Narrow DateInterval->days after DateTimeInterface->diff()

stubs/date.stub

@@ -34,3 +34,19 @@ class DatePeriod implements \IteratorAggregate, \Traversable
 
 	}
 }
+
+interface DateTimeInterface {
+	/** @return \DateInterval&object{days:int} */
+	function diff(\DateTimeInterface $targetObject, bool $absolute = false): \DateInterval
+	{}
+}
+
+class DateTimeImmutable implements DateTimeInterface {
+}
+
+class DateTime implements DateTimeInterface {
+}
+
+/** @return \DateInterval&object{days:int} */
+function date_diff(\DateTimeInterface $baseObject, \DateTimeInterface $targetObject, bool $absolute = false): \DateInterval
+{}

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;
+}

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'], []);
+	}
+
 }