[WIP] Builtin declaration merging

lib/core.js

@@ -908,3 +908,5 @@ declare var console: {
     warn(...data: Array<any>): void,
     ...
 };
+
+declare var globalThis: $GlobalThis;

src/commands/commandUtils.ml

@@ -1095,6 +1095,7 @@ let make_options ~flowconfig_name ~flowconfig ~lazy_mode ~root (options_flags: O
     opt_no_saved_state = options_flags.no_saved_state;
     opt_arch;
     opt_abstract_locations;
+    opt_declaration_merging = FlowConfig.declaration_merging flowconfig;
     opt_include_suppressions = options_flags.include_suppressions;
     opt_trust_mode = Option.value options_flags.trust_mode ~default:(FlowConfig.trust_mode flowconfig);
     opt_recursion_limit = FlowConfig.recursion_limit flowconfig;

src/commands/config/flowConfig.ml

@@ -43,6 +43,7 @@ module Opts = struct
   type opt_error = int * error_kind
 
   type t = {
+    declaration_merging: bool;
     abstract_locations: bool;
     all: bool;
     emoji: bool;
@@ -134,6 +135,7 @@ module Opts = struct
     |> SSet.add ".webm"
 
   let default_options = {
+    declaration_merging = false;
     abstract_locations = false;
     all = false;
     emoji = false;
@@ -617,6 +619,9 @@ module Opts = struct
     "experimental.abstract_locations",
       boolean (fun opts v -> Ok { opts with abstract_locations = v });
 
+    "experimental.declaration_merging",
+      boolean (fun opts v -> Ok { opts with declaration_merging = v });
+
     "no_flowlib",
       boolean (fun opts v -> Ok { opts with no_flowlib = v });
 
@@ -1014,6 +1019,7 @@ let includes config = config.includes
 let libs config = config.libs
 
 (* options *)
+let declaration_merging c = c.options.Opts.declaration_merging
 let abstract_locations c = c.options.Opts.abstract_locations
 let all c = c.options.Opts.all
 let emoji c = c.options.Opts.emoji

src/commands/config/flowConfig.mli

@@ -38,6 +38,7 @@ val includes: config -> string list
 val libs: config -> string list
 
 (* options *)
+val declaration_merging: config -> bool
 val abstract_locations: config -> bool
 val all: config -> bool
 val emoji: config -> bool

src/common/options.ml

@@ -58,6 +58,7 @@ type trust_mode =
   | SilentTrust
 
 type t = {
+  opt_declaration_merging : bool;
   opt_abstract_locations : bool;
   opt_all : bool;
   opt_debug : bool;
@@ -119,6 +120,7 @@ type t = {
   opt_type_asserts: bool;
 }
 
+let declaration_merging opts = opts.opt_declaration_merging
 let all opts = opts.opt_all
 let arch opts = opts.opt_arch
 let max_literal_length opts = opts.opt_max_literal_length

src/flow_dot_js.ml

@@ -143,6 +143,7 @@ let stub_metadata ~root ~checked = { Context.
   include_suppressions = false;
 
   (* global *)
+  declaration_merging = false;
   max_literal_length = 100;
   enable_const_params = false;
   enable_enums = true;

src/services/inference/init_js.ml

@@ -66,10 +66,17 @@ let parse_lib_file ~reader options file =
 let load_lib_files ~sig_cx ~options ~reader files =
 
   let verbose = Options.verbose options in
+  let declaration_merging = Options.declaration_merging options in
+
+  let files = if declaration_merging then
+    files
+  else
+    List.rev files
+  in
 
   (* iterate in reverse override order *)
   let%lwt (_, result) =
-    List.rev files
+    files
     |> Lwt_list.fold_left_s (
       fun (exclude_syms, results) file ->
 

src/typing/context.ml

@@ -29,6 +29,7 @@ type metadata = {
   strict_local: bool;
 
   (* global *)
+  declaration_merging: bool;
   max_literal_length: int;
   enable_const_params: bool;
   enable_enums: bool;
@@ -156,6 +157,7 @@ let metadata_of_options options = {
   strict_local = false;
 
   (* global *)
+  declaration_merging = Options.declaration_merging options;
   max_literal_length = Options.max_literal_length options;
   enable_const_params = Options.enable_const_params options;
   enable_enums = Options.enums options;
@@ -271,6 +273,7 @@ let trust_constructor cx = cx.trust_constructor
 let cx_with_trust cx trust = { cx with trust_constructor = trust }
 
 let metadata cx = cx.metadata
+let declaration_merging cx = cx.metadata.declaration_merging
 let max_literal_length cx = cx.metadata.max_literal_length
 let enable_const_params cx =
   cx.metadata.enable_const_params || cx.metadata.strict || cx.metadata.strict_local

src/typing/context.mli

@@ -29,6 +29,7 @@ type metadata = {
   strict: bool;
   strict_local: bool;
   (* global *)
+  declaration_merging: bool;
   max_literal_length: int;
   enable_const_params: bool;
   enable_enums: bool;
@@ -71,6 +72,7 @@ val find_module_sig: sig_t -> string -> Type.t
 (* accessors *)
 val all_unresolved: t -> ISet.t IMap.t
 val metadata: t -> metadata
+val declaration_merging: t -> bool
 val max_literal_length: t -> int
 val enable_const_params: t -> bool
 val enable_enums: t -> bool

src/typing/coverage.ml

@@ -189,6 +189,7 @@ class visitor = object (self)
     | FunProtoApplyT _
     | FunProtoBindT _
     | FunProtoCallT _
+    | GlobalThisT _
     | InternalT _
     | KeysT _
     | MaybeT _

src/typing/debug_js.ml

@@ -173,6 +173,7 @@ and _json_of_t_impl json_cx t = Hh_json.(
   | FunProtoApplyT _
   | FunProtoBindT _
   | FunProtoCallT _
+  | GlobalThisT _
     -> []
 
   | DefT (_, _, FunT (static, proto, funtype)) -> [
@@ -497,6 +498,7 @@ and _json_of_use_t_impl json_cx t = Hh_json.(
     ]
 
   | SetPropT (_, _, name, _, t, _)
+  | OverwritePropT (_, _, name, t)
   | GetPropT (_, _, name, t)
   | MatchPropT (_, _, name, t)
   | TestPropT (_, _, name, t) -> [
@@ -561,6 +563,11 @@ and _json_of_use_t_impl json_cx t = Hh_json.(
       "rightType", _json_of_t json_cx r
     ]
 
+  | MergeTypesT (_, _, _, _, l, r) -> [
+      "leftType", _json_of_t json_cx l;
+      "rightType", _json_of_t json_cx r
+    ]
+
   | ComparatorT (_, _, t) -> [
       "type", _json_of_t json_cx t
     ]
@@ -1796,7 +1803,8 @@ let rec dump_t_ (depth, tvars) cx t =
   | FunProtoT _
   | FunProtoApplyT _
   | FunProtoBindT _
-  | FunProtoCallT _ -> p t
+  | FunProtoCallT _
+  | GlobalThisT _ -> p t
   | DefT (_, trust, PolyT (_, tps, c, id)) -> p ~trust:(Some trust) ~extra:(spf "%s [%s] #%d"
       (kid c)
       (String.concat "; " (Core_list.map ~f:(fun tp -> tp.name) (Nel.to_list tps)))
@@ -2097,6 +2105,9 @@ and dump_use_t_ (depth, tvars) cx t =
       (if Context.trust_tracking cx then string_of_trust_rep (lookup_trust cx) trust else "")
       (kid t)
   | UseT (use_op, t) -> spf "UseT (%s, %s)" (string_of_use_op use_op) (kid t)
+  | MergeTypesT (_, _, _, _, x, y) -> p ~extra:(spf "%s, %s"
+      (kid x)
+      (kid y)) t
   | AdderT (use_op, _, _, x, y) -> p ~extra:(spf "%s, %s, %s"
       (string_of_use_op use_op)
       (kid x)
@@ -2229,6 +2240,10 @@ and dump_use_t_ (depth, tvars) cx t =
       (string_of_use_op use_op)
       (propref prop)
       (kid ptype)) t
+  | OverwritePropT (use_op, _, prop, ptype) -> p ~extra:(spf "%s, (%s), %s"
+      (string_of_use_op use_op)
+      (propref prop)
+      (kid ptype)) t
   | SetPrivatePropT (_, _, prop, _, _, ptype, _) -> p ~extra:(spf "(%s), %s"
       (prop)
       (kid ptype)) t

src/typing/env.ml

@@ -274,7 +274,9 @@ let trunc_env =
 
 (* initialize a new environment (once per module) *)
 let init_env ?(exclude_syms=SSet.empty) cx module_scope =
-  set_exclude_symbols exclude_syms;
+  if not (Context.declaration_merging cx) then
+    set_exclude_symbols exclude_syms;
+
   havoc_current_activation ();
   let global_scope = Scope.fresh ~var_scope_kind:Global () in
   push_var_scope cx global_scope;

src/typing/flow_js.ml

@@ -3732,6 +3732,9 @@ let rec __flow cx ((l: Type.t), (u: Type.use_t)) trace =
       let reason = reason_of_t l in
       rec_flow cx trace (l, UseT (use_op, fix_this_class cx trace reason (r, i)))
 
+    | l, MergeTypesT (reason, flip, propref, original_t, r, u) ->
+      merge_builtin cx trace reason flip propref original_t l r u
+
     (** This rule is hit when a polymorphic type appears outside a
         type application expression - i.e. not followed by a type argument list
         delimited by angle brackets.
@@ -5244,6 +5247,13 @@ let rec __flow cx ((l: Type.t), (u: Type.use_t)) trace =
       Option.iter ~f:(fun t -> rec_flow_t cx trace (AnyT.untyped reason_op, t)) prop_t;
       rec_flow cx trace (t, UseT (use_op, AnyT.untyped reason_op))
 
+    | DefT (_, _, ObjT { props_tmap; _ }), OverwritePropT (_, _, Named (_, name), t) ->
+      let p = Field (None, t, Polarity.Neutral) in
+      Context.set_prop cx props_tmap name p;
+
+    | ModuleT (_, { exports_tmap; _ }, _), OverwritePropT (_, _, Named (_, name), t) ->
+      Context.set_export cx exports_tmap name (None, t);
+
     | DefT (reason_obj, _, ObjT o), MatchPropT (use_op, reason_op, propref, proptype) ->
         match_obj_prop cx trace ~use_op o propref reason_obj reason_op proptype
 
@@ -6522,6 +6532,14 @@ let rec __flow cx ((l: Type.t), (u: Type.use_t)) trace =
     | FunProtoCallT reason, _ ->
       rec_flow cx trace (FunProtoT reason, u)
 
+    | GlobalThisT _,
+      SetPropT (_, _, Named (_, name), _, tin, _) ->
+      overwrite_builtin cx ~trace name tin;
+
+    | GlobalThisT _, _ ->
+      let t = builtins cx in
+      rec_flow cx trace (t, u)
+
     | _, LookupT (_, _, _, propref, lookup_action) ->
       let use_op = use_op_of_lookup_action lookup_action in
       add_output cx ~trace (Error_message.EIncompatibleProp {
@@ -6932,6 +6950,7 @@ and empty_success flavor u =
      either by specially propagating it or selecting cases, etc. *)
   | _, UseT (_, ExactT _)
   | _, AdderT _
+  | _, MergeTypesT _
   | _, AndT _
   | _, OrT _
   (* Propagation cases: these cases don't use the fact that the LHS is
@@ -7037,6 +7056,7 @@ and empty_success flavor u =
   | _, SetElemT _
   | _, SetPrivatePropT _
   | _, SetPropT _
+  | _, OverwritePropT _
   | _, SpecializeT _
   | _, SubstOnPredT _
   | _, SuperT _
@@ -7132,6 +7152,7 @@ and any_propagated cx trace any u =
      true
 
   | AdderT _
+  | MergeTypesT _
   | AndT _
   | ArrRestT _
   | BecomeT _
@@ -7192,6 +7213,7 @@ and any_propagated cx trace any u =
   | SentinelPropTestT _
   | SetElemT _
   | SetPropT _
+  | OverwritePropT _
   | ModuleExportsAssignT _
   | SpecializeT _
   | SubstOnPredT _ (* Should be impossible. We only generate these with OpenPredTs. *)
@@ -7308,7 +7330,8 @@ and any_propagated_use cx trace use_op any l =
   | FunProtoCallT _
   | FunProtoT _
   | ObjProtoT _
-  | NullProtoT _ ->
+  | NullProtoT _
+  | GlobalThisT _ ->
       true
 
   (* Handled already in __flow *)
@@ -7729,6 +7752,7 @@ and check_polarity cx ?trace polarity = function
   | FunProtoApplyT _
   | FunProtoBindT _
   | FunProtoCallT _
+  | GlobalThisT _
   | EvalT _
   | InternalT (ExtendsT _)
   | InternalT (ChoiceKitT _)
@@ -11112,10 +11136,73 @@ and extract_non_spread cx ~trace = function
     add_output cx ~trace (Error_message.(EUnsupportedSyntax (loc, SpreadArgument)));
     AnyT.error reason
 
+and merge_builtin cx trace reason flip propref original_t l r u =
+  let string_reason = string_of_reason reason in
+  ignore string_reason;
+  if needs_resolution r then begin
+    (* TODO: this doesn't work without writing to property first *)
+    rec_flow cx trace (u, OverwritePropT (unknown_use, reason, propref, l));
+    (* *)
+
+    rec_flow cx trace (r, MergeTypesT (reason, not flip, propref, original_t, l, u))
+  end else begin
+    let (l, r) = if flip then (r, l) else (l, r) in
+    begin match l, r with
+
+    (* polymorphic interface ~> polymorphic class *)
+    | DefT (_, _, PolyT (_, _, DefT (_, _, ClassT (DefT (_, _, InstanceT (_, _, _, { structural = true; own_props = o1_id; _ })))), _)),
+      DefT (_, _, PolyT (_, _, ThisClassT (_, DefT (_, _, InstanceT (_, _, _, { structural = false; own_props = o2_id; _ }))), _)) ->
+      let o1 = Context.find_props cx o1_id in
+      let o2 = Context.find_props cx o2_id in
+      ignore o1;
+      ignore o2;
+      Context.add_property_map cx o2_id (SMap.union o1 o2);
+
+    (* interface ~> class *)
+    | DefT (_, _, ClassT (DefT (_, _, InstanceT (_, _, _, { structural = true; own_props = o1_id; _ })))),
+      ThisClassT (_, DefT (_, _, InstanceT (_, _, _, { structural = false; own_props = o2_id; _ }))) ->
+      let o1 = Context.find_props cx o1_id in
+      let o2 = Context.find_props cx o2_id in
+      ignore o1;
+      ignore o2;
+      Context.add_property_map cx o2_id (SMap.union o1 o2);
+
+    (* module ~> module *)
+    | ModuleT (_, {exports_tmap = exports1_id; _}, _), ModuleT (_, {exports_tmap = exports2_id; _}, _) ->
+      let exports1 = Context.find_exports cx exports1_id in
+      let exports2 = Context.find_exports cx exports2_id in
+      let exports = SMap.union exports1 exports2 in
+      Context.add_export_map cx exports2_id exports;
+      ignore @@ SMap.union ~combine:(fun key (_, t1) (_, t2) ->
+        rec_flow cx trace (t2, MergeTypesT (reason, false, Named (reason_of_t t1, key), t2, t1, r));
+        None
+      ) exports1 exports2;
+      ()
+
+    (* any ~> any *)
+    | _, _ ->
+      ()
+    end
+  end
+
 and set_builtin cx ?trace x t =
+  let reason = builtin_reason (RCustom x) in
+  let obj = builtins cx in
+  let propref = Named (reason, x) in
+  let old_t = Tvar.mk_derivable_where cx reason (fun t ->
+    flow_opt cx ?trace (obj, GetPropT (unknown_use, reason, propref, t));
+  )
+  in
+  if Context.declaration_merging cx then
+    flow_opt cx ?trace (t, MergeTypesT (reason, false, propref, t, old_t, obj))
+  else
+    flow_opt cx ?trace (obj, SetPropT (unknown_use, reason, propref, Normal, t, None))
+
+and overwrite_builtin cx ?trace x t =
   let reason = builtin_reason (RCustom x) in
   let propref = Named (reason, x) in
-  flow_opt cx ?trace (builtins cx, SetPropT (unknown_use, reason, propref, Normal, t, None))
+  let obj = builtins cx in
+  flow_opt cx ?trace (obj, OverwritePropT (unknown_use, reason, propref, t))
 
 (* Wrapper functions around __flow that manage traces. Use these functions for
    all recursive calls in the implementation of __flow. *)

src/typing/members.ml

@@ -431,6 +431,7 @@ let rec extract_type cx this_t = match this_t with
   | FunProtoBindT _
   | FunProtoCallT _
   | FunProtoT _
+  | GlobalThisT _
   | KeysT (_, _)
   | DefT (_, _, MixedT _)
   | NullProtoT _

src/typing/resolvableTypeJob.ml

@@ -251,6 +251,7 @@ and collect_of_type ?log_unresolved cx acc = function
   | FunProtoT _
   | NullProtoT _
   | ObjProtoT _
+  | GlobalThisT _
   | CustomFunT (_, _)
 
   | ExistsT _