Add #[NoDiscard] to CarbonImmutable modify(), next(), previous() and __call()

On PHP 8.5, DateTimeImmutable::modify() warns when its result is
discarded, but CarbonImmutable overrides it and lost the attribute.
Override modify(), next(), previous() and __call() in CarbonImmutable
with #[NoDiscard] so a discarded result warns again. The attribute is
ignored on older PHP versions, and mutable Carbon is unchanged.

Existing immutable tests that discarded results now keep them; in
IsTest::testIsSameMonth this was a real slip, the loop never moved $dt.

Fixes #3377

tests/CarbonImmutable/DiffTest.php

@@ -1686,7 +1686,7 @@ class DiffTest extends AbstractTestCase
     {
         $from = Carbon::parse('2021-03-27 20:00 Europe/Warsaw');
         $to = Carbon::parse('2021-03-27 20:00 Europe/London');
-        $from->floatDiffInRealDays($to);
+        $diff = $from->floatDiffInRealDays($to);
 
         $this->assertSame('2021-03-27 20:00:00 Europe/Warsaw', $from->format('Y-m-d H:i:s e'));
         $this->assertSame('2021-03-27 20:00:00 Europe/London', $to->format('Y-m-d H:i:s e'));
@@ -1751,7 +1751,7 @@ class DiffTest extends AbstractTestCase
     {
         $from = Carbon::parse('2021-03-27 20:00 Europe/Warsaw');
         $to = Carbon::parse('2021-03-27 20:00 Europe/London');
-        $from->floatDiffInUtcDays($to);
+        $diff = $from->floatDiffInUtcDays($to);
 
         $this->assertSame('2021-03-27 20:00:00 Europe/Warsaw', $from->format('Y-m-d H:i:s e'));
         $this->assertSame('2021-03-27 20:00:00 Europe/London', $to->format('Y-m-d H:i:s e'));

tests/CarbonImmutable/GenericMacroTest.php

@@ -55,7 +55,7 @@ class GenericMacroTest extends AbstractTestCaseWithOldNow
         $message = null;
 
         try {
-            $now->barBiz();
+            $result = $now->barBiz();
         } catch (BadMethodCallException $exception) {
             $message = $exception->getMessage();
         }

tests/CarbonImmutable/IsTest.php

@@ -413,9 +413,9 @@ class IsTest extends AbstractTestCase
         $this->assertTrue(Carbon::now()->isSameMonth(Carbon::now()));
         $dt = Carbon::now();
         for ($year = 1990; $year < Carbon::now()->year; $year++) {
-            $dt->modify($year.$dt->format('-m-').'01');
+            $dt = $dt->modify($year.$dt->format('-m-').'01');
             $this->assertTrue(Carbon::now()->isSameMonth($dt, false));
-            $dt->modify($year.$dt->format('-m-').'28');
+            $dt = $dt->modify($year.$dt->format('-m-').'28');
             $this->assertTrue(Carbon::now()->isSameMonth($dt, false));
         }
     }

tests/CarbonImmutable/MacroTest.php

@@ -117,7 +117,7 @@ class MacroTest extends AbstractTestCaseWithOldNow
 
         /** @var mixed $date */
         $date = Carbon::now();
-        $date->nonExistingMacro();
+        $result = $date->nonExistingMacro();
     }
 
     public function testTraitMixin()

tests/CarbonImmutable/ModifyTest.php

@@ -249,6 +249,6 @@ class ModifyTest extends AbstractTestCase
             '$anchorDay is not compatible with overflow',
         ));
 
-        Carbon::parse('2023-02-28')->addYearsWithOverflow(2, anchorDay: 29);
+        $date = Carbon::parse('2023-02-28')->addYearsWithOverflow(2, anchorDay: 29);
     }
 }

tests/CarbonImmutable/NoDiscardTest.php

@@ -0,0 +1,62 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * This file is part of the Carbon package.
+ *
+ * (c) Brian Nesbitt <brian@nesbot.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Tests\CarbonImmutable;
+
+use Carbon\Carbon as MutableCarbon;
+use Carbon\CarbonImmutable as Carbon;
+use ErrorException;
+use Generator;
+use PHPUnit\Framework\Attributes\DataProvider;
+use PHPUnit\Framework\Attributes\RequiresPhp;
+use Tests\AbstractTestCase;
+
+#[RequiresPhp('>=8.5.0')]
+class NoDiscardTest extends AbstractTestCase
+{
+    public static function dataForDiscardedCalls(): Generator
+    {
+        yield ['modify', ['+1 day']];
+        yield ['next', []];
+        yield ['previous', []];
+        yield ['addDays', [2]];
+    }
+
+    #[DataProvider('dataForDiscardedCalls')]
+    public function testDiscardedResultTriggersWarning(string $method, array $arguments): void
+    {
+        $this->expectException(ErrorException::class);
+        $this->expectExceptionMessage(
+            "The return value of method Carbon\\CarbonImmutable::$method() should either be used or intentionally ignored by casting it as (void)",
+        );
+
+        $date = Carbon::parse('2026-10-02 12:00');
+
+        $this->withErrorAsException(static function () use ($date, $method, $arguments) {
+            $date->$method(...$arguments);
+        });
+    }
+
+    public function testMutableResultCanBeDiscarded(): void
+    {
+        $date = MutableCarbon::parse('2026-10-02 12:00');
+
+        $this->withErrorAsException(static function () use ($date) {
+            $date->modify('+1 day');
+            $date->next();
+            $date->addDays(2);
+        });
+
+        $this->assertSame('2026-10-12 00:00:00', $date->format('Y-m-d H:i:s'));
+    }
+}

tests/CarbonImmutable/SettersTest.php

@@ -52,7 +52,7 @@ class SettersTest extends AbstractTestCase
         ));
 
         $d = Carbon::now();
-        $d->setHours(Month::February);
+        $d = $d->setHours(Month::February);
     }
 
     public function testMonthEnum()
@@ -74,7 +74,7 @@ class SettersTest extends AbstractTestCase
         ));
 
         $d = Carbon::parse('2023-10-25 21:14:51');
-        $d->setMonth(2.5);
+        $d = $d->setMonth(2.5);
     }
 
     public function testMonthFloatPassWithZeroDecimalPart()

tests/CarbonPeriod/GettersTest.php

@@ -90,7 +90,7 @@ class GettersTest extends AbstractTestCase
     {
         $period = CarbonPeriodFactory::withStartIntervalEnd(static::$periodClass);
 
-        $period->getStartDate()->subDays(3);
+        $date = $period->getStartDate()->subDays(3);
 
         $this->assertSame('2012-07-01', $period->getStartDate()->format('Y-m-d'));
     }
@@ -99,7 +99,7 @@ class GettersTest extends AbstractTestCase
     {
         $period = CarbonPeriodFactory::withStartIntervalEnd(static::$periodClass);
 
-        $period->getEndDate()->addDays(3);
+        $date = $period->getEndDate()->addDays(3);
 
         $this->assertSame('2012-07-15', $period->getEndDate()->format('Y-m-d'));
     }