Fix in_array false positive with possibly-empty non-constant array

- For non-constant arrays that might be empty (isIterableAtLeastOnce is not yes),
  return null (indeterminate) when the needle type is compatible with the haystack
  value type, since the array could be empty making in_array return false
- Still correctly reports "always false" when needle type is incompatible with
  haystack value type regardless of array emptiness
- Added regression test for the reported issue

Closes phpstan/phpstan#13799

Author: VincentLanglet

src/Rules/Comparison/ImpossibleCheckTypeHelper.php

@@ -141,6 +141,11 @@ public function findSpecifiedType(
 
 							return null;
 						}
+
+						if (!$isNeedleSupertype->no()) {
+							// Array might be empty, so in_array can return false
+							return null;
+						}
 					}
 
 					if (!$haystackType instanceof ConstantArrayType || count($haystackType->getValueTypes()) > 0) {

tests/PHPStan/Rules/Comparison/ImpossibleCheckTypeFunctionCallRuleTest.php

@@ -1195,4 +1195,10 @@ public function testBug13566(): void
 		]);
 	}
 
+	public function testBug13799(): void
+	{
+		$this->treatPhpDocTypesAsCertain = true;
+		$this->analyse([__DIR__ . '/data/bug-13799.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Comparison/data/bug-13799.php

@@ -0,0 +1,22 @@
+<?php declare(strict_types = 1);
+
+namespace Bug13799;
+
+/**
+ * @phpstan-impure
+ * @return list<'a'|'b'>
+ */
+function get_whitelist(): array {
+	$s = [];
+	if (rand(0, 1)) {
+		$s[] = 'a';
+	}
+	if (rand(0, 1)) {
+		$s[] = 'b';
+	}
+	return $s;
+}
+
+if (in_array('a', get_whitelist(), true)) {
+	echo 'ok';
+}