[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, 1 deletions

diff --git a/vernac/comFixpoint.ml b/vernac/comFixpoint.ml
index a794c2db0..1466fa243 100644
--- a/vernac/comFixpoint.ml
+++ b/vernac/comFixpoint.ml
@@ -225,7 +225,8 @@ let interp_recursive ~program_mode ~cofix fixl notations =
   (* Instantiate evars and check all are resolved *)
   let sigma = solve_unif_constraints_with_heuristics env_rec sigma in
   let sigma, _ = nf_evars_and_universes sigma in
-  let fixdefs = List.map (fun c -> Option.map EConstr.(to_constr sigma) c) fixdefs in
+  (* XXX: We still have evars here in Program *)
+  let fixdefs = List.map (fun c -> Option.map EConstr.(to_constr ~abort_on_undefined_evars:false sigma) c) fixdefs in
   let fixtypes = List.map EConstr.(to_constr sigma) fixtypes in
   let fixctxs = List.map (fun (_,ctx) -> ctx) fixctxs in