Fix null falling into object branch in untagged variant switch (#8251)

When pattern matching on untagged variants with both an Object case and
a null/wildcard case, null (typeof "object") would incorrectly match
the object branch. Emit a null check before the typeof switch when both
conditions are present.

Fixes #8251

Signed-Off-By: Cristiano Calcagno <cristiano.calcagno@gmail.com>
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: cristianoc

CHANGELOG.md

@@ -21,6 +21,7 @@
 #### :bug: Bug fix
 
 - Fix compiler crash (`Fatal error: Parmatch.all_record_args`) when matching empty dict/record patterns. https://github.com/rescript-lang/rescript/pull/8246
+- Fix `null` falling into the object branch instead of the wildcard when pattern matching on untagged variants with both `Object` and `null` cases. https://github.com/rescript-lang/rescript/pull/8253
 
 #### :memo: Documentation
 

compiler/core/lam_compile.ml

@@ -814,6 +814,10 @@ let compile output_prefix =
       | _ -> false
     in
     let clause_is_not_typeof (tag, _) = tag_is_not_typeof tag in
+    let clause_is_object_typeof = function
+      | Ast_untagged_variants.Untagged ObjectType, _ -> true
+      | _ -> false
+    in
     let switch ?default ?declaration e clauses =
       let not_typeof_clauses, typeof_clauses =
         List.partition clause_is_not_typeof clauses
@@ -827,7 +831,17 @@ let compile output_prefix =
             (E.emit_check (IsInstanceOf (instance_type, Expr e)))
             switch_body
             ~else_:[build_if_chain rest]
-        | _ -> S.string_switch ?default ?declaration (E.typeof e) typeof_clauses
+        | _ ->
+          let typeof_switch () =
+            S.string_switch ?default ?declaration (E.typeof e) typeof_clauses
+          in
+          if has_null_case && List.exists clause_is_object_typeof typeof_clauses
+          then
+            match default with
+            | Some default_body ->
+              S.if_ (E.is_null e) default_body ~else_:[typeof_switch ()]
+            | None -> typeof_switch ()
+          else typeof_switch ()
       in
       build_if_chain not_typeof_clauses
     in

tests/tests/src/js_json_test.mjs

@@ -367,24 +367,44 @@ Mocha.describe("Js_json_test", () => {
     Test_utils.eq("File \"js_json_test.res\", line 314, characters 7-14", Js_json.decodeArray({}), undefined);
     Test_utils.eq("File \"js_json_test.res\", line 315, characters 7-14", Js_json.decodeArray(1.23), undefined);
   });
+  Mocha.test("JSON Array/Object switch falls through to wildcard on null", () => {
+    let classifyArrayOrObject = json => {
+      if (Array.isArray(json)) {
+        return json.length;
+      }
+      if (json === null) {
+        return;
+      }
+      switch (typeof json) {
+        case "object" :
+          Js_dict.get(json, "x");
+          return 0;
+        default:
+          return;
+      }
+    };
+    Test_utils.eq("File \"js_json_test.res\", line 328, characters 7-14", classifyArrayOrObject(null), undefined);
+    Test_utils.eq("File \"js_json_test.res\", line 329, characters 7-14", classifyArrayOrObject([1]), 1);
+    Test_utils.eq("File \"js_json_test.res\", line 330, characters 7-14", classifyArrayOrObject({}), 0);
+  });
   Mocha.test("JSON decodeBoolean", () => {
-    Test_utils.eq("File \"js_json_test.res\", line 319, characters 7-14", Js_json.decodeBoolean("test"), undefined);
-    Test_utils.eq("File \"js_json_test.res\", line 320, characters 7-14", Js_json.decodeBoolean(true), true);
-    Test_utils.eq("File \"js_json_test.res\", line 321, characters 7-14", Js_json.decodeBoolean([]), undefined);
-    Test_utils.eq("File \"js_json_test.res\", line 322, characters 7-14", Js_json.decodeBoolean(null), undefined);
-    Test_utils.eq("File \"js_json_test.res\", line 323, characters 7-14", Js_json.decodeBoolean({}), undefined);
-    Test_utils.eq("File \"js_json_test.res\", line 324, characters 7-14", Js_json.decodeBoolean(1.23), undefined);
+    Test_utils.eq("File \"js_json_test.res\", line 334, characters 7-14", Js_json.decodeBoolean("test"), undefined);
+    Test_utils.eq("File \"js_json_test.res\", line 335, characters 7-14", Js_json.decodeBoolean(true), true);
+    Test_utils.eq("File \"js_json_test.res\", line 336, characters 7-14", Js_json.decodeBoolean([]), undefined);
+    Test_utils.eq("File \"js_json_test.res\", line 337, characters 7-14", Js_json.decodeBoolean(null), undefined);
+    Test_utils.eq("File \"js_json_test.res\", line 338, characters 7-14", Js_json.decodeBoolean({}), undefined);
+    Test_utils.eq("File \"js_json_test.res\", line 339, characters 7-14", Js_json.decodeBoolean(1.23), undefined);
   });
   Mocha.test("JSON decodeNull", () => {
-    Test_utils.eq("File \"js_json_test.res\", line 328, characters 7-14", Js_json.decodeNull("test"), undefined);
-    Test_utils.eq("File \"js_json_test.res\", line 329, characters 7-14", Js_json.decodeNull(true), undefined);
-    Test_utils.eq("File \"js_json_test.res\", line 330, characters 7-14", Js_json.decodeNull([]), undefined);
-    Test_utils.eq("File \"js_json_test.res\", line 331, characters 7-14", Js_json.decodeNull(null), null);
-    Test_utils.eq("File \"js_json_test.res\", line 332, characters 7-14", Js_json.decodeNull({}), undefined);
-    Test_utils.eq("File \"js_json_test.res\", line 333, characters 7-14", Js_json.decodeNull(1.23), undefined);
+    Test_utils.eq("File \"js_json_test.res\", line 343, characters 7-14", Js_json.decodeNull("test"), undefined);
+    Test_utils.eq("File \"js_json_test.res\", line 344, characters 7-14", Js_json.decodeNull(true), undefined);
+    Test_utils.eq("File \"js_json_test.res\", line 345, characters 7-14", Js_json.decodeNull([]), undefined);
+    Test_utils.eq("File \"js_json_test.res\", line 346, characters 7-14", Js_json.decodeNull(null), null);
+    Test_utils.eq("File \"js_json_test.res\", line 347, characters 7-14", Js_json.decodeNull({}), undefined);
+    Test_utils.eq("File \"js_json_test.res\", line 348, characters 7-14", Js_json.decodeNull(1.23), undefined);
   });
   Mocha.test("JSON serialize/deserialize identity", () => {
-    let idtest = obj => Test_utils.eq("File \"js_json_test.res\", line 339, characters 27-34", obj, Js_json.deserializeUnsafe(Js_json.serializeExn(obj)));
+    let idtest = obj => Test_utils.eq("File \"js_json_test.res\", line 354, characters 27-34", obj, Js_json.deserializeUnsafe(Js_json.serializeExn(obj)));
     idtest(undefined);
     idtest({
       hd: [

tests/tests/src/js_json_test.res

@@ -315,6 +315,21 @@ describe(__MODULE__, () => {
     eq(__LOC__, J.decodeArray(J.number(1.23)), None)
   })
 
+  test("JSON Array/Object switch falls through to wildcard on null", () => {
+    let classifyArrayOrObject = (json: J.t) =>
+      switch json {
+      | J.Array(items) => Some(items->Js.Array2.length)
+      | J.Object(dict) =>
+        ignore(Js.Dict.get(dict, "x"))
+        Some(0)
+      | _ => None
+      }
+
+    eq(__LOC__, classifyArrayOrObject(J.null), None)
+    eq(__LOC__, classifyArrayOrObject(J.array([J.number(1.)])), Some(1))
+    eq(__LOC__, classifyArrayOrObject(J.object_(Js.Dict.empty())), Some(0))
+  })
+
   test("JSON decodeBoolean", () => {
     eq(__LOC__, J.decodeBoolean(J.string("test")), None)
     eq(__LOC__, J.decodeBoolean(J.boolean(true)), Some(true))