gh-156963: Raise TypeError when concatenating complexstr with another type (GH-156964)

complexstr_concat() is the sq_concat slot and returned NotImplemented
for a non-complexstr operand.  PyNumber_Add() returns an sq_concat
result verbatim, so the singleton reached Python code instead of a
TypeError, and an augmented assignment rebound the name to it.

Author: fedonman

Lib/test/test_curses.py

@@ -796,6 +796,9 @@ def test_complexstr(self):
         self.assertEqual(str(s[1:]), 'bc')
         self.assertEqual(str(s[::-1]), 'cbA')
         self.assertEqual(str(s + curses.complexstr(['Z'])), 'AbcZ')
+        # Concatenating anything else raises instead of returning NotImplemented.
+        self.assertRaises(TypeError, lambda: s + 'Z')
+        self.assertRaises(TypeError, lambda: s + cc('Z'))
         # The empty complexstr.
         self.assertEqual(len(curses.complexstr([])), 0)
         self.assertEqual(str(curses.complexstr('')), '')

Modules/_cursesmodule.c

@@ -1590,7 +1590,10 @@ complexstr_concat(PyObject *a, PyObject *b)
 {
     cursesmodule_state *state = get_cursesmodule_state_by_cls(Py_TYPE(a));
     if (!Py_IS_TYPE(b, state->complexstr_type)) {
-        Py_RETURN_NOTIMPLEMENTED;
+        PyErr_Format(PyExc_TypeError,
+                     "can only concatenate complexstr to complexstr, not %T",
+                     b);
+        return NULL;
     }
     PyCursesComplexStrObject *sa = _PyCursesComplexStrObject_CAST(a);
     PyCursesComplexStrObject *sb = _PyCursesComplexStrObject_CAST(b);