evd: restrict_evar with candidates, can raise NoCandidatesLeft

When restricting an evar with candidates, raise an exception if this
restriction would leave the evar without candidates, i.e. unsolvable.

- evarutil: mark restricted evars as "cleared"

They would otherwise escape being catched by the [advance] function
of clenv, and result in dangling evars not being registered to the shelf.

- engine: restrict_evar marks it cleared, update the future goals

We make the new evar a future goal and remove the old one.
If we did nothing, [unshelve tac] would work correctly as it
uses [Proofview.advance] to find the shelved goals, going through
the cleared evar. But [Unshelve] would fail as it expects only
undefined evars on the shelf and throws away the defined ones.

4 files changed, 40 insertions, 9 deletions

diff --git a/engine/evarutil.ml b/engine/evarutil.ml
index 82be4791f..cdc2897c8 100644
--- a/engine/evarutil.ml
+++ b/engine/evarutil.ml
@@ -426,10 +426,6 @@ let push_rel_context_to_named_context ?hypnaming env sigma typ =
 
 let default_source = Loc.tag @@ Evar_kinds.InternalHole
 
-let restrict_evar evd evk filter ?src candidates =
-  let evd, evk' = Evd.restrict evk filter ?candidates ?src evd in
-  Evd.declare_future_goal evk' evd, evk'
-
 let new_pure_evar_full evd evi =
   let (evd, evk) = Evd.new_evar evd evi in
   let evd = Evd.declare_future_goal evk evd in
@@ -532,11 +528,33 @@ let generalize_evar_over_rels sigma (ev,args) =
 type clear_dependency_error =
 | OccurHypInSimpleClause of Id.t option
 | EvarTypingBreak of existential
+| NoCandidatesLeft of Evar.t
 
 exception ClearDependencyError of Id.t * clear_dependency_error * GlobRef.t option
 
 exception Depends of Id.t
 
+let set_of_evctx l =
+  List.fold_left (fun s decl -> Id.Set.add (NamedDecl.get_id decl) s) Id.Set.empty l
+
+let filter_effective_candidates evd evi filter candidates =
+  let ids = set_of_evctx (Filter.filter_list filter (evar_context evi)) in
+  List.filter (fun a -> Id.Set.subset (collect_vars evd a) ids) candidates
+
+let restrict_evar evd evk filter ?src candidates =
+  let evar_info = Evd.find_undefined evd evk in
+  let candidates = Option.map (filter_effective_candidates evd evar_info filter) candidates in
+  match candidates with
+  | Some [] -> raise (ClearDependencyError (*FIXME*)(Id.of_string "blah", (NoCandidatesLeft evk), None))
+  | _ ->
+     let evd, evk' = Evd.restrict evk filter ?candidates ?src evd in
+     (** Mark new evar as future goal, removing previous one,
+         circumventing Proofview.advance but making Proof.run_tactic catch these. *)
+     let future_goals = Evd.save_future_goals evd in
+     let future_goals = Evd.filter_future_goals (fun evk' -> not (Evar.equal evk evk')) future_goals in
+     let evd = Evd.restore_future_goals evd future_goals in
+     (Evd.declare_future_goal evk' evd, evk')
+
 let rec check_and_clear_in_constr env evdref err ids global c =
   (* returns a new constr where all the evars have been 'cleaned'
      (ie the hypotheses ids have been removed from the contexts of
@@ -606,7 +624,9 @@ let rec check_and_clear_in_constr env evdref err ids global c =
               let origfilter = Evd.evar_filter evi in
               let filter = Evd.Filter.apply_subfilter origfilter filter in
               let evd = !evdref in
-              let (evd,_) = restrict_evar evd evk filter None in
+              let candidates = Evd.evar_candidates evi in
+              let candidates = Option.map (List.map EConstr.of_constr) candidates in
+              let (evd,_) = restrict_evar evd evk filter candidates in
               evdref := evd;
               Evd.existential_value0 !evdref ev
 
diff --git a/engine/evarutil.mli b/engine/evarutil.mli
index c17f3d168..1003c4feb 100644
--- a/engine/evarutil.mli
+++ b/engine/evarutil.mli
@@ -65,9 +65,6 @@ val new_type_evar :
 
 val new_Type : ?rigid:rigid -> env -> evar_map -> evar_map * constr
 
-val restrict_evar : evar_map -> Evar.t -> Filter.t ->
-  ?src:Evar_kinds.t Loc.located -> constr list option -> evar_map * Evar.t
-
 (** Polymorphic constants *)
 
 val new_global : evar_map -> GlobRef.t -> evar_map * constr
@@ -223,9 +220,18 @@ raise OccurHypInSimpleClause if the removal breaks dependencies *)
 type clear_dependency_error =
 | OccurHypInSimpleClause of Id.t option
 | EvarTypingBreak of Constr.existential
+| NoCandidatesLeft of Evar.t
 
 exception ClearDependencyError of Id.t * clear_dependency_error * GlobRef.t option
 
+(** Restrict an undefined evar according to a (sub)filter and candidates.
+    The evar will be defined if there is only one candidate left,
+@raise ClearDependencyError NoCandidatesLeft if the filter turns the candidates
+  into an empty list. *)
+
+val restrict_evar : evar_map -> Evar.t -> Filter.t ->
+  ?src:Evar_kinds.t Loc.located -> constr list option -> evar_map * Evar.t
+
 val clear_hyps_in_evi : env -> evar_map -> named_context_val -> types ->
   Id.Set.t -> evar_map * named_context_val * types
 
diff --git a/engine/evd.ml b/engine/evd.ml
index 0c9c3a29b..7e0c397b2 100644
--- a/engine/evd.ml
+++ b/engine/evd.ml
@@ -171,6 +171,8 @@ let evar_context evi = named_context_of_val evi.evar_hyps
 let evar_filtered_context evi =
   Filter.filter_list (evar_filter evi) (evar_context evi)
 
+let evar_candidates evi = evi.evar_candidates
+
 let evar_hyps evi = evi.evar_hyps
 
 let evar_filtered_hyps evi = match Filter.repr (evar_filter evi) with
@@ -620,6 +622,7 @@ let merge_universe_context evd uctx' =
 let set_universe_context evd uctx' =
   { evd with universes = uctx' }
 
+(* TODO: make unique *)
 let add_conv_pb ?(tail=false) pb d =
   (** MS: we have duplicates here, why? *)
   if tail then {d with conv_pbs = d.conv_pbs @ [pb]}
diff --git a/engine/evd.mli b/engine/evd.mli
index c40e925d8..83fd00065 100644
--- a/engine/evd.mli
+++ b/engine/evd.mli
@@ -113,6 +113,7 @@ val evar_filtered_context : evar_info -> (econstr, etypes) Context.Named.pt
 val evar_hyps : evar_info -> named_context_val
 val evar_filtered_hyps : evar_info -> named_context_val
 val evar_body : evar_info -> evar_body
+val evar_candidates : evar_info -> constr list option
 val evar_filter : evar_info -> Filter.t
 val evar_env :  evar_info -> env
 val evar_filtered_env :  evar_info -> env
@@ -243,7 +244,8 @@ val evars_reset_evd  : ?with_conv_pbs:bool -> ?with_univs:bool ->
 val restrict : Evar.t-> Filter.t -> ?candidates:econstr list ->
   ?src:Evar_kinds.t located -> evar_map -> evar_map * Evar.t
 (** Restrict an undefined evar into a new evar by filtering context and
-    possibly limiting the instances to a set of candidates *)
+    possibly limiting the instances to a set of candidates (candidates
+    are filtered according to the filter) *)
 
 val is_restricted_evar : evar_info -> Evar.t option
 (** Tell if an evar comes from restriction of another evar, and if yes, which *)