diff --git a/src/action.ml b/src/action.ml
index 110c6739..d465f079 100644
--- a/src/action.ml
+++ b/src/action.ml
@@ -106,6 +106,13 @@ struct
       List [Sexp.unsafe_atom_of_string "diff"; path file1; path file2]
     | Diff { optional = true; file1; file2 } ->
       List [Sexp.unsafe_atom_of_string "diff?"; path file1; path file2]
+    | Merge_files_into (srcs, extras, target) ->
+      List
+        [ Sexp.unsafe_atom_of_string "merge-files-into"
+        ; List (List.map ~f:path srcs)
+        ; List (List.map ~f:string extras)
+        ; path target
+        ]
 
   let run prog args = Run (prog, args)
   let chdir path t = Chdir (path, t)
@@ -165,6 +172,11 @@ module Make_mapper
     | Digest_files x -> Digest_files (List.map x ~f:(f_path ~dir))
     | Diff { optional; file1; file2 } ->
       Diff { optional; file1 = f_path ~dir file1; file2 = f_path ~dir file2 }
+    | Merge_files_into (sources, extras, target) ->
+      Merge_files_into
+        (List.map sources ~f:(f_path ~dir),
+         List.map extras ~f:(f_string ~dir),
+         f_path ~dir target)
 end
 
 module Prog = struct
@@ -449,6 +461,11 @@ module Unexpanded = struct
              ; file1 = E.path ~dir ~f file1
              ; file2 = E.path ~dir ~f file2
              }
+      | Merge_files_into (sources, extras, target) ->
+        Merge_files_into
+          (List.map ~f:(E.path ~dir ~f) sources,
+           List.map ~f:(E.string ~dir ~f) extras,
+           E.path ~dir ~f target)
   end
 
   module E = struct
@@ -548,6 +565,11 @@ module Unexpanded = struct
            ; file1 = E.path ~dir ~f file1
            ; file2 = E.path ~dir ~f file2
            }
+    | Merge_files_into (sources, extras, target) ->
+      Merge_files_into
+        (List.map sources ~f:(E.path ~dir ~f),
+         List.map extras ~f:(E.string ~dir ~f),
+         E.path ~dir ~f target)
 end
 
 let fold_one_step t ~init:acc ~f =
@@ -570,7 +592,8 @@ let fold_one_step t ~init:acc ~f =
   | Remove_tree _
   | Mkdir _
   | Digest_files _
-  | Diff _ -> acc
+  | Diff _
+  | Merge_files_into _ -> acc
 
 include Make_mapper(Ast)(Ast)
 
@@ -864,6 +887,22 @@ let rec exec t ~ectx ~dir ~env ~stdout_to ~stderr_to =
       end;
       Print_diff.print file1 file2
     end
+  | Merge_files_into (sources, extras, target) ->
+    let lines =
+      List.fold_left
+        ~init:(String_set.of_list extras)
+        ~f:(fun set source_path ->
+          Path.to_string source_path
+          |> Io.lines_of_file
+          |> String_set.of_list
+          |> String_set.union set
+        )
+        sources
+    in
+    Io.write_lines
+      (Path.to_string target)
+      (String_set.to_list lines);
+    Fiber.return ()
 
 and redirect outputs fn t ~ectx ~dir ~env ~stdout_to ~stderr_to =
   let fn = Path.to_string fn in
@@ -980,6 +1019,8 @@ module Infer = struct
       | Digest_files l -> List.fold_left l ~init:acc ~f:(+<)
       | Diff { optional; file1; file2 } ->
         if optional then acc else acc +< file1 +< file2
+      | Merge_files_into (sources, _extras, target) ->
+        List.fold_left sources ~init:acc ~f:(+<) +@ target
       | Echo _
       | System _
       | Bash _
diff --git a/src/action_intf.ml b/src/action_intf.ml
index 5937cc07..e9fb0709 100644
--- a/src/action_intf.ml
+++ b/src/action_intf.ml
@@ -40,6 +40,7 @@ module type Ast = sig
     | Mkdir          of path
     | Digest_files   of path list
     | Diff           of Diff.t
+    | Merge_files_into of path list * string list * path
 end
 
 module type Helpers = sig
diff --git a/src/build.ml b/src/build.ml
index 78ae7c9d..a64ed303 100644
--- a/src/build.ml
+++ b/src/build.ml
@@ -278,3 +278,9 @@ let mkdir dir =
 let progn ts =
   all ts >>^ fun actions ->
   Action.Progn actions
+
+let merge_files_dyn ~target =
+  dyn_paths (arr fst)
+  >>^ (fun (sources, extras) ->
+    Action.Merge_files_into (sources, extras, target))
+  >>> action_dyn ~targets:[target] ()
diff --git a/src/build.mli b/src/build.mli
index 1268815d..01777221 100644
--- a/src/build.mli
+++ b/src/build.mli
@@ -234,3 +234,5 @@ val merge_lib_deps : lib_deps -> lib_deps -> lib_deps
 
 (**/**)
 val paths_for_rule : Path.Set.t -> ('a, 'a) t
+
+val merge_files_dyn : target:Path.t -> (Path.t list * string list, Action.t) t
diff --git a/src/ocamldep.ml b/src/ocamldep.ml
index b5f6865f..fc4a12e1 100644
--- a/src/ocamldep.ml
+++ b/src/ocamldep.ml
@@ -58,7 +58,15 @@ module Dep_graphs = struct
     Ml_kind.Dict.make_both (Dep_graph.dummy m)
 end
 
-let parse_deps ~dir ~file ~(unit : Module.t)
+let parse_module_names ~(unit : Module.t) ~modules words =
+  List.filter_map words ~f:(fun m ->
+    let m = Module.Name.of_string m in
+    if m = unit.name then
+      None
+    else
+      Module.Name.Map.find modules m)
+
+let parse_deps ~dir ~file ~unit
       ~modules ~alias_module ~lib_interface_module lines =
   let invalid () =
     die "ocamldep returned unexpected output for %s:\n\
@@ -81,12 +89,7 @@ let parse_deps ~dir ~file ~(unit : Module.t)
       let deps =
         String.extract_blank_separated_words
           (String.sub line ~pos:(i + 1) ~len:(String.length line - (i + 1)))
-        |> List.filter_map ~f:(fun m ->
-          let m = Module.Name.of_string m in
-          if m = unit.name then
-            None
-          else
-            Module.Name.Map.find modules m)
+        |> parse_module_names ~unit ~modules
       in
       (match lib_interface_module with
        | None -> ()
@@ -109,12 +112,9 @@ let parse_deps ~dir ~file ~(unit : Module.t)
              Module.Name.pp unit.name (Path.to_string dir)
              Module.Name.pp m.name
              Module.Name.pp m.name);
-      let deps =
-        match alias_module with
-        | None -> deps
-        | Some m -> m :: deps
-      in
-      deps
+      match alias_module with
+      | None -> deps
+      | Some m -> m :: deps
 
 let rules ~(ml_kind:Ml_kind.t) ~dir ~modules
       ?(already_used=Module.Name.Set.empty)
@@ -124,17 +124,42 @@ let rules ~(ml_kind:Ml_kind.t) ~dir ~modules
       match Module.file ~dir unit ml_kind with
       | None -> Build.return []
       | Some file ->
-        let ocamldep_output = Path.extend_basename file ~suffix:".d" in
+        let all_deps_path file =
+          Path.extend_basename file ~suffix:".all-deps"
+        in
         let context = SC.context sctx in
+        let all_deps_file = all_deps_path file in
+        let ocamldep_output = Path.extend_basename file ~suffix:".d" in
         if not (Module.Name.Set.mem already_used unit.name) then
-          SC.add_rule sctx
-            (Build.run ~context (Ok context.ocamldep)
-               [A "-modules"; Ml_kind.flag ml_kind; Dep file]
-               ~stdout_to:ocamldep_output);
-        Build.memoize (Path.to_string ocamldep_output)
-          (Build.lines_of ocamldep_output
-           >>^ parse_deps ~dir ~file ~unit ~modules ~alias_module
-                 ~lib_interface_module))
+          begin
+            SC.add_rule sctx
+              ( Build.run ~context (Ok context.ocamldep)
+                  [A "-modules"; Ml_kind.flag ml_kind; Dep file]
+                  ~stdout_to:ocamldep_output
+              );
+            let build_paths dependencies =
+              let dependency_file_path m =
+                Option.map
+                  (Module.file ~dir m Ml_kind.Intf)
+                 ~f:all_deps_path
+              in
+              List.filter_map dependencies ~f:dependency_file_path
+            in
+            SC.add_rule sctx
+              ( Build.lines_of ocamldep_output
+                >>^ parse_deps
+                      ~dir ~file ~unit ~modules ~alias_module
+                      ~lib_interface_module
+                >>^ (fun modules ->
+                  (build_paths modules,
+                   List.map modules ~f:(fun m ->
+                     Module.Name.to_string (Module.name m))
+                  ))
+                >>> Build.merge_files_dyn ~target:all_deps_file)
+          end;
+        Build.memoize (Path.to_string all_deps_file)
+          ( Build.lines_of all_deps_file
+          >>^ parse_module_names ~unit ~modules))
   in
   let per_module =
     match alias_module with
diff --git a/test/blackbox-tests/test-cases/github660/run.t b/test/blackbox-tests/test-cases/github660/run.t
index 5cf01919..ed5c2704 100644
--- a/test/blackbox-tests/test-cases/github660/run.t
+++ b/test/blackbox-tests/test-cases/github660/run.t
@@ -5,9 +5,8 @@ When there are explicit interfaces, modules must be rebuilt.
   hello
   $ echo 'let x = 1' >> explicit-interfaces/lib_sub.ml
   $ jbuilder runtest --root explicit-interfaces --display quiet -j1 2>&1 | grep -v Entering | grep -v ocamlopt
-  File "_none_", line 1:
-  Error: Files .main.eobjs/main.cmx and .main.eobjs/lib_sub.cmx
-         make inconsistent assumptions over implementation Lib_sub
+          main alias runtest
+  hello
 
 When there are no interfaces, the situation is the same, but it is not possible
 to rely on these.
diff --git a/test/blackbox-tests/test-cases/menhir/run.t b/test/blackbox-tests/test-cases/menhir/run.t
index 35d348a7..8dc89d99 100644
--- a/test/blackbox-tests/test-cases/menhir/run.t
+++ b/test/blackbox-tests/test-cases/menhir/run.t
@@ -8,10 +8,10 @@
       ocamldep src/test_base.ml.d
         menhir src/test_menhir1.{ml,mli}
       ocamldep src/test_menhir1.ml.d
+      ocamldep src/test_base.mli.d
       ocamldep src/test_menhir1.mli.d
         ocamlc src/.test.eobjs/test_menhir1.{cmi,cmti}
         ocamlc src/.test.eobjs/lexer1.{cmi,cmo,cmt}
-      ocamldep src/test_base.mli.d
         ocamlc src/.test.eobjs/test_base.{cmi,cmti}
         ocamlc src/.test.eobjs/lexer2.{cmi,cmo,cmt}
         ocamlc src/.test.eobjs/test.{cmi,cmo,cmt}