From 030ef2f015e5c592ea7599f0f98a715873c1e4d0 Mon Sep 17 00:00:00 2001
From: Pierre-Marie Pédrot
Date: Sun, 24 Jan 2016 15:15:16 +0100
Subject: Fixing bug #3826: "Incompatible module types" is uninformative.

---
 toplevel/himsg.ml | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/toplevel/himsg.ml b/toplevel/himsg.ml
index 7ddfd46cd1..89c33cb6fe 100644
--- a/toplevel/himsg.ml
+++ b/toplevel/himsg.ml
@@ -890,7 +890,17 @@ let explain_is_a_functor () =
   str "Illegal use of a functor."
 
 let explain_incompatible_module_types mexpr1 mexpr2 =
-  str "Incompatible module types."
+  let open Declarations in
+  let rec get_arg = function
+  | NoFunctor _ -> 0
+  | MoreFunctor (_, _, ty) -> succ (get_arg ty)
+  in
+  let len1 = get_arg mexpr1.mod_type in
+  let len2 = get_arg mexpr2.mod_type in
+  if len1 <> len2 then
+    str "Incompatible module types: module expects " ++ int len2 ++
+      str " arguments, found " ++ int len1 ++ str "."
+  else str "Incompatible module types."
 
 let explain_not_equal_module_paths mp1 mp2 =
   str "Non equal modules."
-- 
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