fix(codegen): support nested patterns inside tuple patterns (WASM)

The core-Wasm backend rejected any tuple sub-pattern that wasn't a plain
variable or wildcard (UnsupportedFeature "Only variable and wildcard patterns
supported in tuple patterns") — which stdlib/option.affine and result.affine
hit. gen_pattern now recurses per tuple element: each element is loaded into a
temp local and matched against its sub-pattern, with the per-element test bools
ANDed together. Every gen_pattern result is one-bool-net with net-zero binds,
so the combination is stack-safe; binds register via the threaded ctx (the same
mechanism constructor-argument patterns use).

Verified under node: `match (a,b) { (0,y)=>y, (x,0)=>x+100, (x,y)=>x+y }`
selects the right arm and binds correctly — (0,5)->5, (7,0)->107, (3,4)->7.
option.affine / result.affine now get PAST this gap (they next hit a separate
`panic`-builtin gap in the Wasm backend, tracked in #607). dune test 461 green;
adds a Wasm nested-tuple regression test.

Part of #607.

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

Author: claude

lib/codegen.ml

@@ -2281,37 +2281,41 @@ and gen_pattern (ctx : context) (scrutinee_local : int) (pat : pattern)
       Ok (ctx_final, full_code, [])
 
   | PatTuple sub_patterns ->
-    (* Tuple pattern: (a, b, c) *)
-    (* scrutinee is a pointer to [elem0: i32][elem1: i32][elem2: i32]... *)
-
-    (* Bind each element to sub-pattern *)
-    let rec bind_elements ctx_acc offset patterns =
+    (* Tuple pattern: (p0, p1, ...). scrutinee is a pointer to
+       [elem0: i32][elem1: i32]...  Each element is loaded into a temp local
+       and matched RECURSIVELY against its sub-pattern, so nested patterns
+       (literals, constructors, tuples) work — not just var/wildcard.
+
+       Stack discipline: every [gen_pattern] result leaves exactly one bool
+       (net) and its binds are net-zero (see the PatVar/PatLit/PatCon cases),
+       so each element's test bool ANDs cleanly with the accumulator. The
+       per-element load is itself net-zero (LocalGet +1, I32Load 0, LocalSet
+       -1), so it never disturbs the accumulated bool. Binds register in the
+       threaded ctx via [alloc_local] (same mechanism PatCon args use), so the
+       returned binding list stays [] and the arm body resolves names via ctx. *)
+    let rec match_elements ctx_acc idx patterns ~first =
       match patterns with
-      | [] -> Ok (ctx_acc, [])
+      | [] ->
+        (* Empty tuple, or no elements left: an empty match is vacuously true
+           only when it is the whole pattern (first); otherwise the caller has
+           already left the accumulated bool on the stack. *)
+        if first then Ok (ctx_acc, [I32Const 1l]) else Ok (ctx_acc, [])
       | pat :: rest ->
-        begin match pat with
-          | PatVar id ->
-            (* Allocate local for this element *)
-            let (ctx', elem_idx) = alloc_local ctx_acc id.name in
-            (* Load element from tuple *)
-            let load_code = [
-              LocalGet scrutinee_local;
-              I32Load (2, offset);
-              LocalSet elem_idx;
-            ] in
-            let* (ctx_final, rest_code) = bind_elements ctx' (offset + 4) rest in
-            Ok (ctx_final, load_code @ rest_code)
-          | PatWildcard _ ->
-            (* Skip this element *)
-            bind_elements ctx_acc (offset + 4) rest
-          | _ ->
-            Error (UnsupportedFeature "Only variable and wildcard patterns supported in tuple patterns")
-        end
+        let (ctx1, elem_tmp) =
+          alloc_local ctx_acc (Printf.sprintf "__tuple_elem_%d" idx) in
+        let load_elem = [
+          LocalGet scrutinee_local;
+          I32Load (2, idx * 4);
+          LocalSet elem_tmp;
+        ] in
+        let* (ctx2, sub_test, _sub_binds) = gen_pattern ctx1 elem_tmp pat in
+        (* For every element after the first, AND its bool with the running
+           accumulator already on the stack. *)
+        let combine = if first then [] else [I32And] in
+        let* (ctx3, rest_code) = match_elements ctx2 (idx + 1) rest ~first:false in
+        Ok (ctx3, load_elem @ sub_test @ combine @ rest_code)
     in
-
-    let* (ctx_final, binding_code) = bind_elements ctx 0 sub_patterns in
-    (* Tuple patterns always match (no tag to check) *)
-    let match_code = binding_code @ [I32Const 1l] in
+    let* (ctx_final, match_code) = match_elements ctx 0 sub_patterns ~first:true in
     Ok (ctx_final, match_code, [])
 
   | PatRecord (field_pats, _has_wildcard) ->

test/test_stdlib_aot.ml

@@ -305,8 +305,44 @@ let dup_ctor_tests =
     Alcotest.test_case "declared Option does not duplicate preamble ctor (JS)"
       `Quick test_js_no_duplicate_option_ctor ]
 
+(* ---- WASM: nested patterns inside a tuple pattern --------------------------
+
+   The core-Wasm backend previously rejected any tuple sub-pattern that wasn't
+   a plain variable or wildcard (`UnsupportedFeature "Only variable and
+   wildcard patterns supported in tuple patterns"` — what stdlib/option.affine
+   hit). gen_pattern now recurses per element, so literals/constructors/nested
+   tuples work. This asserts such a program reaches a Wasm module; runtime
+   correctness (correct arm selection + binding) is verified under node in the
+   PR's manual check. *)
+let nested_tuple_src = {|
+module nested_tuple;
+pub fn classify(a: Int, b: Int) -> Int {
+  let t = (a, b);
+  match t {
+    (0, y) => y,
+    (x, 0) => x + 100,
+    (x, y) => x + y,
+  }
+}
+|}
+
+let test_nested_tuple_patterns_wasm () =
+  match Parse_driver.parse_string ~file:"<nested_tuple>" nested_tuple_src with
+  | exception e ->
+    Alcotest.failf "nested-tuple parse raised: %s" (Printexc.to_string e)
+  | prog ->
+    (match pipeline_to_wasm prog with
+     | Ok _ -> ()
+     | Error m ->
+       Alcotest.failf "nested tuple patterns must codegen to Wasm: %s" m)
+
+let tuple_pattern_tests =
+  [ Alcotest.test_case "nested (literal/var) tuple patterns -> Wasm" `Quick
+      test_nested_tuple_patterns_wasm ]
+
 let tests =
   [ ("STAGE-A AOT smoke (#136)", aot_smoke_tests);
     ("STAGE-A multi-module integration (#137)", integration_tests);
     ("cross-module constructor linking, Wasm (#138)", xmod_constructor_tests);
-    ("Deno-ESM / JS no duplicate Option/Result constructor", dup_ctor_tests) ]
+    ("Deno-ESM / JS no duplicate Option/Result constructor", dup_ctor_tests);
+    ("Wasm nested tuple patterns", tuple_pattern_tests) ]