Remove the forward class hint feature.

It was not documented, not properly tested and thus likely buggy. Judging
from the code alone I spotted already one potential bug. Further more it was
prominently making use of the infamous "arbitrary term as hint" feature.

Since the only user in our CI seems to be a math-classes file that introduced
the feature under a claim of "cleanup", I believe we can safely remove it
without anyone noticing.

11 files changed, 15 insertions, 116 deletions

diff --git a/pretyping/typeclasses.ml b/pretyping/typeclasses.ml
index adb9c5299f..78f04c99e5 100644
--- a/pretyping/typeclasses.ml
+++ b/pretyping/typeclasses.ml
@@ -11,7 +11,6 @@
 (*i*)
 open Names
 open Globnames
-open Term
 open Constr
 open Vars
 open Evd
@@ -42,7 +41,7 @@ let get_solve_one_instance, solve_one_instance_hook = Hook.make ()
 let resolve_one_typeclass ?(unique=get_typeclasses_unique_solutions ()) env evm t =
   Hook.get get_solve_one_instance env evm t unique
 
-type direction = Forward | Backward
+type direction = Backward
 
 (* This module defines type-classes *)
 type typeclass = {
@@ -156,66 +155,6 @@ let load_class cl =
 
 (** Build the subinstances hints. *)
 
-let check_instance env sigma c =
-  try
-    let (evd, c) = resolve_one_typeclass env sigma
-      (Retyping.get_type_of env sigma c) in
-      not (Evd.has_undefined evd)
-  with e when CErrors.noncritical e -> false
-
-let build_subclasses ~check env sigma glob { hint_priority = pri } =
-  let _id = Nametab.basename_of_global glob in
-  let _next_id =
-    let i = ref (-1) in
-      (fun () -> incr i;
-        Nameops.add_suffix _id ("_subinstance_" ^ string_of_int !i))
-  in
-  let ty, ctx = Typeops.type_of_global_in_context env glob in
-  let inst, ctx = UnivGen.fresh_instance_from ctx None in
-  let ty = Vars.subst_instance_constr inst ty in
-  let ty = EConstr.of_constr ty in
-  let sigma = Evd.merge_context_set Evd.univ_rigid sigma ctx in
-  let rec aux pri c ty path =
-      match class_of_constr env sigma ty with
-      | None -> []
-      | Some (rels, ((tc,u), args)) ->
-        let instapp =
-          Reductionops.whd_beta env sigma (EConstr.of_constr (appvectc c (Context.Rel.to_extended_vect mkRel 0 rels)))
-        in
-        let instapp = EConstr.Unsafe.to_constr instapp in
-        let projargs = Array.of_list (args @ [instapp]) in
-        let projs = List.map_filter
-          (fun (n, b, proj) ->
-           match b with
-           | None -> None
-           | Some (Backward, _) -> None
-           | Some (Forward, info) ->
-             let proj = Option.get proj in
-             let rels = List.map (fun d -> Termops.map_rel_decl EConstr.Unsafe.to_constr d) rels in
-             let u = EConstr.EInstance.kind sigma u in
-             let body = it_mkLambda_or_LetIn (mkApp (mkConstU (proj,u), projargs)) rels in
-               if check && check_instance env sigma (EConstr.of_constr body) then None
-               else
-                 let newpri =
-                   match pri, info.hint_priority with
-                   | Some p, Some p' -> Some (p + p')
-                   | Some p, None -> Some (p + 1)
-                   | _, _ -> None
-                 in
-                   Some (GlobRef.ConstRef proj, { info with hint_priority = newpri }, body)) tc.cl_projs
-        in
-        let declare_proj hints (cref, info, body) =
-          let path' = cref :: path in
-          let ty = Retyping.get_type_of env sigma (EConstr.of_constr body) in
-          let rest = aux pri body ty path' in
-            hints @ (path', info, body) :: rest
-        in List.fold_left declare_proj [] projs
-  in
-  let term = Constr.mkRef (glob, inst) in
-    (*FIXME subclasses should now get substituted for each particular instance of
-      the polymorphic superclass *)
-    aux pri term ty [glob]
-
 (*
  * interface functions
  *)
diff --git a/pretyping/typeclasses.mli b/pretyping/typeclasses.mli
index 9de8083276..c0e71f26c4 100644
--- a/pretyping/typeclasses.mli
+++ b/pretyping/typeclasses.mli
@@ -13,7 +13,7 @@ open Constr
 open Evd
 open Environ
 
-type direction = Forward | Backward
+type direction = Backward
 
 (* Core typeclasses hints *)
 type 'a hint_info_gen =
@@ -127,11 +127,3 @@ val classes_transparent_state : unit -> TransparentState.t
 
 val solve_all_instances_hook : (env -> evar_map -> evar_filter -> bool -> bool -> bool -> evar_map) Hook.t
 val solve_one_instance_hook : (env -> evar_map -> EConstr.types -> bool -> evar_map * EConstr.constr) Hook.t
-
-(** Build the subinstances hints for a given typeclass object.
-    check tells if we should check for existence of the
-    subinstances and add only the missing ones. *)
-
-val build_subclasses : check:bool -> env -> evar_map -> GlobRef.t ->
-                       hint_info ->
-                       (GlobRef.t list * hint_info * constr) list
diff --git a/tactics/class_tactics.ml b/tactics/class_tactics.ml
index b4d7e7d7f0..ed92a85a12 100644
--- a/tactics/class_tactics.ml
+++ b/tactics/class_tactics.ml
@@ -483,17 +483,7 @@ let make_resolve_hyp env sigma st only_classes pri decl =
     if keep then
       let id = GlobRef.VarRef id in
       let name = PathHints [id] in
-      let hints =
-        if is_class then
-          let hints = build_subclasses ~check:false env sigma id empty_hint_info in
-            (List.map_append
-             (fun (path,info,c) ->
-              let h = IsConstr (EConstr.of_constr c, None) [@ocaml.warning "-3"] in
-              make_resolves env sigma ~name:(PathHints path) info ~check:true h)
-               hints)
-        else []
-      in
-        (hints @ make_resolves env sigma pri ~name ~check:false (IsGlobRef id))
+        (make_resolves env sigma pri ~name ~check:false (IsGlobRef id))
     else []
 
 let make_hints g (modes,st) only_classes sign =
diff --git a/test-suite/bugs/closed/bug_2928.v b/test-suite/bugs/closed/bug_2928.v
deleted file mode 100644
index 21e92ae20c..0000000000
--- a/test-suite/bugs/closed/bug_2928.v
+++ /dev/null
@@ -1,11 +0,0 @@
-Class Equiv A := equiv: A -> A -> Prop.
-Infix "=" := equiv : type_scope.
-
-Class Associative {A} f `{Equiv A} := associativity x y z : f x (f y z) = f (f x y) z.
-
-Class SemiGroup A op `{Equiv A} := { sg_ass :>> Associative op }.
-
-Class SemiLattice A op `{Equiv A} :=
-    { semilattice_sg :>> SemiGroup A op
-    ; redundant : Associative op
-    }.
diff --git a/test-suite/output/UnivBinders.out b/test-suite/output/UnivBinders.out
index 163ed15606..d8d3f696b7 100644
--- a/test-suite/output/UnivBinders.out
+++ b/test-suite/output/UnivBinders.out
@@ -67,9 +67,9 @@ mono
 The command has indeed failed with message:
 Universe u already exists.
 bobmorane = 
-let tt := Type@{UnivBinders.33} in
-let ff := Type@{UnivBinders.35} in tt -> ff
-     : Type@{max(UnivBinders.32,UnivBinders.34)}
+let tt := Type@{UnivBinders.32} in
+let ff := Type@{UnivBinders.34} in tt -> ff
+     : Type@{max(UnivBinders.31,UnivBinders.33)}
 The command has indeed failed with message:
 Universe u already bound.
 foo@{E M N} = 
diff --git a/vernac/classes.ml b/vernac/classes.ml
index b38a249b73..3485d17951 100644
--- a/vernac/classes.ml
+++ b/vernac/classes.ml
@@ -58,13 +58,7 @@ let is_local_for_hint i =
 let add_instance_base inst =
   let locality = if is_local_for_hint inst then Goptions.OptLocal else Goptions.OptGlobal in
   add_instance_hint (Hints.IsGlobRef inst.is_impl) [inst.is_impl] ~locality
-    inst.is_info;
-  List.iter (fun (path, pri, c) ->
-    let h = Hints.IsConstr (EConstr.of_constr c, None) [@ocaml.warning "-3"] in
-    add_instance_hint h path
-                ~locality pri)
-    (build_subclasses ~check:(not (isVarRef inst.is_impl))
-       (Global.env ()) (Evd.from_env (Global.env ())) inst.is_impl inst.is_info)
+    inst.is_info
 
 let mk_instance cl info glob impl =
   let global =
diff --git a/vernac/g_vernac.mlg b/vernac/g_vernac.mlg
index e0550fd744..5b039e76f3 100644
--- a/vernac/g_vernac.mlg
+++ b/vernac/g_vernac.mlg
@@ -469,11 +469,8 @@ GRAMMAR EXTEND Gram
     [ [ id = identref; c=constructor_type -> { c id } ] ]
   ;
   of_type_with_opt_coercion:
-    [ [ ":>>" -> { Some false }
-        | ":>"; ">" -> { Some false }
-        | ":>" -> { Some true }
-        | ":"; ">"; ">" -> { Some false }
-        | ":"; ">" -> { Some true }
+    [ [   ":>" -> { Some () }
+        | ":"; ">" -> { Some () }
         | ":" -> { None } ] ]
   ;
 END
diff --git a/vernac/ppvernac.ml b/vernac/ppvernac.ml
index b73e7c7515..8a98a43ba0 100644
--- a/vernac/ppvernac.ml
+++ b/vernac/ppvernac.ml
@@ -504,8 +504,7 @@ let pr_intarg n = spc () ++ int n
 
 let pr_oc = function
   | None -> str" :"
-  | Some true -> str" :>"
-  | Some false -> str" :>>"
+  | Some () -> str" :>"
 
 let pr_record_field (x, { rf_subclass = oc ; rf_priority = pri ; rf_notation = ntn }) =
   let prx = match x with
diff --git a/vernac/record.ml b/vernac/record.ml
index bd5b71cd6b..79abf94726 100644
--- a/vernac/record.ml
+++ b/vernac/record.ml
@@ -557,7 +557,7 @@ let declare_class def cumulative ubinders univs id idbuild paramimpls params uni
       Impargs.declare_manual_implicits false (GlobRef.ConstRef proj_cst) (List.hd fieldimpls);
       Classes.set_typeclass_transparency (EvalConstRef cst) false false;
       let sub = match List.hd coers with
-        | Some b -> Some ((if b then Backward else Forward), List.hd priorities)
+        | Some () -> Some (Backward, List.hd priorities)
         | None -> None
       in
       [cref, [Name proj_name, sub, Some proj_cst]]
@@ -568,8 +568,7 @@ let declare_class def cumulative ubinders univs id idbuild paramimpls params uni
         params template ~kind:Decls.Method ~name:[|binder_name|] record_data
       in
        let coers = List.map2 (fun coe pri ->
-                              Option.map (fun b ->
-                              if b then Backward, pri else Forward, pri) coe)
+                              Option.map (fun () -> Backward, pri) coe)
           coers priorities
        in
       let map ind =
diff --git a/vernac/vernacentries.ml b/vernac/vernacentries.ml
index fba6800729..60c6d2ec0b 100644
--- a/vernac/vernacentries.ml
+++ b/vernac/vernacentries.ml
@@ -776,7 +776,7 @@ let vernac_inductive ~atts kind indl =
       | _ -> CErrors.user_err Pp.(str "Definitional classes do not support the \"|\" syntax.")
     in
     let (coe, (lid, ce)) = l in
-    let coe' = if coe then Some true else None in
+    let coe' = if coe then Some () else None in
     let f = AssumExpr ((make ?loc:lid.loc @@ Name lid.v), ce),
             { rf_subclass = coe' ; rf_priority = None ; rf_notation = [] ; rf_canonical = true } in
     vernac_record ~template udecl ~cumulative (Class true) ~poly finite [id, bl, c, None, [f]]
diff --git a/vernac/vernacexpr.ml b/vernac/vernacexpr.ml
index d8e17d00e3..721e710e1d 100644
--- a/vernac/vernacexpr.ml
+++ b/vernac/vernacexpr.ml
@@ -106,8 +106,8 @@ type search_restriction =
 
 type verbose_flag   = bool (* true = Verbose;       false = Silent         *)
 type coercion_flag  = bool (* true = AddCoercion    false = NoCoercion     *)
-type instance_flag  = bool option
-  (* Some true = Backward instance; Some false = Forward instance, None = NoInstance *)
+type instance_flag  = unit option
+  (* Some () = Backward instance, None = NoInstance *)
 
 type export_flag    = bool (* true = Export;        false = Import         *)