Added regression test

Author: staabm

tests/PHPStan/Analyser/nsrt/bug7856.php

@@ -0,0 +1,16 @@
+<?php
+
+namespace Bug7856;
+
+use function PHPStan\Testing\assertType;
+
+function doFoo() {
+	$intervals = ['+1week', '+1months', '+6months', '+17months'];
+	$periodEnd = new DateTimeImmutable('-1year');
+	$endDate = new DateTimeImmutable('+1year');
+
+	do {
+		assertType("array{'+1week', '+1months', '+6months', '+17months'}|array{0: literal-string&non-falsy-string, 1?: literal-string&non-falsy-string, 2?: '+17months'}", $intervals);
+		$periodEnd = $periodEnd->modify(array_shift($intervals));
+	} while (count($intervals) > 0 && $periodEnd->format('U') < $endDate);
+}