add test for mixed __toString()

Author: samsonasik

tests/PHPStan/Analyser/nsrt/bug-to-string-type.php

@@ -0,0 +1,32 @@
+<?php
+
+namespace BugToStringType;
+
+class ParentClassWithToStringMixedReturn
+{
+    public function __toString()
+    {
+        return 'a';
+    }
+}
+
+class WithParentMixedReturn extends ParentClassWithToStringMixedReturn
+{
+    public function __toString()
+    {
+        return 'value';
+    }
+}
+
+class Consumer extends WithParentMixedReturn
+{
+    public function __toString()
+    {
+        return 'value';
+    }
+}
+
+function test(Consumer $test): void
+{
+	assertType('mixed', $test->__toString());
+}