fix(form-core): support value types with a custom equals() method in deep equality

.changeset/evaluate-custom-equals.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/form-core': patch
+---
+
+Support value types that expose a custom `equals()` method (Temporal types, Luxon `DateTime`, etc.) in the deep-equality check. Previously two equal instances were reported as unequal because they have no own enumerable keys, so a field backed by such a value stayed dirty even after being reset to its original value (#2195).

packages/form-core/src/utils.ts

@@ -514,6 +514,20 @@ export function evaluate<T>(objA: T, objB: T) {
     return true
   }
 
+  // Delegate to a custom value-equality method (Temporal types, Luxon DateTime,
+  // Immutable collections, etc.) when both operands share a constructor that
+  // exposes one. This must run before the no-enumerable-keys guard below, which
+  // would otherwise treat every such value as unequal since they expose their
+  // state through getters/private fields rather than own enumerable keys (#2195).
+  if (
+    objA.constructor === objB.constructor &&
+    typeof (objA as { equals?: unknown }).equals === 'function'
+  ) {
+    return (objA as unknown as { equals: (other: unknown) => boolean }).equals(
+      objB,
+    )
+  }
+
   const keysA = Object.keys(objA)
   const keysB = Object.keys(objB)
 

packages/form-core/tests/utils.spec.ts

@@ -677,6 +677,30 @@ describe('evaluate', () => {
     expect(undefinedFalse).toEqual(false)
   })
 
+  it('uses a custom equals() method for value types like Temporal (#2195)', () => {
+    // Value types (Temporal.PlainDate, Luxon DateTime, ...) expose their state
+    // through private fields/getters, so they have no own enumerable keys and a
+    // non-plain prototype. Without delegating to `equals()`, two equal instances
+    // are reported as unequal, marking a reset field as permanently dirty.
+    class PlainDate {
+      #iso: string
+      constructor(iso: string) {
+        this.#iso = iso
+      }
+      equals(other: PlainDate) {
+        return this.#iso === other.#iso
+      }
+    }
+
+    expect(Object.keys(new PlainDate('2026-07-31'))).toEqual([])
+    expect(
+      evaluate(new PlainDate('2026-07-31'), new PlainDate('2026-07-31')),
+    ).toBe(true)
+    expect(
+      evaluate(new PlainDate('2026-07-31'), new PlainDate('2020-01-01')),
+    ).toBe(false)
+  })
+
   it('should test equality between arrays', () => {
     const arrayTrue = evaluate([], [])
     expect(arrayTrue).toEqual(true)