[nullsafe] Simplify signature of InferredNullability.join

Summary:
This function can return `None` if the result is equal to the first
argument of join (why first?). It is unclear if it was an optimization
attempt of over-complicated logic.

Reviewed By: artempyanykh

Differential Revision: D17876561

fbshipit-source-id: 9628fb86e

infer/src/nullsafe/InferredNullability.ml

@@ -9,8 +9,6 @@ open! IStd
 
 type t = {nullability: Nullability.t; origin: TypeOrigin.t} [@@deriving compare]
 
-let equal = [%compare.equal: t]
-
 let get_nullability {nullability} = nullability
 
 let is_nonnull {nullability} = match nullability with Nullable -> false | Nonnull -> true
@@ -50,9 +48,7 @@ let join t1 t2 =
          *)
         TypeOrigin.join t1.origin t2.origin
   in
-  let result = {nullability= joined_nullability; origin= joined_origin} in
-  (* TODO: make the result return non optional value *)
-  if equal result t1 then None else Some result
+  {nullability= joined_nullability; origin= joined_origin}
 
 
 let get_origin t = t.origin

infer/src/nullsafe/InferredNullability.mli

@@ -43,7 +43,7 @@ val of_annotated_nullability : AnnotatedNullability.t -> TypeOrigin.t -> t
 val get_origin : t -> TypeOrigin.t
 (** The simple explanation of how was nullability inferred.  *)
 
-val join : t -> t -> t option
+val join : t -> t -> t
 (** This is what happens with nullability when we join two flows in CFG,
     e.g.
     {[

infer/src/nullsafe/typeState.ml

@@ -39,18 +39,16 @@ let pp fmt typestate =
 let map_join m1 m2 =
   let range_join _exp range1_opt range2_opt =
     Option.both range1_opt range2_opt
-    |> Option.map
-         ~f:(fun (((typ1, inferred_nullability1) as range1), (_, inferred_nullability2)) ->
-           InferredNullability.join inferred_nullability1 inferred_nullability2
-           |> Option.value_map ~default:range1 ~f:(fun ta' ->
-                  ( (* Java does not support local type inference (for codebases and Java version nullsafe is currently aimed for).
+    |> Option.map ~f:(fun ((typ1, inferred_nullability1), (_, inferred_nullability2)) ->
+           (* Unlike nullability that Nullsafe infers, Java does not support local type inference
+              (for codebases and Java version nullsafe is currently aimed for).
               The real type does not depend on types being joined; it is determined by the corresponding type declaration instead.
               We don't really use type information for reasons not related to things like diagnostic, and using one of types
               serves as a good proxy.
               Let's take the left one.
            *)
-                    typ1
-                  , ta' ) ) )
+           let joined_type = typ1 in
+           (joined_type, InferredNullability.join inferred_nullability1 inferred_nullability2) )
   in
   if phys_equal m1 m2 then m1 else M.merge range_join m1 m2