be_final then
+        (* In a final class, `this` is known statically and so is safe here *)
+        acc
+      else
+        this#invalid acc r "the late static bound `this` type"
 
     method! on_trefinement acc r _ty _ = this#invalid acc r "type refinement"
   end

hphp/hack/test/typecheck/reified_generics/this_is_like_id_in_final_classes.php

@@ -0,0 +1,20 @@
+<?hh
+
+type Id_of<+T> = int;
+
+final class C1 {
+  const type T = Id_of<this>;
+}
+
+class C2 {
+  const type T = Id_of<this>;
+}
+
+function take_refied<reify T>(): void {}
+
+function test(): void {
+  // OK
+  take_refied<C1::T>();
+  // error
+  take_refied<C2::T>();
+}

hphp/hack/test/typecheck/reified_generics/this_is_like_id_in_final_classes.php.exp

@@ -0,0 +1,6 @@
+ERROR: File "this_is_like_id_in_final_classes.php", line 19, characters 15-19:
+Invalid reified hint (Typing[4305])
+  File "this_is_like_id_in_final_classes.php", line 10, characters 24-27:
+  This is the late static bound `this` type, it cannot be used as a reified type argument
+  File "this_is_like_id_in_final_classes.php", line 13, characters 28-28:
+  `T` is reified