fix(v4): report a union option that only checks rejected (#6662)

Since #6652 an oversized array, set or map fails its size bound before
its elements are parsed, and the issue aborts so nothing reads the
unparsed value. A union treated that abort like a type mismatch and fell
back to invalid_union, where 4.6.5 reported the option's too_big.

When no option can continue, the union now falls back to the options
whose issues all came from checks, and reports that option if it is the
only one.

Fixes #6659

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: colinhacks

packages/zod/src/v4/classic/tests/union.test.ts

@@ -180,6 +180,17 @@ test("surface continuable errors only if they exist", () => {
   `);
 });
 
+test("surface an option a check stopped when none can continue", () => {
+  // the size cap aborts before parsing elements
+  const schema = z.union([z.array(z.string()).max(2), z.string()]);
+  expect(schema.safeParse(["a", "b", "c"]).error!.issues).toMatchObject([
+    { code: "too_big", origin: "array", path: [] },
+  ]);
+  expect(z.union([z.int(), z.number().max(5)]).safeParse(7.5).error!.issues).toMatchObject([{ code: "too_big" }]);
+  const both = z.union([z.array(z.string()).max(2), z.array(z.number()).max(1)]);
+  expect(both.safeParse(["a", "b", "c"]).error!.issues).toMatchObject([{ code: "invalid_union" }]);
+});
+
 // z.xor() tests
 test("z.xor() - exactly one match succeeds", () => {
   const schema = z.xor([z.string(), z.number()]);

packages/zod/src/v4/core/schemas.ts

@@ -2466,10 +2466,14 @@ function handleUnionResults(results: ParsePayload[], final: ParsePayload, inst:
     }
   }
 
-  const nonaborted = results.filter((r) => !util.aborted(r));
-  if (nonaborted.length === 1) {
-    final.value = nonaborted[0].value;
-    return nonaborted[0];
+  let matched = results.filter((r) => !util.aborted(r));
+  // failing that, an option only checks rejected still matched the input's type
+  if (!matched.length) {
+    matched = results.filter((r) => !r.aborted && r.issues.every((iss) => iss.continue !== undefined));
+  }
+  if (matched.length === 1) {
+    final.value = matched[0].value;
+    return matched[0];
   }
 
   final.issues.push({