[flow] Show signature verification errors for builtin type sig

Summary:
It turns out we are not exposing these errors at all. For libdefs it's mostly fine since things like `const foo = new Set()` is already banned, but it will be important for the next diff where I raise new kinds of errors.

Changelog: [errors] Signature verification errors will now show up for libdef files

Reviewed By: gkz

Differential Revision: D70303552

fbshipit-source-id: ce37ed169b8c7ecc47a801f302e4a887b371080f

src/flow_dot_js.ml

@@ -83,7 +83,8 @@ let load_lib_files files =
       locs_to_dirtify = [];
     }
   in
-  Merge_js.merge_lib_files ~sig_opts asts |> snd
+  let (_, _, master_cx) = Merge_js.merge_lib_files ~sig_opts asts in
+  master_cx
 
 let stub_metadata ~root ~checked =
   {

src/services/inference/init_js.ml

@@ -45,7 +45,7 @@ let load_lib_files ~ccx ~options ~reader files =
   let (builtin_exports, master_cx, cx_opt) =
     if ok then
       let sig_opts = Type_sig_options.builtin_options options in
-      let (builtins, master_cx) = Merge_js.merge_lib_files ~sig_opts ordered_asts in
+      let (builtins, builtin_errors, master_cx) = Merge_js.merge_lib_files ~sig_opts ordered_asts in
       let cx_opt =
         match master_cx with
         | Context.EmptyMasterContext -> None
@@ -66,6 +66,7 @@ let load_lib_files ~ccx ~options ~reader files =
               (fun mref -> Context.MissingModule mref)
               mk_builtins
           in
+          Context.reset_errors cx builtin_errors;
           Some cx
       in
       (Exports.of_builtins builtins, master_cx, cx_opt)

src/typing/__tests__/type_hint_test.ml

@@ -181,7 +181,8 @@ end = struct
         locs_to_dirtify = [];
       }
     in
-    Merge_js.merge_lib_files ~sig_opts asts |> snd
+    let (_, _, master_cx) = Merge_js.merge_lib_files ~sig_opts asts in
+    master_cx
 
   let master_cx_ref = ref None
 

src/typing/merge_js.ml

@@ -866,14 +866,36 @@ let module_type_copied dst_cx src_cx ({ Type.module_export_types; module_global_
   { m with Type.module_global_types_tmap }
 
 let merge_lib_files ~sig_opts ordered_asts =
-  let (_builtin_errors, builtin_locs, builtins) =
+  let (builtin_errors, builtin_locs, builtins) =
     Type_sig_utils.parse_and_pack_builtins sig_opts ordered_asts
   in
+  let builtin_errors =
+    builtin_errors
+    |> Base.List.filter_map ~f:(function
+           | Type_sig.SigError e ->
+             let e =
+               Signature_error.map
+                 (fun l -> l |> Type_sig_collections.Locs.get builtin_locs |> ALoc.of_loc)
+                 e
+             in
+             let msg = Error_message.ESignatureVerification e in
+             let source_file =
+               Base.Option.value_exn
+                 (msg |> Error_message.loc_of_msg |> Base.Option.bind ~f:ALoc.source)
+             in
+             Some (Flow_error.error_of_msg ~source_file msg)
+           | Type_sig.CheckError -> None
+           )
+    |> Flow_error.ErrorSet.of_list
+  in
   match ordered_asts with
-  | [] -> (builtins, Context.EmptyMasterContext)
+  | [] -> (builtins, builtin_errors, Context.EmptyMasterContext)
   | fst_ast :: _ ->
     let builtin_leader_file_key = Base.Option.value_exn (fst_ast |> fst |> Loc.source) in
-    (builtins, Context.NonEmptyMasterContext { builtin_leader_file_key; builtin_locs; builtins })
+    ( builtins,
+      builtin_errors,
+      Context.NonEmptyMasterContext { builtin_leader_file_key; builtin_locs; builtins }
+    )
 
 let mk_builtins metadata master_cx =
   match master_cx with

src/typing/merge_js.mli

@@ -28,6 +28,8 @@ val copy_into :
 val merge_lib_files :
   sig_opts:Type_sig_options.t ->
   (Loc.t, Loc.t) Flow_ast.Program.t list ->
-  Type_sig_collections.Locs.index Packed_type_sig.Builtins.t * Context.master_context
+  Type_sig_collections.Locs.index Packed_type_sig.Builtins.t
+  * Flow_error.ErrorSet.t
+  * Context.master_context
 
 val mk_builtins : Context.metadata -> Context.master_context -> Context.t -> Builtins.t