Allow using cumulativity without forcing strict constraints.

Previously [fun x : Ind@{i} => x : Ind@{j}] with Ind some cumulative
inductive would try to generate a constraint [i = j] and use
cumulativity only if this resulted in an inconsistency. This is
confusingly different from the behaviour with [Type] and means
cumulativity can only be used to lift between universes related by
strict inequalities. (This isn't a kernel restriction so there might
be some workaround to send the kernel the right constraints, but
not in a nice way.)

See modified test for more details of what is now possible.

Technical notes:

When universe constraints were inferred by comparing the shape of
terms without reduction, cumulativity was not used and so too-strict
equality constraints were generated. Then in order to use cumulativity
we had to make this comparison fail to fall back to full conversion.

When unifiying 2 instances of a cumulative inductive type, if there
are any Irrelevant universes we try to unify them if they are
flexible.

1 files changed, 2 insertions, 1 deletions

diff --git a/plugins/ltac/extratactics.ml4 b/plugins/ltac/extratactics.ml4
index 7d43f1986..61632e388 100644
--- a/plugins/ltac/extratactics.ml4
+++ b/plugins/ltac/extratactics.ml4
@@ -859,8 +859,9 @@ END
 
 let eq_constr x y = 
   Proofview.Goal.enter begin fun gl ->
+    let env = Tacmach.New.pf_env gl in
     let evd = Tacmach.New.project gl in
-      match EConstr.eq_constr_universes evd x y with
+      match EConstr.eq_constr_universes env evd x y with
       | Some _ -> Proofview.tclUNIT () 
       | None -> Tacticals.New.tclFAIL 0 (str "Not equal")
   end