[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, 5 insertions, 3 deletions

diff --git a/vernac/comProgramFixpoint.ml b/vernac/comProgramFixpoint.ml
index b95741ca4..745f1df1d 100644
--- a/vernac/comProgramFixpoint.ml
+++ b/vernac/comProgramFixpoint.ml
@@ -229,7 +229,8 @@ let build_wellfounded (recname,pl,n,bl,arityc,body) poly r measure notation =
   in
   (* XXX: Capturing sigma here... bad bad *)
   let hook = Lemmas.mk_hook (hook sigma) in
-  let fullcoqc = EConstr.to_constr sigma def in
+  (* XXX: Grounding non-ground terms here... bad bad *)
+  let fullcoqc = EConstr.to_constr ~abort_on_undefined_evars:false sigma def in
   let fullctyp = EConstr.to_constr sigma typ in
   Obligations.check_evars env sigma;
   let evars, _, evars_def, evars_typ =
@@ -261,9 +262,10 @@ let do_program_recursive local poly fixkind fixl ntns =
   let collect_evars id def typ imps =
     (* Generalize by the recursive prototypes  *)
     let def =
-      EConstr.to_constr evd (Termops.it_mkNamedLambda_or_LetIn (EConstr.of_constr def) rec_sign)
+      EConstr.to_constr ~abort_on_undefined_evars:false evd (Termops.it_mkNamedLambda_or_LetIn (EConstr.of_constr def) rec_sign)
     and typ =
-      EConstr.to_constr evd (Termops.it_mkNamedProd_or_LetIn (EConstr.of_constr typ) rec_sign)
+      (* Worrying... *)
+      EConstr.to_constr ~abort_on_undefined_evars:false evd (Termops.it_mkNamedProd_or_LetIn (EConstr.of_constr typ) rec_sign)
     in
     let evm = collect_evars_of_term evd def typ in
     let evars, _, def, typ =