From f75e1b5d8a4f679c839ffa327baa590198c69640 Mon Sep 17 00:00:00 2001
From: Kenji Maillard
Date: Mon, 4 Nov 2019 23:29:40 +0100
Subject: Prevent redefinition of Ltac2 types; fix #10097

---
 test-suite/bugs/closed/bug_10097.v | 14 ++++++++++++++
 user-contrib/Ltac2/tac2entries.ml  |  9 +++++++++
 2 files changed, 23 insertions(+)
 create mode 100644 test-suite/bugs/closed/bug_10097.v

diff --git a/test-suite/bugs/closed/bug_10097.v b/test-suite/bugs/closed/bug_10097.v
new file mode 100644
index 0000000000..12f2d4cc58
--- /dev/null
+++ b/test-suite/bugs/closed/bug_10097.v
@@ -0,0 +1,14 @@
+From Ltac2 Require Import Ltac2.
+
+(* #10097 *)
+Ltac2 Type s := [X(int)].
+Fail Ltac2 Type s.
+Fail Ltac2 Type s := [].
+
+Ltac2 Type r := [..].
+Fail Ltac2 Type r := [].
+
+Module Other.
+  Ltac2 Type s.
+  Ltac2 Type r := [].
+End Other.
diff --git a/user-contrib/Ltac2/tac2entries.ml b/user-contrib/Ltac2/tac2entries.ml
index 92bc49346f..b8853f3755 100644
--- a/user-contrib/Ltac2/tac2entries.ml
+++ b/user-contrib/Ltac2/tac2entries.ml
@@ -374,6 +374,15 @@ let register_typedef ?(local = false) isrec types =
   | ({loc;v=id}, _) :: _ ->
     user_err ?loc (str "Multiple definition of the type name " ++ Id.print id)
   in
+  let () =
+    let check_existing_type ({v=id},_) =
+      let qid = Libnames.make_qualid (Lib.current_dirpath false) id in
+      try let _ = Tac2env.locate_type qid in
+        user_err (str "Multiple definition of the type name " ++ pr_qualid qid)
+      with Not_found -> ()
+    in
+    List.iter check_existing_type types
+  in
   let check ({loc;v=id}, (params, def)) =
     let same_name {v=id1} {v=id2} = Id.equal id1 id2 in
     let () = match List.duplicates same_name params with
-- 
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