Extend rank check

Summary: The current check looks for type parameters of higher rank when adding a bound to a type variable but not higher rank type variables. This omission allows higher rank generics to escape their scope. This diff extends the rank check to cover both cases and modifies the error message for the second case

Reviewed By: andrewjkennedy

Differential Revision: D86223152

fbshipit-source-id: eff5b85810db51f37d8953c0a5c9dc9d7a42c8f0

Author: Michael Thomas

hphp/hack/src/typing/typing_error.ml

@@ -1893,7 +1893,7 @@ and Secondary : sig
         tvar_pos: Pos_or_decl.t;
         pos_with_generic: Pos_or_decl.t;
         generic_reason: Typing_reason.t;
-        generic_name: string;
+        generic_name: string option;
       }
   [@@deriving show]
 end = struct
@@ -2212,7 +2212,7 @@ end = struct
         tvar_pos: Pos_or_decl.t;
         pos_with_generic: Pos_or_decl.t;
         generic_reason: Typing_reason.t;
-        generic_name: string;
+        generic_name: string option;
       }
   [@@deriving show]
 end

hphp/hack/src/typing/typing_error.mli

@@ -1732,7 +1732,7 @@ and Secondary : sig
         tvar_pos: Pos_or_decl.t;
         pos_with_generic: Pos_or_decl.t;
         generic_reason: Typing_reason.t;
-        generic_name: string;
+        generic_name: string option;
       }
   [@@deriving show]
 end

hphp/hack/src/typing/typing_error_utils.ml

@@ -6604,16 +6604,24 @@ end = struct
     create ~code:Error_code.EnumClassLabelUnknown ~reasons ()
 
   let higher_rank_tparam_escape
-      tvar_pos pos_with_generic generic_reason generic_name =
+      tvar_pos pos_with_generic generic_reason generic_name_opt =
     let reasons =
-      lazy
-        ((pos_with_generic, "Expected a first-class polymorphic function type.")
-         :: Typing_reason.to_string
-              (Format.sprintf
-                 "A higher rank generic `%s` will escape its scope"
-                 generic_name)
-              generic_reason
-        @ [(tvar_pos, "Please make this type explicit")])
+      match generic_name_opt with
+      | Some generic_name ->
+        let msg = "Expected a first-class polymorphic function type." in
+        let prefix =
+          Format.sprintf
+            "A higher rank generic `%s` will escape its scope"
+            generic_name
+        in
+        lazy
+          ((pos_with_generic, msg)
+           :: Typing_reason.to_string prefix generic_reason
+          @ [(tvar_pos, "Please make this type explicit")])
+      | _ ->
+        let msg = "A higher rank generic will escape its scope" in
+        let pos = Typing_reason.to_pos generic_reason in
+        lazy [(pos, msg); (tvar_pos, "Please make this type explicit")]
     in
     create ~code:Error_code.RigidTVarEscape ~reasons ()
 

hphp/hack/src/typing/typing_subtype.ml

@@ -845,23 +845,25 @@ end = struct
   end
 
   (* Find the first generic occurring in [ty] with a rank higher than [rank]  *)
-  let find_higher_rank_generic env subtype_env ty rank =
+  let check_rank env subtype_env ty rank =
     (* To avoid this potentially expensive check we record when the sub- or
        supertype _may_ contain a higher rank type parameter. *)
     if Subtype_env.get_check_rank subtype_env then
       let p ty =
         match get_node ty with
         | Tgeneric name -> Env.rank_of_tparam env name > rank
+        | Tvar tv -> Env.rank_of_tvar env tv > rank
         | _ -> false
       in
       let ty_opt = Typing_defs_core.find_locl_ty ty ~p in
       Option.map ty_opt ~f:(fun ty ->
           match deref ty with
-          | (r, Tgeneric name) -> (r, name)
+          | (r, Tgeneric name) -> (r, Some name)
+          | (r, Tvar _) -> (r, None)
           | _ ->
             (* We are guaranteed that if the result if [Some(...)] then the type
                is a [Tgeneric] so something is seriously wrong here *)
-            failwith "find_higher_rank_generic: unexpected type")
+            failwith "check_rank: unexpected type")
     else
       None
 
@@ -1100,7 +1102,7 @@ end = struct
     | (r_sub, Tvar id) -> begin
       (* Ensure that higher-ranked type parameters don't escape their scope *)
       let tvar_rank = Env.rank_of_tvar env id in
-      match find_higher_rank_generic env subtype_env ty_super tvar_rank with
+      match check_rank env subtype_env ty_super tvar_rank with
       | Some (generic_reason, generic_name) ->
         let error =
           Typing_error.Secondary.Higher_rank_tparam_escape
@@ -1195,7 +1197,7 @@ end = struct
               tvar_pos = Typing_reason.to_pos r_super;
               pos_with_generic = Typing_reason.to_pos r_generic;
               generic_reason = r_generic;
-              generic_name = name_sub;
+              generic_name = Some name_sub;
             }
         in
         let fail = Subtype_env.fail_with_secondary_error subtype_env error in
@@ -3403,7 +3405,7 @@ end = struct
         (r_super, Tvar var_super_id) ) -> begin
       (* Ensure that higher-ranked type parameters don't escape their scope *)
       let tvar_rank = Env.rank_of_tvar env var_super_id in
-      match find_higher_rank_generic env subtype_env ty_sub tvar_rank with
+      match check_rank env subtype_env ty_sub tvar_rank with
       | Some (generic_reason, generic_name) ->
         let error =
           Typing_error.Secondary.Higher_rank_tparam_escape
@@ -4060,7 +4062,7 @@ end = struct
             tvar_pos = Typing_reason.to_pos r_sub;
             pos_with_generic = Typing_reason.to_pos r_super;
             generic_reason = r_super;
-            generic_name = tp_sup;
+            generic_name = Some tp_sup;
           }
       in
       let fail = Subtype_env.fail_with_secondary_error subtype_env error in
@@ -9034,7 +9036,7 @@ end = struct
               tvar_pos = Typing_reason.to_pos r_super;
               pos_with_generic = Typing_reason.to_pos r_generic;
               generic_reason = r_generic;
-              generic_name = nm;
+              generic_name = Some nm;
             }
         in
         let fail = Subtype_env.fail_with_secondary_error subtype_env error in

hphp/hack/test/typecheck/first_class_polymorphism/secret_resource.php

@@ -0,0 +1,57 @@
+<?hh
+
+class SecretResource {
+  public function __construct(private string $secret_path): void {}
+
+  public async function doSomething(): Awaitable<string> {
+    return "lolololol";
+  }
+
+  public function close(): void {}
+}
+
+class Handle<TScope> {
+  public function __construct(private SecretResource $resource) {}
+
+  public async function doSomething(): Awaitable<string> {
+    return await $this->resource->doSomething();
+  }
+
+  public function close(): void {
+    $this->resource->close();
+  }
+}
+
+function withSecretResourceUnsafe<TScope, T>(
+  string $path,
+  (function(Handle<TScope>): T) $f,
+): T {
+  $handle = new Handle(new SecretResource($path));
+  try {
+    return $f($handle);
+  } finally {
+    $handle->close();
+  }
+}
+
+function withSecretResourceSafe<T>(
+  string $path,
+  (function<TScope>(Handle<TScope>): T) $f,
+): T {
+  $handle = new Handle(new SecretResource($path));
+  try {
+    return $f($handle);
+  } finally {
+    $handle->close();
+  }
+}
+
+function tryAndGetTheHandle(): void {
+  $escaping_handle =
+    withSecretResourceUnsafe("tell_nobody.txt", $handle ==> $handle);
+
+  $nope = withSecretResourceSafe(
+    "tell_nobody.txt",
+    function<TScope>(Handle<TScope> $handle): Handle<TScope> ==> $handle,
+  );
+}

hphp/hack/test/typecheck/first_class_polymorphism/secret_resource.php.exp

@@ -0,0 +1,10 @@
+ERROR: File "secret_resource.php", line 39, characters 3-39:
+Cannot use unstable feature: `polymorphic_function_hints` (Parsing[1002])
+ERROR: File "secret_resource.php", line 55, characters 5-60:
+Cannot use unstable feature: `polymorphic_lambda` (Parsing[1002])
+ERROR: File "secret_resource.php", line 55, characters 5-72:
+Invalid argument (Typing[4110])
+  File "secret_resource.php", line 55, characters 54-59:
+  A higher rank generic will escape its scope
+  File "secret_resource.php", line 53, characters 11-32:
+  Please make this type explicit