fix deprecations / add tests

Author: staabm

src/Analyser/MutatingScope.php

@@ -150,6 +150,7 @@
 use function array_values;
 use function count;
 use function explode;
+use function function_exists;
 use function get_class;
 use function implode;
 use function in_array;
@@ -160,6 +161,7 @@
 use function ltrim;
 use function md5;
 use function sprintf;
+use function str_increment;
 use function str_starts_with;
 use function strlen;
 use function strtolower;
@@ -1736,6 +1738,8 @@ static function (Node $node, Scope $scope) use ($arrowScope, &$arrowFunctionImpu
 						// see https://github.com/php/php-src/issues/19597
 						if ($varValue === '') {
 							$varValue = '1';
+						} elseif (is_string($varValue) && function_exists('str_increment')) {
+							$varValue = str_increment($varValue);
 						} elseif (!is_bool($varValue)) {
 							++$varValue;
 						}

src/Type/Php/StrIncrementDecrementFunctionReturnTypeExtension.php

@@ -6,13 +6,15 @@
 use PHPStan\Analyser\Scope;
 use PHPStan\DependencyInjection\AutowiredService;
 use PHPStan\Reflection\FunctionReflection;
+use PHPStan\ShouldNotHappenException;
 use PHPStan\Type\Constant\ConstantStringType;
 use PHPStan\Type\DynamicFunctionReturnTypeExtension;
 use PHPStan\Type\ErrorType;
 use PHPStan\Type\Type;
 use PHPStan\Type\TypeCombinator;
 use function chr;
 use function count;
+use function function_exists;
 use function implode;
 use function in_array;
 use function is_float;
@@ -21,6 +23,7 @@
 use function is_string;
 use function ord;
 use function preg_match;
+use function str_increment;
 use function str_split;
 use function stripos;
 
@@ -103,6 +106,14 @@ private function increment(string $s): string
 			}
 		}
 
+		if ($s === '') {
+			throw new ShouldNotHappenException();
+		}
+
+		if (function_exists('str_increment')) {
+			return str_increment($s);
+		}
+
 		return (string) ++$s;
 	}
 

tests/PHPStan/Analyser/nsrt/pre-inc.php

@@ -0,0 +1,23 @@
+<?php
+
+namespace PreInc;
+
+use function PHPStan\Testing\assertType;
+
+function doFoo() {
+	$s = '';
+	++$s;
+	assertType("'1'", $s);
+}
+
+function doFoo2() {
+	$s = '123';
+	++$s;
+	assertType("'124'", $s);
+}
+
+function doFooBar() {
+	$s = 'abc';
+	++$s;
+	assertType("'abd'", $s);
+}