Support dynamic for splats in tuples

Summary: Tuple types of the form `(t, ...T)` support dynamic if `t` does and `T` does.

Differential Revision: D66009281

fbshipit-source-id: e935d6252c1bc48f2fde2c83b2861444db7db18f

hphp/hack/src/typing/typing_subtype.ml

@@ -4186,8 +4186,12 @@ end = struct
                 ~rhs:{ super_like = false; super_supportdyn = false; ty_super }
             in
             param_props env &&& ret_prop
-        | (_, Ttuple { t_required; t_extra = Textra { t_optional; t_variadic } })
-          ->
+        | (_, Ttuple { t_required; t_extra }) ->
+          let extras =
+            match t_extra with
+            | Textra { t_variadic; t_optional } -> t_variadic :: t_optional
+            | Tsplat t_splat -> [t_splat]
+          in
           List.fold_left
             ~init:(env, TL.valid)
             ~f:(fun res ty_sub ->
@@ -4198,9 +4202,7 @@ end = struct
                     ~lhs:{ sub_supportdyn; ty_sub }
                     ~rhs:
                       { super_like = false; super_supportdyn = false; ty_super })
-            ((t_variadic :: t_optional) @ t_required)
-        (* TODO splats in tuples *)
-        | (_, Ttuple { t_extra = Tsplat _; _ }) -> invalid env ~fail
+            (extras @ t_required)
         | ( _,
             Tshape
               {

hphp/hack/test/typecheck/splat/tuple_splat_dynamic.bad.php

@@ -0,0 +1,12 @@
+<?hh
+
+<<file: __EnableUnstableFeatures('open_tuples', 'type_splat')>>
+
+function expect_dyn(dynamic $_):void { }
+
+<<__NoAutoDynamic>>
+class C { }
+
+function test<<<__NoAutoBound>> T as (C...)>((int, ...T) $tup):void {
+  expect_dyn($tup);
+}

hphp/hack/test/typecheck/splat/tuple_splat_dynamic.bad.php.exp

@@ -0,0 +1,6 @@
+ERROR: File "tuple_splat_dynamic.bad.php", line 11, characters 14-17:
+Invalid argument (Typing[4110])
+  File "tuple_splat_dynamic.bad.php", line 5, characters 21-27:
+  Expected `dynamic`
+  File "tuple_splat_dynamic.bad.php", line 10, characters 46-56:
+  Type `(int, ...T)` is not a subtype of `dynamic` under dynamic-aware subtyping

hphp/hack/test/typecheck/splat/tuple_splat_dynamic.good.php

@@ -0,0 +1,12 @@
+<?hh
+
+<<file: __EnableUnstableFeatures('open_tuples', 'type_splat')>>
+
+function expect_dyn(dynamic $_):void { }
+
+<<__SupportDynamicType>>
+class C { }
+
+function test<T as (C...)>((int, ...T) $tup):void {
+  expect_dyn($tup);
+}

hphp/hack/test/typecheck/splat/tuple_splat_dynamic.good.php.exp

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