fix crash in Python object to C++ string conversion

--HG--
extra : amend_source : 6a5e7668208a89296264b05bbfe7640514318a1d

CHANGES.rst

@@ -109,6 +109,8 @@ Bugs fixed
 
 * Fixed a bug which prevented overriding const methods of C++ classes.
 
+* Fixed a crash when converting Python objects to C++ strings fails.
+
 Other changes
 -------------
 

Cython/Utility/CppConvert.pyx

@@ -7,7 +7,7 @@ cdef extern from *:
     cdef cppclass string "std::string":
         string()
         string(char* c_str, size_t size)
-    cdef char* __Pyx_PyObject_AsStringAndSize(object, Py_ssize_t*)
+    cdef char* __Pyx_PyObject_AsStringAndSize(object, Py_ssize_t*) except NULL
 
 @cname("{{cname}}")
 cdef string {{cname}}(object o) except *:

tests/run/cpp_stl_string.pyx

@@ -8,6 +8,17 @@ b_asdf = b'asdf'
 b_asdg = b'asdg'
 b_s = b's'
 
+def test_conversion(py_obj):
+    """
+    >>> test_conversion(b_asdf) == b_asdf or test_conversion(b_asdf)
+    True
+    >>> test_conversion(123)  # doctest: +ELLIPSIS
+    Traceback (most recent call last):
+    TypeError: expected ..., int found
+    """
+    cdef string s = py_obj
+    return s
+
 def test_indexing(char *py_str):
     """
     >>> test_indexing(b_asdf)