Fix

Author: VincentLanglet

src/Type/Php/FilterFunctionReturnTypeHelper.php

@@ -191,7 +191,7 @@ public function getType(Type $inputType, ?Type $filterType, ?Type $flagsType): T
 		}
 
 		if ($exactType === null || $hasOptions->maybe() || (!$inputType->equals($type) && $inputType->isSuperTypeOf($type)->yes())) {
-			if ($defaultType->isSuperTypeOf($type)->no()) {
+			if (!$defaultType->isSuperTypeOf($type)->yes()) {
 				$type = TypeCombinator::union($type, $defaultType);
 			}
 		}

tests/PHPStan/Analyser/nsrt/filter-var.php

@@ -174,6 +174,10 @@ public function randomFlag($mixed, bool $bool) {
 		assertType('int|false|null', filter_var($mixed, FILTER_VALIDATE_INT, [
 			'flags' => $bool ? FILTER_NULL_ON_FAILURE : FILTER_FLAG_NONE,
 		]));
+
+		assertType('bool|null', filter_var($mixed, FILTER_VALIDATE_BOOLEAN, [
+			'flags' => $bool ? FILTER_NULL_ON_FAILURE : FILTER_FLAG_NONE,
+		]));
 	}
 
 }