Remove [[call]] property from function statics

Summary:
A function is modeled as a FunT with statics, but since all functions are
objects it is also fair to consider the function as its statics. For this
reason, we add a [[call]] property to the statics with the type of the function
itself.

This recursion is mostly fine, but polymorphic functions behave badly. The FunT
is wrapped in a PolyT, but the [[call]] property is also wrapped in PolyT. This
causes the type instantiations for the function and it's [[call]] property to be
unrelated, when the two are essentially the same type.

The trick here is to ensure that FunT types are not decomposed into their
statics if they will be compared against another function or callable thing.

I observed this behavior investigating something else, but it was not the cause
of any specific bad behavior.

Reviewed By: panagosg7

Differential Revision: D14783679

fbshipit-source-id: 8ce44ac0df985776b85368d886a2ace625bd37f8

src/typing/flow_js.ml

@@ -5683,6 +5683,13 @@ let rec __flow cx ((l: Type.t), (u: Type.use_t)) trace =
     (* You can use a function as a callable object *)
     (***********************************************)
 
+    | DefT (_, _, FunT _), UseT (use_op, DefT (_, _, (
+        ObjT {call_t = Some id; _} |
+        InstanceT (_, _, _, {inst_call_t = Some id; _})
+      ))) ->
+      let t = Context.find_call cx id in
+      rec_flow cx trace (l, UseT (use_op, t))
+
     (* FunT ~> ObjT *)
 
     (* Previously, call properties were stored in the props map, and were

src/typing/func_sig.ml

@@ -98,7 +98,7 @@ let functiontype cx this_t {reason; kind; tparams; fparams; return_t; knot; _} =
   let make_trust = Context.trust_constructor cx in
   let static =
     let proto = FunProtoT reason in
-    Obj_type.mk_with_proto cx reason ~call:knot proto
+    Obj_type.mk_with_proto cx reason proto
   in
   let prototype =
     let reason = replace_reason_const RPrototype reason in