Narrow to non-empty-string via Strings::length

extension.neon

@@ -29,6 +29,7 @@ parameters:
 		- stubs/Utils/Html.stub
 		- stubs/Utils/Paginator.stub
 		- stubs/Utils/Random.stub
+		- stubs/Utils/Strings.stub
 	universalObjectCratesClasses:
 		- Nette\Application\UI\ITemplate
 		- Nette\Application\UI\Template

stubs/Utils/Strings.stub

@@ -0,0 +1,17 @@
+<?php
+
+namespace Nette\Utils;
+
+use Nette;
+
+
+/**
+ * String tools library.
+ */
+class Strings
+{
+	/** @phpstan-assert-if-true non-empty-string $s */
+	public static function length(string $s): int
+	{}
+
+}

tests/Type/Nette/MultiplierTest.php → tests/Type/Nette/TypeInferenceTest.php

@@ -8,7 +8,7 @@
 use function class_exists;
 use function version_compare;
 
-class MultiplierTest extends TypeInferenceTestCase
+class TypeInferenceTest extends TypeInferenceTestCase
 {
 
 	public function dataFileAsserts(): iterable
@@ -26,6 +26,8 @@ public function dataFileAsserts(): iterable
 		} else {
 			yield from self::gatherAssertTypes(__DIR__ . '/data/multiplier.php');
 		}
+
+		yield from $this->gatherAssertTypes(__DIR__ . '/data/strings.php');
 	}
 
 	/**

tests/Type/Nette/data/strings.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace StringsTypesNarrowing;
+
+use Nette\Utils\Strings;
+use function PHPStan\Testing\assertType;
+
+function doFoo(string $string) {
+	assertType('string', $string);
+	if (Strings::length($string)) {
+		assertType('non-empty-string', $string);
+	} else {
+		assertType('string', $string);
+	}
+	assertType('string', $string);
+
+	if (Strings::length($string) === 0) {
+		assertType('string', $string);
+	}
+	assertType('string', $string);
+}