Fix issue with slashes after ? in type

src/Psalm/Type.php

@@ -404,7 +404,7 @@ abstract class Type
     public static function tokenize($string_type, $ignore_space = true)
     {
         // remove all unacceptable characters
-        $string_type = preg_replace('/\?(?=[a-zA-Z])/', 'null|', $string_type);
+        $string_type = preg_replace('/\?(?=[\\\\a-zA-Z])/', 'null|', $string_type);
 
         $type_tokens = [''];
         $was_char = false;

tests/AnnotationTest.php

@@ -769,6 +769,17 @@ class AnnotationTest extends TestCase
                     '$e' => 'callable():string',
                 ],
             ],
+            'slashAfter?' => [
+                '<?php
+                    namespace ns;
+
+                    /** @param ?\stdClass $s */
+                    function foo($s) : void {
+                    }
+
+                    foo(null);
+                    foo(new \stdClass);',
+            ],
         ];
     }
 

tests/TypeParseTest.php

@@ -61,6 +61,14 @@ class TypeParseTest extends TestCase
         $this->assertSame('null|string', (string) Type::parseString('?string'));
     }
 
+    /**
+     * @return void
+     */
+    public function testNullableFullyQualified()
+    {
+        $this->assertSame('null|stdClass', (string) Type::parseString('?\\stdClass'));
+    }
+
     /**
      * @return void
      */