> I understand this code as: sizeof(wchar_t) == sizeof(Py_UNICODE). If I 
> misunderstood the code, it's a a heap overflow :-)

Yep, sorry.  You're right.

>> A test would be good, too.
>
>PyUnicode_FromWideChar() is not a public API. Should I write a function 
>in _testcapi?

I was actually thinking of a test for the "No such file or directory" 
bug that's mentioned at the start of the discussion.