Look transitively through nullable chains.

This causes `??T` to be treated as `?T` for the purpose of null checking.

Reviewed By: @gabelevi

Differential Revision: D1250188

Author: evincarofautumn

hphp/hack/src/typing/typing.ml

@@ -2297,7 +2297,11 @@ and non_null env ty =
   match ty with
   | _, Toption ty ->
       let env, ty = Env.expand_type env ty in
-      env, ty
+      (* When "??T" appears in the typing environment due to implicit
+       * typing, the recursion here ensures that it's treated as
+       * isomorphic to "?T"; that is, all nulls are created equal.
+       *)
+      non_null env ty
   | r, Tunresolved tyl ->
       let env, tyl = lfold non_null env tyl in
       (* We need to flatten the unresolved types, otherwise we could

hphp/hack/test/more_tests/double_nullable.php

@@ -0,0 +1,17 @@
+<?hh
+
+function bar(): array<int, ?int> {
+  return array(123 => null, 456 => 789);
+}
+
+function idx2<Tk, Tv>(?Indexish<Tk, Tv> $collection, ?Tk $index): ?Tv {
+  return idx($collection, $index);
+}
+
+function foo(): int {
+  $a = idx2(bar(), 123);
+  if ($a !== null) {
+    return $a;
+  }
+  return 345;
+}

hphp/hack/test/more_tests/double_nullable.php.exp

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