From c3b470f60455b7712a5b86b9cc4bf6b4c33dce9a Mon Sep 17 00:00:00 2001
From: Markus Staab <maggus.staab@googlemail.com>
Date: Sun, 3 May 2026 10:38:02 +0200
Subject: [PATCH 1/4] Narrow DateInterval->days after DateTimeInterface->diff()

---
 stubs/date.stub                               | 22 +++++++++++++
 tests/PHPStan/Analyser/nsrt/bug-14428.php     | 31 +++++++++++++++++++
 .../Rules/Functions/ReturnTypeRuleTest.php    |  7 +++++
 3 files changed, 60 insertions(+)
 create mode 100644 tests/PHPStan/Analyser/nsrt/bug-14428.php

diff --git a/stubs/date.stub b/stubs/date.stub
index e58c7f0682a..cddb6af9317 100644
--- a/stubs/date.stub
+++ b/stubs/date.stub
@@ -34,3 +34,25 @@ class DatePeriod implements \IteratorAggregate, \Traversable
 
 	}
 }
+
+interface DateTimeInterface {
+	/** @return \DateInterval&object{days:int} */
+	function diff(\DateTimeInterface $targetObject, bool $absolute = false): \DateInterval
+	{}
+}
+
+interface DateTimeImmutable {
+	/** @return \DateInterval&object{days:int} */
+	function diff(\DateTimeInterface $targetObject, bool $absolute = false): \DateInterval
+	{}
+}
+
+interface DateTime {
+	/** @return \DateInterval&object{days:int} */
+	function diff(\DateTimeInterface $targetObject, bool $absolute = false): \DateInterval
+	{}
+}
+
+/** @return \DateInterval&object{days:int} */
+function date_diff(\DateTimeInterface $targetObject, bool $absolute = false): \DateInterval
+{}
diff --git a/tests/PHPStan/Analyser/nsrt/bug-14428.php b/tests/PHPStan/Analyser/nsrt/bug-14428.php
new file mode 100644
index 00000000000..f59ce94f3df
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-14428.php
@@ -0,0 +1,31 @@
+<?php
+
+namespace Bug14428;
+
+use DateInterval;
+use DateTime;
+use DateTimeImmutable;
+use DateTimeInterface;
+use function PHPStan\Testing\assertType;
+
+function doImpure(DateInterval &$a): void {
+}
+
+function getDateTimeInterfacediff(DateTimeInterface $a, DateTimeInterface $b): void {
+	$interval = $a->diff($b);
+	assertType('int', $interval->days);
+	doImpure($interval);
+	assertType('int|false', $interval->days);
+}
+
+function getDateTimeImmutablediff(DateTimeImmutable $a, DateTimeImmutable $b): int {
+	return $a->diff($b)->days;
+}
+
+function getDatetimediff(DateTime $a, DateTime $b): int {
+	return $a->diff($b)->days;
+}
+
+function getDateDiffDays(DateTime $a, DateTime $b): int {
+	return date_diff($a, $b)->days;
+}
diff --git a/tests/PHPStan/Rules/Functions/ReturnTypeRuleTest.php b/tests/PHPStan/Rules/Functions/ReturnTypeRuleTest.php
index 0c23f0e6b0a..272fc1a39e9 100644
--- a/tests/PHPStan/Rules/Functions/ReturnTypeRuleTest.php
+++ b/tests/PHPStan/Rules/Functions/ReturnTypeRuleTest.php
@@ -431,4 +431,11 @@ public function testBug13000(): void
 		$this->analyse([__DIR__ . '/data/bug-13000.php'], []);
 	}
 
+	public function testBug14428(): void
+	{
+		$this->checkNullables = true;
+		$this->checkExplicitMixed = false;
+		$this->analyse([__DIR__ . '/../../Analyser/nsrt/bug-14428.php'], []);
+	}
+
 }

From 1efd33bdb035f9d4c26d569b604305eaff39e368 Mon Sep 17 00:00:00 2001
From: Markus Staab <maggus.staab@googlemail.com>
Date: Sun, 3 May 2026 10:39:51 +0200
Subject: [PATCH 2/4] Update date.stub

---
 stubs/date.stub | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/stubs/date.stub b/stubs/date.stub
index cddb6af9317..aeadf6ef55e 100644
--- a/stubs/date.stub
+++ b/stubs/date.stub
@@ -41,13 +41,13 @@ interface DateTimeInterface {
 	{}
 }
 
-interface DateTimeImmutable {
+class DateTimeImmutable {
 	/** @return \DateInterval&object{days:int} */
 	function diff(\DateTimeInterface $targetObject, bool $absolute = false): \DateInterval
 	{}
 }
 
-interface DateTime {
+class DateTime {
 	/** @return \DateInterval&object{days:int} */
 	function diff(\DateTimeInterface $targetObject, bool $absolute = false): \DateInterval
 	{}

From a2c906eb1d526c3f7088823dd1ccd33ad631961e Mon Sep 17 00:00:00 2001
From: Markus Staab <maggus.staab@googlemail.com>
Date: Sun, 3 May 2026 10:41:01 +0200
Subject: [PATCH 3/4] Update date.stub

---
 stubs/date.stub | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/stubs/date.stub b/stubs/date.stub
index aeadf6ef55e..0a3b69e0203 100644
--- a/stubs/date.stub
+++ b/stubs/date.stub
@@ -54,5 +54,5 @@ class DateTime {
 }
 
 /** @return \DateInterval&object{days:int} */
-function date_diff(\DateTimeInterface $targetObject, bool $absolute = false): \DateInterval
+function date_diff(\DateTimeInterface $baseObject, \DateTimeInterface $targetObject, bool $absolute = false): \DateInterval
 {}

From 8b0b833680871af673a7b50637e58d4477cce040 Mon Sep 17 00:00:00 2001
From: Markus Staab <maggus.staab@googlemail.com>
Date: Sun, 3 May 2026 10:41:59 +0200
Subject: [PATCH 4/4] simplify

---
 stubs/date.stub | 10 ++--------
 1 file changed, 2 insertions(+), 8 deletions(-)

diff --git a/stubs/date.stub b/stubs/date.stub
index 0a3b69e0203..8191057c642 100644
--- a/stubs/date.stub
+++ b/stubs/date.stub
@@ -41,16 +41,10 @@ interface DateTimeInterface {
 	{}
 }
 
-class DateTimeImmutable {
-	/** @return \DateInterval&object{days:int} */
-	function diff(\DateTimeInterface $targetObject, bool $absolute = false): \DateInterval
-	{}
+class DateTimeImmutable implements DateTimeInterface {
 }
 
-class DateTime {
-	/** @return \DateInterval&object{days:int} */
-	function diff(\DateTimeInterface $targetObject, bool $absolute = false): \DateInterval
-	{}
+class DateTime implements DateTimeInterface {
 }
 
 /** @return \DateInterval&object{days:int} */