Add unit tests

Author: devbanana

tests/PHPStan/Analyser/NodeScopeResolverTest.php

@@ -475,6 +475,8 @@ public function dataFileAsserts(): iterable
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/modulo-operator.php');
 
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/literal-string.php');
+
+		yield from $this->gatherAssertTypes(__DIR__ . '/data/filter-var-returns-non-empty-string.php');
 	}
 
 	/**

tests/PHPStan/Analyser/data/filter-var-returns-non-empty-string.php

@@ -0,0 +1,24 @@
+<?php
+
+namespace FilterVarReturnsNonEmptyString;
+
+use function PHPStan\Testing\assertType;
+
+class Foo
+{
+	/** @param non-empty-string $str */
+	public function run(string $str): void
+	{
+		assertType('non-empty-string', $str);
+
+		$return = filter_var($str, FILTER_DEFAULT);
+		assertType('non-empty-string|false', $return);
+
+		$return = filter_var($str, FILTER_SANITIZE_STRING);
+		assertType('non-empty-string|false', $return);
+
+		$str2 = '';
+		$return = filter_var($str2, FILTER_DEFAULT);
+		assertType('string|false', $return);
+	}
+}