The attached patch (hopefully) silences the signedness warnings generated by Visual Studio and reported on python-dev in <https://mail.python.org/pipermail/python-dev/2014-July/135603.html>. 

This was sloppiness on my part, I even noted the problem in the original ticket and never fixed it. :)

I don't have a local dev environment setup for MSVC and Python, but at least the attached patch cures the signedness errors on Clang. They don't seem to occur at all with GCC on my Mac.

The added casts ensure comparisons uniformly compare in the unsigned domain. It seems "size_t buf_size" is pretty redundant in the original struct, it just introduces lots of casting when it only appears to be required during write_bytes() to avoid signed overflow (undefined behaviour)

Hmmmm, maybe I am missing some context, but why not avoid the casting and do?

diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -47,7 +47,7 @@ typedef struct {
  * exception and returns -1 on failure. Existing state is preserved on failure.
  */
 static int
-unshare(bytesio *self, size_t preferred_size, int truncate)
+unshare(bytesio *self, Py_ssize_t preferred_size, int truncate)
 {
     if (self->initvalue) {
         Py_ssize_t copy_size;

Either patch makes MSVC happy; Meador's gets points from me for simplicity.

Since 2e29d54843a4 this is no longer needed. Thanks for the patches!