fix(codegen): #138 — link imported enum constructors into codegen

A module that imports prelude's Option/Result and applies their
constructors type-checked but failed to compile: codegen raised
UnboundVariable "...Some". Removing the b895374 seeded-builtins band-aid
(front-end half of #138) correctly routed Some/None/Ok/Err through the
module path, but the backends learn variant tags only from TopType decls,
and imported types never reached them.

- Codegen.gen_imports (core-Wasm — the path `compile` uses): previously
  wired up only TopFn (-> wasm import) and TopConst (-> global) and dropped
  imported types. Now also registers the constructor tags / struct layouts
  of imported public types, reusing gen_decl's local-type registration so
  imported and local types share one code path.
- Module_loader.flatten_imports (Deno / JS / Julia / C / Rust / ...): now
  inlines imported public TopType decls (separate namespace from fn/const,
  local-wins, deduped, ordered before consumers) so the prog_decls-iterating
  backends register them too.

Scope: directly-imported constructors lower on every backend. Transitive
re-export stays unimplemented. Unrelated pre-existing core-Wasm gaps
(tuple-pattern codegen; mixed-representation match of a zero-arg variant)
reproduce with purely local enums and are out of scope.

Adds a Wasm-path multi-module regression test (test_stdlib_aot.ml), the
counterpart to the #137 Deno-path integration.

Closes #138

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Lz7pRcec2Z3tVtaAhvB3M8

Author: claude

docs/history/MODULE-SYSTEM-PROGRESS.md

@@ -210,7 +210,8 @@ type context = {
 | Visibility checking | ✅ | Public/PubCrate filtering |
 | Symbol registration | ✅ | Symbols added to table |
 | Type information transfer | ✅ | **FIXED** |
-| Re-exports | ❌ | Not implemented |
+| Cross-module constructor codegen | ✅ | Directly-imported enum constructors (`use prelude::{Option, Some, None}`) lower on every backend (#138) |
+| Re-exports (transitive) | ❌ | A module surfacing names it itself imported (`use option` → prelude's `Option`) — not implemented |
 | Nested modules | ❌ | Not implemented |
 
 ## Known Limitations
@@ -400,3 +401,29 @@ actual objective of #128.
 | #138 | Delete the b895374 seeded-builtins band-aid once the prelude re-export module exists. |
 
 No code change in #132 (decision + documentation only).
+
+### #138 codegen follow-up (2026-06-20)
+
+Removing the `b895374` seeded `Some/None/Ok/Err` builtins (front-end half of
+#138) correctly routed those constructors through the module path, so `check`
+passes — but it surfaced a codegen gap: a consumer that imports prelude's
+`Option`/`Result` and applies their constructors type-checked yet failed to
+compile, because the backends learn variant tags only from `TopType` decls and
+imported types never reached them.
+
+- **Core-Wasm backend** (`Codegen.gen_imports`): wired up only `TopFn`
+  (→ wasm import) and `TopConst` (→ global); imported types were dropped. It now
+  also registers the constructor tags / struct layouts of imported public types,
+  reusing the local-type registration in `gen_decl`.
+- **Other backends** (Deno / JS / Julia / C / Rust / …): `Module_loader.flatten_imports`
+  now inlines imported public `TopType` decls (a separate namespace from
+  fn/const, local-wins, deduped) so the `prog_decls`-iterating codegens see them.
+
+Scope: **directly-imported** constructors lower on every backend. **Transitive
+re-export** (a module re-exposing constructors it itself imported) remains
+unimplemented — see the status table above. Unrelated and still open: the
+core-Wasm pattern-codegen gap for tuple patterns (`UnsupportedFeature "Only
+variable and wildcard patterns supported in tuple patterns"`, which
+`stdlib/option.affine` / `result.affine` hit) and the mixed-representation match
+of a zero-arg variant against a constructor-with-args arm — both reproduce with
+purely local enums and are independent of cross-module linking.

lib/codegen.ml

@@ -3339,7 +3339,65 @@ let gen_imports (loader : Module_loader.t) (imports : import_decl list) (ctx : c
            | _ -> None
          ) lm.mod_program.prog_decls)
   in
+  (* #138: register the constructor tags (and struct field layouts) of
+     imported PUBLIC types. [gen_imports] is the WASM backend's native
+     cross-module import path; it historically wired up only [TopFn]
+     (-> wasm import) and [TopConst] (-> global), silently dropping imported
+     TYPES. Codegen learns variant tags ONLY from [TopType] decls (see
+     [gen_decl]/[variant_tags]), and the WASM compile path feeds the original
+     (un-flattened) [prog] to [generate_module], so applying an imported
+     constructor such as [Some]/[None] from `use prelude::{Option, Some, None}`
+     raised [UnboundVariable] at codegen even though resolve + typecheck
+     passed. We reuse the local-type registration in [gen_decl] so imported
+     and local types share exactly one code path; the non-WASM backends get
+     the same decls inlined via [Module_loader.flatten_imports]. *)
+  let register_imported_types ctx =
+    (* Dedup imported types by name across all imports (a type may be reachable
+       through more than one path). Local [TopType]s — registered afterwards by
+       the [prog_decls] fold in [generate_module] — are prepended after these,
+       so they win on the first-match [List.assoc] lookup. *)
+    let seen = Hashtbl.create 8 in
+    let path_strs path = List.map (fun (id : ident) -> id.name) path in
+    List.fold_left (fun acc imp ->
+      let* ctx = acc in
+      let p = match imp with
+        | ImportSimple (path, _) | ImportList (path, _) | ImportGlob path ->
+          path_strs path
+      in
+      match Module_loader.load_module loader p with
+      | Error _ -> Ok ctx
+      | Ok lm ->
+        let public_types = List.filter_map (function
+          | TopType td when td.td_vis = Public || td.td_vis = PubCrate -> Some td
+          | _ -> None
+        ) lm.mod_program.prog_decls in
+        (* `use M::{..}` selects a type when the list names the type itself or
+           any of its constructors (so `use prelude::{Some}` works without also
+           naming `Option`); `use M` / `use M::*` bring all public types. *)
+        let selected = match imp with
+          | ImportGlob _ | ImportSimple _ -> public_types
+          | ImportList (_, items) ->
+            let wanted = List.map (fun (it : import_item) -> it.ii_name.name) items in
+            List.filter (fun td ->
+              List.mem td.td_name.name wanted ||
+              (match td.td_body with
+               | TyEnum variants ->
+                 List.exists (fun vd -> List.mem vd.vd_name.name wanted) variants
+               | _ -> false)
+            ) public_types
+        in
+        List.fold_left (fun acc td ->
+          let* ctx = acc in
+          if Hashtbl.mem seen td.td_name.name then Ok ctx
+          else begin
+            Hashtbl.add seen td.td_name.name ();
+            gen_decl ctx (TopType td)
+          end
+        ) (Ok ctx) selected
+    ) (Ok ctx) imports
+  in
   let entries = List.concat_map expand_import imports in
+  let* ctx = register_imported_types ctx in
   List.fold_left (fun acc e ->
     let* ctx = acc in
     process_one ctx e

lib/module_loader.ml

@@ -345,4 +345,61 @@ let flatten_imports (loader : t) (prog : program) : program =
         ) select
     ) prog.prog_imports
   in
-  { prog with prog_decls = imported_decls @ prog.prog_decls }
+  (* #138: type declarations are a SEPARATE namespace from fn/const bindings,
+     so they get their own dedup table — a local `fn Foo` must not suppress an
+     imported `type Foo`, and vice versa. Imported public types are inlined too
+     so the prog_decls-iterating backends (Deno / JS / Julia / C / Rust / ...)
+     register their constructors exactly as they would for a local type;
+     without this, applying an imported constructor (`Some`/`None` from
+     `use prelude::{Option, Some, None}`) can reach codegen with no type in
+     scope. Local types win over imported ones, and a type reachable through
+     more than one path is carried only once. The Wasm backend gets the same
+     registration natively in [Codegen.gen_imports]. *)
+  let local_type_names =
+    List.filter_map (function
+      | TopType td -> Some td.td_name.name
+      | _ -> None
+    ) prog.prog_decls
+  in
+  let type_already_in = Hashtbl.create 16 in
+  List.iter (fun n -> Hashtbl.add type_already_in n ()) local_type_names;
+  let imported_types =
+    List.concat_map (fun imp ->
+      let path_strs path = List.map (fun (id : ident) -> id.name) path in
+      let mod_path = match imp with
+        | ImportSimple (p, _) | ImportList (p, _) | ImportGlob p -> path_strs p
+      in
+      match Hashtbl.find_opt loader.loaded mod_path with
+      | None -> []
+      | Some lm ->
+        let public_types = List.filter_map (function
+          | TopType td when td.td_vis = Public || td.td_vis = PubCrate -> Some td
+          | _ -> None
+        ) lm.mod_program.prog_decls in
+        (* `use M::{..}` selects a type when the list names the type itself or
+           any of its constructors; `use M` / `use M::*` bring all public
+           types (the resolver still gates what is referenceable). *)
+        let selected = match imp with
+          | ImportGlob _ | ImportSimple _ -> public_types
+          | ImportList (_, items) ->
+            let wanted = List.map (fun (it : import_item) -> it.ii_name.name) items in
+            List.filter (fun td ->
+              List.mem td.td_name.name wanted ||
+              (match td.td_body with
+               | TyEnum variants ->
+                 List.exists (fun vd -> List.mem vd.vd_name.name wanted) variants
+               | _ -> false)
+            ) public_types
+        in
+        List.filter_map (fun td ->
+          if Hashtbl.mem type_already_in td.td_name.name then None
+          else begin
+            Hashtbl.add type_already_in td.td_name.name ();
+            Some (TopType td)
+          end
+        ) selected
+    ) prog.prog_imports
+  in
+  (* Types precede imported fns/consts and all local decls so the single-pass
+     codegen registers an imported type before any function that uses it. *)
+  { prog with prog_decls = imported_types @ imported_decls @ prog.prog_decls }

test/test_stdlib_aot.ml

@@ -61,6 +61,37 @@ let pipeline_to_deno (prog : Ast.program) : (string, string) result =
            | Error e -> Error (Printf.sprintf "deno-codegen: %s" e)
            | Ok js -> Ok js)))
 
+(** Full AOT pipeline to the core-Wasm backend: resolve -> typecheck ->
+    borrow -> [Codegen.generate_module] (loader-aware). Mirrors
+    [pipeline_to_deno] but targets the backend whose cross-module constructor
+    linking (#138) the test below regression-locks. The Wasm path feeds the
+    original (un-flattened) [prog] to codegen and resolves imported decls
+    natively via [Codegen.gen_imports]. *)
+let pipeline_to_wasm (prog : Ast.program) : (Wasm.wasm_module, string) result =
+  let ld = loader () in
+  match Resolve.resolve_program_with_loader prog ld with
+  | Error (e, sp) ->
+    Error (Printf.sprintf "resolve: %s @ %s"
+             (Resolve.show_resolve_error e) (Span.show sp))
+  | Ok (rctx, itc) ->
+    (match
+       Typecheck.check_program
+         ~import_types:itc.Typecheck.name_types rctx.symbols prog
+     with
+     | Error e ->
+       Error (Printf.sprintf "typecheck: %s" (Typecheck.format_type_error e))
+     | Ok _ ->
+       (match Borrow.check_program rctx.symbols prog with
+        | Error e ->
+          Error (Printf.sprintf "borrow: %s" (Borrow.format_borrow_error e))
+        | Ok () ->
+          let optimized = Opt.fold_constants_program prog in
+          (match Codegen.generate_module ~loader:ld optimized with
+           | Error e ->
+             Error (Printf.sprintf "wasm-codegen: %s"
+                      (Codegen.show_codegen_error e))
+           | Ok m -> Ok m)))
+
 let parse_file_safe path =
   try Ok (Parse_driver.parse_file path)
   with
@@ -140,6 +171,61 @@ let integration_tests =
   [ Alcotest.test_case "string+option+collections together" `Quick
       test_multi_module_integration ]
 
+(* ---- #138: cross-module constructor linking on the core-Wasm backend ----
+
+   A consumer that imports prelude's Option/Result and APPLIES their
+   constructors must reach a runnable artifact. The front-end resolves
+   `Some`/`None`/`Ok`/`Err` through the module path, so `check` passes; before
+   the #138 codegen fix [Codegen.gen_imports] dropped the imported TYPE decls,
+   so the Wasm backend had no variant tag for `Some` and raised
+   [UnboundVariable "...Some"] at codegen. This feeds the imported-constructor
+   decl shape to all three [variant_tags] consumers in [Codegen]: construction
+   with an argument (`Some(x)`, `Ok`/`Err`), the zero-arg form (`None`), and
+   constructor patterns (`match`). It is the Wasm counterpart to the #137
+   Deno-path integration above. *)
+let imported_ctors_src = {|
+module xmod_ctors;
+use prelude::{ Option, Some, None, Result, Ok, Err };
+
+pub fn wrap(x: Int) -> Option<Int> { Some(x) }
+pub fn empty() -> Option<Int> { None }
+
+pub fn unwrap_or(o: Option<Int>, d: Int) -> Int {
+  match o {
+    Some(v) => v,
+    None => d,
+  }
+}
+
+pub fn divide(a: Int, b: Int) -> Result<Int, Int> {
+  if b == 0 { Err(0) } else { Ok(a / b) }
+}
+|}
+
+let test_imported_constructors_wasm () =
+  match Parse_driver.parse_string ~file:"<xmod_ctors>" imported_ctors_src with
+  | exception e ->
+    Alcotest.failf "imported-ctors parse raised: %s" (Printexc.to_string e)
+  | prog ->
+    (match pipeline_to_wasm prog with
+     | Ok m ->
+       let names =
+         List.map (fun (e : Wasm.export) -> e.Wasm.e_name) m.Wasm.exports in
+       List.iter
+         (fun fn ->
+            Alcotest.(check bool)
+              (Printf.sprintf "Wasm module exports %s" fn)
+              true (List.mem fn names))
+         [ "wrap"; "empty"; "unwrap_or"; "divide" ]
+     | Error m ->
+       Alcotest.failf
+         "imported prelude constructors must codegen to Wasm (#138): %s" m)
+
+let xmod_constructor_tests =
+  [ Alcotest.test_case "imported Option/Result constructors -> Wasm" `Quick
+      test_imported_constructors_wasm ]
+
 let tests =
   [ ("STAGE-A AOT smoke (#136)", aot_smoke_tests);
-    ("STAGE-A multi-module integration (#137)", integration_tests) ]
+    ("STAGE-A multi-module integration (#137)", integration_tests);
+    ("cross-module constructor linking, Wasm (#138)", xmod_constructor_tests) ]