3 files changed, 19 insertions, 0 deletions
diff --git a/kernel/subtyping.ml b/kernel/subtyping.ml
index d0d5cb1d5..e95d5d2b5 100644
--- a/kernel/subtyping.ml
+++ b/kernel/subtyping.ml
@@ -118,6 +118,15 @@ let check_inductive cst env mp1 l info1 mp2 mib2 spec2 subst1 subst2 reso1 reso2
       let env = check_polymorphic_instance error env auctx auctx' in
       env, Univ.make_abstract_instance auctx'
     | Cumulative_ind cumi, Cumulative_ind cumi' ->
+      (** Currently there is no way to control variance of inductive types, but
+          just in case we require that they are in a subtyping relation. *)
+      let () =
+        let v = ACumulativityInfo.variance cumi in
+        let v' = ACumulativityInfo.variance cumi' in
+        if not (Array.for_all2 Variance.check_subtype v' v) then
+          CErrors.anomaly Pp.(str "Variance of " ++ KerName.print kn1 ++
+            str " is not compatible with the one of " ++ KerName.print kn2)
+      in
       let auctx = Univ.ACumulativityInfo.univ_context cumi in
       let auctx' = Univ.ACumulativityInfo.univ_context cumi' in
       let env = check_polymorphic_instance error env auctx auctx' in
diff --git a/kernel/univ.ml b/kernel/univ.ml
index fbb047364..c42b66749 100644
--- a/kernel/univ.ml
+++ b/kernel/univ.ml
@@ -759,6 +759,13 @@ struct
     | Invariant, _ | _, Invariant -> Invariant
     | Covariant, Covariant -> Covariant
 
+  let check_subtype x y = match x, y with
+  | (Irrelevant | Covariant | Invariant), Irrelevant -> true
+  | Irrelevant, Covariant -> false
+  | (Covariant | Invariant), Covariant -> true
+  | (Irrelevant | Covariant), Invariant -> false
+  | Invariant, Invariant -> true
+
   let pr = function
     | Irrelevant -> str "*"
     | Covariant -> str "+"
diff --git a/kernel/univ.mli b/kernel/univ.mli
index c45ebe21c..74d1bfd3a 100644
--- a/kernel/univ.mli
+++ b/kernel/univ.mli
@@ -253,6 +253,9 @@ sig
      A'] as opposed to [A' <= A]. *)
   type t = Irrelevant | Covariant | Invariant
 
+  (** [check_subtype x y] holds if variance [y] is also an instance of [x] *)
+  val check_subtype : t -> t -> bool
+
   val sup : t -> t -> t
 
   val pr : t -> Pp.t