Fix a bug in error printing of unif constraints

Conversion problems are in a de Bruijn environment that may
include references to unbound rels (because of the way evars
are created), we patch the env to named all de Bruijn variables
so that error printing does not raise an anomaly. Also fix a minor
printing bug in unsatisfiable constraints error reporting.

HoTT_coq_117.v tests all of this.

1 files changed, 11 insertions, 0 deletions

diff --git a/pretyping/evd.ml b/pretyping/evd.ml
index 0bc688aacf..2f4b8fc12f 100644
--- a/pretyping/evd.ml
+++ b/pretyping/evd.ml
@@ -840,6 +840,7 @@ let set_universe_context evd uctx' =
   { evd with universes = uctx' }
 
 let add_conv_pb ?(tail=false) pb d =
+  (** MS: we have duplicates here, why? *)
   if tail then {d with conv_pbs = d.conv_pbs @ [pb]}
   else {d with conv_pbs = pb::d.conv_pbs}
 
@@ -1888,6 +1889,16 @@ let print_env_short env =
 
 let pr_evar_constraints pbs =
   let pr_evconstr (pbty, env, t1, t2) =
+    let env =
+      (** We currently allow evar instances to refer to anonymous de
+          Bruijn indices, so we protect the error printing code in this
+          case by giving names to every de Bruijn variable in the
+          rel_context of the conversion problem. MS: we should rather
+          stop depending on anonymous variables, they can be used to
+          indicate independency. Also, this depends on a strategy for
+          naming/renaming. *)
+      Namegen.make_all_name_different env
+    in
     print_env_short env ++ spc () ++ str "|-" ++ spc () ++
       print_constr_env env t1 ++ spc () ++
       str (match pbty with