Improve error message for link order error in bytelink

Changes

@@ -3,6 +3,11 @@ Working version
 
 (Changes that can break existing programs are marked with a "*")
 
+### Compiler user-interface and warnings:
+
+- GPR#XXXX: Improve error message for link order error in bytecode
+  (Pierre Chambart)
+
 ### Internal/compiler-libs changes:
 
 - GPR#2229: Env: remove prefix_idents cache

bytecomp/bytelink.ml

@@ -29,6 +29,7 @@ type error =
   | File_exists of filepath
   | Cannot_open_dll of filepath
   | Required_module_unavailable of modname
+  | Wrong_link_order of (modname * modname) list
 
 exception Error of error
 
@@ -86,6 +87,8 @@ let add_ccobjs origin l =
 (* First pass: determine which units are needed *)
 
 let missing_globals = ref Ident.Set.empty
+let provided_globals = ref Ident.Set.empty
+let badly_ordered_dependencies : (string * string) list ref = ref []
 
 let is_required (rel, _pos) =
   match rel with
@@ -95,6 +98,9 @@ let is_required (rel, _pos) =
 
 let add_required compunit =
   let add id =
+    if Ident.Set.mem id !provided_globals then
+      badly_ordered_dependencies :=
+        ((Ident.name id), compunit.cu_name) :: !badly_ordered_dependencies;
     missing_globals := Ident.Set.add id !missing_globals
   in
   List.iter add (Symtable.required_globals compunit.cu_reloc);
@@ -567,7 +573,12 @@ let link objfiles output_name =
   begin
     match Ident.Set.elements missing_modules with
     | [] -> ()
-    | id :: _ -> raise (Error (Required_module_unavailable (Ident.name id)))
+    | id :: _ ->
+        match !badly_ordered_dependencies with
+        | [] ->
+            raise (Error (Required_module_unavailable (Ident.name id)))
+        | l ->
+            raise (Error (Wrong_link_order l))
   end;
   Clflags.ccobjs := !Clflags.ccobjs @ !lib_ccobjs; (* put user's libs last *)
   Clflags.all_ccopts := !lib_ccopts @ !Clflags.all_ccopts;
@@ -694,6 +705,12 @@ let report_error ppf = function
         Location.print_filename file
   | Required_module_unavailable s ->
       fprintf ppf "Required module `%s' is unavailable" s
+  | Wrong_link_order l ->
+      let depends_on ppf (dep, depending) =
+        fprintf ppf "%s depends on %s" depending dep
+      in
+      fprintf ppf "@[<hov 2>Wrong link order: %a@]"
+        (pp_print_list ~pp_sep:(fun ppf () -> fprintf ppf ",@ ") depends_on) l
 
 let () =
   Location.register_error_of_exn

bytecomp/bytelink.mli

@@ -34,6 +34,7 @@ type error =
   | File_exists of filepath
   | Cannot_open_dll of filepath
   | Required_module_unavailable of modname
+  | Wrong_link_order of (modname * modname) list
 
 exception Error of error