[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/vernac/comDefinition.ml b/vernac/comDefinition.ml
index b18a60a1f..9aa61ab46 100644
--- a/vernac/comDefinition.ml
+++ b/vernac/comDefinition.ml
@@ -88,8 +88,8 @@ let interp_definition pl bl poly red_option c ctypopt =
   let evd = Evd.minimize_universes evd in
   (* Substitute evars and universes, and add parameters.
      Note: in program mode some evars may remain. *)
-  let ctx = List.map (EConstr.to_rel_decl evd) ctx in
-  let c = Term.it_mkLambda_or_LetIn (EConstr.to_constr evd c) ctx in
+  let ctx = List.map Termops.(map_rel_decl (to_constr ~abort_on_undefined_evars:false evd)) ctx in
+  let c = Term.it_mkLambda_or_LetIn (EConstr.to_constr ~abort_on_undefined_evars:false evd c) ctx in
   let tyopt = Option.map (fun ty -> Term.it_mkProd_or_LetIn (EConstr.to_constr evd ty) ctx) tyopt in
   (* Keep only useful universes. *)
   let uvars_fold uvars c =