Relax variance restrictions on constraints on class type parameters

Summary:
Covariant and contravariant parameters can safely appear anywhere in "as" and "super" constraints on
class type parameters. Currently an "as" constraint is treated as a negative (contravariant) position,
and a "super" constraint as a positive (covariant) position. This is necessary for constraints on *method*
type parameters but there is no soundness requirement for this check on class type parameters.

(See http://research.microsoft.com/pubs/64042/ecoop06.pdf for a soundness theorem that proves it.)

Reviewed By: dlreeves

Differential Revision: D3235459

fb-gh-sync-id: 483a61d9ddc1201c31a9ef870daaa4a366cc5693
fbshipit-source-id: 483a61d9ddc1201c31a9ef870daaa4a366cc5693

hphp/hack/src/typing/typing_variance.ml

@@ -140,9 +140,9 @@ let reason_to_string ~sign (_, descr, variance) =
         (variance_to_string variance)
         name
   | Rconstraint_super ->
-      "`super` constraints are covariant"
+      "`super` constraints on method type parameters are covariant"
   | Rconstraint_as ->
-      "`as` constraints are contravariant"
+      "`as` constraints on method type parameters are contravariant"
 
 let detailed_message variance pos stack =
   match stack with
@@ -384,6 +384,8 @@ and class_method tcopt root _method_name method_ env =
             end ~init:env in
           let env =
             List.fold_left ~f:(fun_param tcopt root) ~init:env ft_params in
+          let env =
+            List.fold_left ~f:(fun_tparam tcopt root) ~init:env ft_tparams in
           let env = fun_ret tcopt root env ft_ret in
           env
       | _ -> assert false
@@ -394,6 +396,12 @@ and fun_param tcopt root env (_, (reason, _ as ty)) =
   let variance = Vcontravariant [reason_contravariant] in
   type_ tcopt root variance env ty
 
+and fun_tparam tcopt root env (_, _, cstr_opt) =
+  begin match cstr_opt with
+  | None -> env
+  | Some cstr -> constraint_ tcopt root env cstr
+  end
+
 and fun_ret tcopt root env (reason, _ as ty) =
   let pos = Reason.to_pos reason in
   let reason_covariant = pos, Rfun_return, Pcovariant in
@@ -419,7 +427,7 @@ and type_ tcopt root variance env (reason, ty) =
       (* `this` constraints are bivariant (otherwise any class that used the
        * `this` type would not be able to use covariant type params) *)
       env
-  | Tgeneric (name, cstr_opt) ->
+  | Tgeneric (name, _) ->
       let pos = Reason.to_pos reason in
       (* This section makes the position more precise.
        * Say we find a return type that is a tuple (int, int, T).
@@ -438,10 +446,7 @@ and type_ tcopt root variance env (reason, ty) =
         | x -> x
       in
       let env = add_variance env name variance in
-      begin match cstr_opt with
-        | None -> env
-        | Some cstr -> constraint_ tcopt root env cstr
-      end
+      env
   | Toption ty ->
       type_ tcopt root variance env ty
   | Tprim _ -> env
@@ -481,7 +486,8 @@ and type_ tcopt root variance env (reason, ty) =
         type_ tcopt root variance env ty
       end ty_map env
 
-(* `as` constraints must be contravariant and `super` constraints covariant. To
+(* `as` constraints on method type parameters must be contravariant
+ * and `super` constraints on method type parameters are covariant. To
  * see why, suppose that we allow the wrong variance:
  *
  *   class Foo<+T> {

hphp/hack/test/typecheck/covariance_as_constraint_1.php.exp

@@ -3,4 +3,4 @@ Illegal usage of a covariant type parameter (Typing[4120])
 File "covariance_as_constraint_1.php", line 3, characters 12-12:
 This is where the parameter was declared as covariant (+)
 File "covariance_as_constraint_1.php", line 4, characters 29-29:
-`as` constraints are contravariant
+`as` constraints on method type parameters are contravariant

hphp/hack/test/typecheck/covariance_as_constraint_3.php

@@ -0,0 +1,10 @@
+<?hh // strict
+// Copyright 2004-present Facebook. All Rights Reserved.
+
+interface Box<+T> {
+  public function get(): T;
+}
+
+interface BoxFactory<+T, +Tbox as Box<T>> {
+  public function make(): Tbox;
+}

hphp/hack/test/typecheck/covariance_as_constraint_3.php.exp

@@ -0,0 +1 @@
+No errors

hphp/hack/test/typecheck/covariance_super_constraint.php.exp

@@ -3,4 +3,4 @@ Illegal usage of a contravariant type parameter (Typing[4121])
 File "covariance_super_constraint.php", line 3, characters 12-12:
 This is where the parameter was declared as contravariant (-)
 File "covariance_super_constraint.php", line 4, characters 32-32:
-`super` constraints are covariant
+`super` constraints on method type parameters are covariant