[nullsafe] Don't join types in a fancy way.

Summary:
The goal of this logic is unclear:
1/ See the comments
2/ I can not see the scenario where classes and proper types can be
joined in a legit Java program
3/ Even it if was the case, I don't see how this heuristic is justified.
So I assume it is not.

Reviewed By: artempyanykh

Differential Revision: D17876568

fbshipit-source-id: c9c6cd604

infer/src/absint/PatternMatch.ml

@@ -9,14 +9,6 @@ open! IStd
 
 (** Module for Pattern matching. *)
 
-let type_is_object typ =
-  match typ.Typ.desc with
-  | Tptr ({desc= Tstruct name}, _) ->
-      Typ.Name.equal name Typ.Name.Java.java_lang_object
-  | _ ->
-      false
-
-
 (** Holds iff the predicate holds on a supertype of the named type, including the type itself *)
 let rec supertype_exists tenv pred name =
   match Tenv.lookup tenv name with

infer/src/absint/PatternMatch.mli

@@ -132,9 +132,6 @@ val type_get_class_name : Typ.t -> Typ.Name.t option
 val type_is_class : Typ.t -> bool
 (** Is the type a class type *)
 
-val type_is_object : Typ.t -> bool
-(** Is the type java.lang.Object *)
-
 val get_fields_nullified : Procdesc.t -> Typ.Fieldname.Set.t
 (** return the set of instance fields that are assigned to a null literal in [procdesc] *)
 

infer/src/nullsafe/typeState.ml

@@ -36,17 +36,21 @@ let pp fmt typestate =
   pp_map typestate
 
 
-let type_join typ1 typ2 = if PatternMatch.type_is_object typ1 then typ2 else typ1
-
 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), (typ2, inferred_nullability2)) ->
+         ~f:(fun (((typ1, inferred_nullability1) as range1), (_, inferred_nullability2)) ->
            InferredNullability.join inferred_nullability1 inferred_nullability2
            |> Option.value_map ~default:range1 ~f:(fun ta' ->
-                  let typ' = type_join typ1 typ2 in
-                  (typ', ta') ) )
+                  ( (* 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' ) ) )
   in
   if phys_equal m1 m2 then m1 else M.merge range_join m1 m2