Change str.add type-checking behavior

In CPython, str.add directly throws a TypeError since it is implemented
as a sq_concat.  sqlalchemy relies on being able to do str()+MyStr() and
use MyStr.__radd__ to handle the addition.  We were throwing a TypeError
but not implementing it in sq_concat.

So instead, return NotImplemented instead of directly throwing a TypeError.
This makes '+' more compatible but str.__add__ a bit less compatible.

src/runtime/str.cpp

@@ -346,10 +346,10 @@ extern "C" Box* strAdd(BoxedString* lhs, Box* _rhs) {
             checkAndThrowCAPIException();
             return rtn;
         } else {
-            // Note: this is deliberately not returning NotImplemented, even though
-            // that would be more usual.  I assume this behavior of CPython's is
-            // for backwards compatibility.
-            raiseExcHelper(TypeError, "cannot concatenate 'str' and '%s' objects", getTypeName(_rhs));
+            // This is a compatibility break with CPython, which has their sq_concat method
+            // directly throw a TypeError.  Since we're not implementing this as a sq_concat,
+            // return NotImplemented for now.
+            return NotImplemented;
         }
     }
 

test/tests/augassign_reverse.py

@@ -10,7 +10,7 @@ def f():
     i += IntLike()
     print i
     try:
-        i + 1
+        1 + i
     except TypeError, e:
         print e
 f()

test/tests/str_adding.py

-# expected: fail
-
 # str has some weird adding behavior that cannot be replicated with Python code.
 # Usually, if an __add__ throws an error, then __radd__ isn't tried.
 # But string addition is defined in sq_concat, which is tried after __add__ and
@@ -13,10 +11,8 @@ class D(object):
     def __radd__(self, lhs):
         return 3.14
 
-try:
-    print "".__add__(D())
-except TypeError as e:
-    print e
+print "" + D()
+
 try:
     print C() + D()
 except TypeError as e:

test/tests/str_adding_compat.py

+# expected: fail
+
+# CPython's str.__add__ directly throws a TypeError.
+# Ours (and PyPy's) returns NotImplemented.
+
+try:
+    print "".__add__(1)
+except TypeError as e:
+    print e