remove "fast-path" for (i)adding strings

These were just an artifact of the old unicode concatenation hack and likely
just penalized other kinds of adding. Also, this fixes __(i)add__ on string
subclasses.

1 files changed, 12 insertions, 0 deletions

diff --git a/Lib/test/test_unicode.py b/Lib/test/test_unicode.py
index b903fbe9..a527dff 100644
--- a/Lib/test/test_unicode.py
+++ b/Lib/test/test_unicode.py
@@ -1760,6 +1760,18 @@ class UnicodeTest(string_tests.CommonTest,
         self.assertEqual(size, nchar)
         self.assertEqual(wchar, nonbmp + '\0')
 
+    def test_subclass_add(self):
+        class S(str):
+            def __add__(self, o):
+                return "3"
+        self.assertEqual(S("4") + S("5"), "3")
+        class S(str):
+            def __iadd__(self, o):
+                return "3"
+        s = S("1")
+        s += "4"
+        self.assertEqual(s, "3")
+
 
 class StringModuleTest(unittest.TestCase):
     def test_formatter_parser(self):