fix(typecheck): generalize parametric enum ctor schemes via shared param tyvars (#128)

register_type_decl's TyEnum branch built result_ty tyvars with an
anonymous fresh_tyvar 0 and never bound the declared param names, so
(a) field types and the result type used disconnected vars and
(b) generalize was a no-op at level 0 -> imported parametric ctor
schemes (prelude Ok/Err/Some) were monomorphic with shared vars,
collapsing payload types across use-sites on the import path.

Bind param names to fresh tyvars one level deeper so result and field
types share them, generalize at the outer level, and scope the param
names so they don't leak into sibling type decls.

Fixes stdlib/result.affine; stdlib 14->15/19; 233/233 dune test, zero regression.

Refs #128

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

lib/typecheck.ml

@@ -1527,35 +1527,49 @@ let register_type_decl (ctx : context) (td : type_decl) : unit result =
       let* () = check_kind ctx ty KType in
       Ok ty
     | TyEnum variants ->
-      (* Register each variant as a constructor *)
-      let result_ty = match td.td_type_params with
+      (* Register each variant as a constructor.
+
+         Bind the declared type-param names (e.g. T, E) to fresh tyvars in
+         the type env so that BOTH the result type and the field types
+         resolve to the *same* variables (previously the result tyvars were
+         anonymous `fresh_tyvar 0` and each field `T` became its own
+         unrelated var via lower_type_expr's TyVar fallback).
+
+         The param tyvars are created one level deeper than the current
+         context so that `generalize` (which quantifies vars with
+         level > ctx.level) actually quantifies them — at level 0 it was a
+         no-op, leaving imported ctor schemes monomorphic with shared vars
+         (the import-path bug behind result.affine's Unify ((_->_), T)). *)
+      let param_names =
+        List.map (fun (tp : type_param) -> tp.tp_name.name) td.td_type_params
+      in
+      enter_level ctx;
+      let param_tvs = List.map (fun n ->
+        let tv = fresh_tyvar ctx.level in
+        Hashtbl.replace ctx.type_env n tv;
+        tv
+      ) param_names in
+      let result_ty = match param_tvs with
         | [] -> TCon td.td_name.name
-        | params ->
-          let tparams = List.map (fun tp ->
-            match tp.tp_name.name with
-            | _ ->
-              let tv = fresh_tyvar 0 in
-              (* Note: In a full impl we'd map param name to tv in a local env *)
-              tv
-          ) params in
-          TApp (TCon td.td_name.name, tparams)
+        | tvs -> TApp (TCon td.td_name.name, tvs)
       in
-      List.iter (fun (vd : variant_decl) ->
+      let ctors = List.map (fun (vd : variant_decl) ->
         let ctor_ty = List.fold_right (fun field_te acc ->
           TArrow (lower_type_expr ctx field_te, QOmega, acc, EPure)
         ) vd.vd_fields result_ty in
-        (* If it has type params, we should really bind a TForall scheme *)
-        let sc = match td.td_type_params with
+        (vd.vd_name.name, ctor_ty)
+      ) variants in
+      exit_level ctx;
+      (* Don't let this decl's param names leak into sibling type decls. *)
+      List.iter (fun n -> Hashtbl.remove ctx.type_env n) param_names;
+      List.iter (fun (name, ctor_ty) ->
+        let sc = match param_names with
           | [] -> { sc_tyvars = []; sc_effvars = []; sc_rowvars = []; sc_body = ctor_ty }
           | _ -> generalize ctx ctor_ty
         in
-        Hashtbl.replace ctx.constructor_env vd.vd_name.name ctor_ty;
-        (* Also bind as a variable for ExprVar references *)
-        if vd.vd_fields = [] then
-          bind_scheme ctx vd.vd_name.name sc
-        else
-          bind_scheme ctx vd.vd_name.name sc
-      ) variants;
+        Hashtbl.replace ctx.constructor_env name ctor_ty;
+        bind_scheme ctx name sc
+      ) ctors;
       Ok (TCon td.td_name.name)
     | TyExtern ->
       (* Opaque host-supplied type. Register a TCon so user code can name it