Support concurrent blocks in reactive code

Summary: We need to whitelist awaitall as counting as being immediately awaited.

Reviewed By: vladima

Differential Revision: D13329905

fbshipit-source-id: 292afc3fd7d73544a4f9702b2e7011efc79bd81f

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

@@ -464,6 +464,13 @@ let check = object(self)
     disallow_static_or_global ~is_static:true el;
     super#on_Static_var s el
 
+  method! on_Awaitall ((env, ctx) as s) _ els =
+    let allow_awaitable_s = (env, allow_awaitable ctx) in
+    List.iter els ~f:(fun (lhs, rhs) ->
+      Option.iter lhs ~f:(super#on_expr s);
+      super#on_expr allow_awaitable_s rhs
+    )
+
   method! on_expr (env, ctx) expr =
     let check_reactivity =
       ctx.reactivity <> Nonreactive &&

hphp/hack/test/typecheck/concurrent/concurrent_rx.php

@@ -0,0 +1,16 @@
+<?hh // strict
+
+<<__Rx>>
+async function genx(): Awaitable<int> { return 42; }
+<<__Rx>>
+async function geny(): Awaitable<void> {}
+
+<<__Rx>>
+async function foo(): Awaitable<int> {
+  concurrent {
+    $x = await genx();
+    await geny();
+    $y = await genx();
+  }
+  return $x + $y;
+}

hphp/hack/test/typecheck/concurrent/concurrent_rx.php.exp

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