[econstr] Forbid calling `to_constr` in open terms.

We forbid calling `EConstr.to_constr` on terms that are not evar-free,
as to progress towards enforcing the invariant that `Constr.t` is
evar-free. [c.f. #6308]

Due to compatibility constraints we provide an optional parameter to
`to_constr`, `abort` which can be used to overcome this restriction
until we fix all parts of the code.

Now, grepping for `~abort:false` should return the questionable
parts of the system.

Not a lot of places had to be fixed, some comments:

- problems with the interface due to `Evd/Constr` [`Evd.define` being
  the prime example] do seem real!
- inductives also look bad with regards to `Constr/EConstr`.
- code in plugins needs work.

A notable user of this "feature" is `Obligations/Program` that seem to
like to generate kernel-level entries with free evars, then to scan
them and workaround this problem by generating constants.

1 files changed, 2 insertions, 2 deletions

diff --git a/pretyping/vnorm.ml b/pretyping/vnorm.ml
index 3c9b8bc33..f314ae0d6 100644
--- a/pretyping/vnorm.ml
+++ b/pretyping/vnorm.ml
@@ -381,8 +381,8 @@ let cbv_vm env sigma c t  =
   if Termops.occur_meta sigma c then
     CErrors.user_err Pp.(str "vm_compute does not support metas.");
   (** This evar-normalizes terms beforehand *)
-  let c = EConstr.to_constr sigma c in
-  let t = EConstr.to_constr sigma t in
+  let c = EConstr.to_constr ~abort_on_undefined_evars:false sigma c in
+  let t = EConstr.to_constr ~abort_on_undefined_evars:false sigma t in
   let v = Vconv.val_of_constr env c in
   EConstr.of_constr (nf_val env sigma v t)