From 2ac4c4d9ce4d371ceb53039e66474b7e23e3606d Mon Sep 17 00:00:00 2001
From: Andrew Kennedy <akenn@fb.com>
Date: Sat, 13 Apr 2019 23:17:19 -0700
Subject: [PATCH] Treat array as subtype of Traversable<any>

Summary:
Currently, the untyped array type `array` is a subtype of `Traversable<t>` and `KeyedTraversable<t,u>` for *any* `t` and `u`.

This diff makes a small change: it is now treated as a subtype of `Traversable<_>` and `KeyedTraversable<_,_>` where `_` is the "any" type. This leads to better propagation of `_`, rather than instantiation with `[unresolved]` (under legacy checking) or `nothing` (in new inference), or unresolved type variables that later produce errors (new inference). See the new test for an example of this latter behaviour.

Reviewed By: dlreeves

Differential Revision: D14916991

fbshipit-source-id: 66d044a0bcf5462b60c3de5ba2b1bc2f056c9862
---
 hphp/hack/src/typing/typing_subtype.ml                       | 10 +++++++---
 hphp/hack/test/tast/array_filter.php.exp                     |  4 ++--
 hphp/hack/test/typecheck/new_inference/untyped_array.php     |  8 ++++++++
 hphp/hack/test/typecheck/new_inference/untyped_array.php.exp |  1 +
 4 files changed, 18 insertions(+), 5 deletions(-)
 create mode 100644 hphp/hack/test/typecheck/new_inference/untyped_array.php
 create mode 100644 hphp/hack/test/typecheck/new_inference/untyped_array.php.exp

diff --git a/hphp/hack/src/typing/typing_subtype.ml b/hphp/hack/src/typing/typing_subtype.ml
index f81bc73d223..90156b1c3d2 100644
--- a/hphp/hack/src/typing/typing_subtype.ml
+++ b/hphp/hack/src/typing/typing_subtype.ml
@@ -802,8 +802,9 @@ and simplify_subtype
           coll = SN.Rx.cTraversable ||
           coll = SN.Collections.cContainer) ->
       (match akind with
-        (* array <: Traversable<t> and emptyarray <: Traversable<t> for any t *)
-      | AKany -> valid ()
+        (* array <: Traversable<_> and emptyarray <: Traversable<t> for any t *)
+      | AKany ->
+        simplify_subtype ~seen_generic_params ~deep ~this_ty (fst ety_sub, Tany) tv_super env
       | AKempty -> valid ()
       (* vec<tv> <: Traversable<tv_super>
        * iff tv <: tv_super
@@ -823,7 +824,10 @@ and simplify_subtype
          || coll = SN.Collections.cKeyedContainer) ->
     let r = fst ety_sub in
       (match akind with
-      | AKany -> valid ()
+      | AKany ->
+        env |>
+        simplify_subtype ~seen_generic_params ~deep ~this_ty (fst ety_sub, Tany) tk_super &&&
+        simplify_subtype ~seen_generic_params ~deep ~this_ty (fst ety_sub, Tany) tv_super
       | AKempty -> valid ()
       | AKvarray tv
       | AKvec tv ->
diff --git a/hphp/hack/test/tast/array_filter.php.exp b/hphp/hack/test/tast/array_filter.php.exp
index 0a4baf3119c..b6226570f39 100644
--- a/hphp/hack/test/tast/array_filter.php.exp
+++ b/hphp/hack/test/tast/array_filter.php.exp
@@ -70,7 +70,7 @@
            (Expr
               (([13:3-31], array),
                (Call (Cnormal,
-                  (([13:3-15], (function<Tv>(Container<nothing> $input, ?(function(nothing): bool) $callback): array)),
+                  (([13:3-15], (function<Tv>(Container<_> $input, ?(function(_): bool) $callback): array)),
                    (Id ([13:3-15], "\\array_filter"))),
                   [],
                   [(([13:16-30], array), (Lvar ([13:16-30], $untyped_array)))
@@ -163,7 +163,7 @@
            (Expr
               (([22:3-35], array),
                (Call (Cnormal,
-                  (([22:3-15], (function<Tv>(Container<nothing> $input, ?(function(nothing): bool) $callback): array)),
+                  (([22:3-15], (function<Tv>(Container<_> $input, ?(function(_): bool) $callback): array)),
                    (Id ([22:3-15], "\\array_filter"))),
                   [],
                   [(([22:16-30], array), (Lvar ([22:16-30], $untyped_array)));
diff --git a/hphp/hack/test/typecheck/new_inference/untyped_array.php b/hphp/hack/test/typecheck/new_inference/untyped_array.php
new file mode 100644
index 00000000000..6aebd050475
--- /dev/null
+++ b/hphp/hack/test/typecheck/new_inference/untyped_array.php
@@ -0,0 +1,8 @@
+<?hh // partial
+
+function foo(array $m):void {
+   $x = new Vector($m);
+   $x[0]->foo();
+   $y = new Map($m);
+   $y['a']->foo();
+}
diff --git a/hphp/hack/test/typecheck/new_inference/untyped_array.php.exp b/hphp/hack/test/typecheck/new_inference/untyped_array.php.exp
new file mode 100644
index 00000000000..4269126fceb
--- /dev/null
+++ b/hphp/hack/test/typecheck/new_inference/untyped_array.php.exp
@@ -0,0 +1 @@
+No errors
-- 
2.11.4.GIT