fix(checkout): use original per-day rates for period-switch detection

Preserve $orig_old_price_per_day / $orig_new_price_per_day before
search_for_same_period_plans() normalises the rates, then compare those
originals in the $is_period_switch_to_shorter check. Previously the
normalised (plan-variation) rates were used, which could mis-classify
a period switch when the normalised rates happened to diverge from the
true membership vs. cart rates.

Also update test_get_billing_next_charge_date_null_membership_guard to
actually exercise the null-membership guard: force cart_type to
'downgrade' via ReflectionProperty so the downgrade branch in
get_billing_next_charge_date() is reached with a null membership, proving
the guard prevents a fatal call to is_active() on null.

Resolves #892

Author: superdav42

inc/checkout/class-cart.php

@@ -1319,6 +1319,14 @@ protected function build_from_membership($membership_id): bool {
 		$old_price_per_day = $days_in_old_cycle > 0 ? $membership->get_amount() / $days_in_old_cycle : $membership->get_amount();
 		$new_price_per_day = $days_in_new_cycle > 0 ? $this->get_recurring_total() / $days_in_new_cycle : $this->get_recurring_total();
 
+		/*
+		 * Preserve the true per-day rates before search_for_same_period_plans()
+		 * may normalise them. The shorter-period override must compare the raw
+		 * membership vs. cart rates, not the normalised plan-variation rates.
+		 */
+		$orig_old_price_per_day = $old_price_per_day;
+		$orig_new_price_per_day = $new_price_per_day;
+
 		$is_same_product = $membership->get_plan_id() === $this->plan_id;
 
 		/**
@@ -1357,11 +1365,14 @@ protected function build_from_membership($membership_id): bool {
 		 * fits this profile is treated as a scheduled downgrade regardless of
 		 * whether the nominal per-period amounts suggest an "upgrade".
 		 *
+		 * We use the original rates ($orig_*) captured before search_for_same_period_plans()
+		 * may have normalised them, so we compare the actual membership vs. cart rates.
+		 *
 		 * @since 2.6.2
 		 */
 		$is_period_switch_to_shorter = ! $membership->is_free()
 			&& $days_in_old_cycle > $days_in_new_cycle
-			&& $old_price_per_day < $new_price_per_day;
+			&& $orig_old_price_per_day < $orig_new_price_per_day;
 
 		/*
 		 * If is the same product and the customer will start to pay less,

tests/WP_Ultimo/Checkout/Cart_Test.php

@@ -3034,11 +3034,11 @@ public function test_yearly_plan_to_monthly_plan_different_products_is_downgrade
 	 * null and cart_type happens to be 'downgrade' (defensive null guard).
 	 */
 	public function test_get_billing_next_charge_date_null_membership_guard() {
-		// Build a cart with cart_type=downgrade but no membership_id.
-		// The cart will default to type 'new' internally (no membership), but
-		// we can force the scenario by temporarily mocking; instead just verify
-		// that the guard at get_billing_next_charge_date does not throw on a
-		// null membership (regression test for the explicit null check added).
+		// Build a cart with no membership_id, then force cart_type to 'downgrade'
+		// via Reflection so the null-membership guard in get_billing_next_charge_date()
+		// is actually exercised. Before the guard was added this would fatal with
+		// "Call to member function is_active() on null" when cart_type was 'downgrade'
+		// and $this->membership was null.
 		$plan = $this->create_plan(['amount' => 30.00]);
 
 		$cart = new Cart([
@@ -3047,9 +3047,12 @@ public function test_get_billing_next_charge_date_null_membership_guard() {
 			'duration_unit' => 'month',
 		]);
 
-		// Call get_billing_next_charge_date() on a cart that has no membership.
-		// Before the fix this would fatal with "Call to member function is_active() on null"
-		// if cart_type was forced to 'downgrade'. With the null guard it must not throw.
+		// Force the cart into the downgrade branch while membership stays null.
+		$ref = new \ReflectionProperty(Cart::class, 'cart_type');
+		$ref->setAccessible(true);
+		$ref->setValue($cart, 'downgrade');
+
+		// Must return an int without throwing despite membership being null.
 		$this->assertIsInt($cart->get_billing_next_charge_date());
 
 		$plan->delete();