Proposed Changes

deepEqual returns false positives for plain, non-circular structures that reuse an object reference.

const shared = { value: 1 };
deepEqual({ first: shared, second: shared }, { first: { value: 1 }, second: { value: 2 } });
// => true, expected false (second differs: 1 vs 2)

Root cause: the circular-reference guard introduced in #12914 keeps one shared WeakSet of every object ever visited and short-circuits to true when either side has been seen before. The first comparison of shared vs { value: 1 } adds both to the set, so when the traversal reaches second, shared is already "visited" and { value: 2 } is never actually compared. Equality is decided by visit history rather than by structure.

This is user-visible through the values prop: useForm compares incoming values against the previous ones with deepEqual, so a rerender whose values reuse one object reference is silently ignored and the form keeps stale values (repro included as a useForm test).

This PR tracks visited pairs (WeakMap<object, WeakSet<object>>) instead of individual objects, which is the standard approach for cycle detection in structural equality. Termination for circular structures is still guaranteed: each (object1, object2) pair is recorded before recursing and there are finitely many pairs, so any cycle eventually revisits the same pair and short-circuits.

Fixes # (no open issue; the regression was introduced by #12914)

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing tests pass locally with my changes

Verified locally: new unit tests + the useForm regression test fail on master and pass with the fix; the existing circular-reference tests from #12914 still pass; jest on deepEqual/useForm/formState/setValue suites → 173 passed; pnpm test:type, pnpm lint, pnpm build (incl. ESM/CJS export assertions) and pnpm api-extractor all pass; dist/index.cjs.js stays within the bundlewatch budget (12.25 kB gzip < 12.5 kB).