Fix a signedness compiler warning in vector.to_py (GH-5438)

When compiling cythonized code which uses `std::vector` we get the
following compiler warning on GCC 8 and Python 3.9 (which is turned into
an error in our case):

```
my_file.cpp: In function ‘PyObject* __pyx_convert_vector_to_py_int(const std::vector<int>&)’:
my_file.cpp:4716:33: warning: comparison of integer expressions of different signedness: ‘Py_ssize_t’ {aka ‘long int’} and ‘size_t’ {aka ‘long unsigned int’} [-Wsign-compare]
   for (__pyx_t_5 = 0; __pyx_t_5 < __pyx_t_4; __pyx_t_5+=1) {
                       ~~~~~~~~~~^~~~~~~~~~~
```

The generated code in question is as follows:

```
/* "vector.to_py":75
 *     cdef object item
 *
 *     for i in range(v.size()):             # <<<<<<<<<<<<<<
 *         item = v[i]
 *         Py_INCREF(item)
 */
  __pyx_t_3 = __pyx_v_v.size();
  __pyx_t_4 = __pyx_t_3;
  for (__pyx_t_5 = 0; __pyx_t_5 < __pyx_t_4; __pyx_t_5+=1) {
    __pyx_v_i = __pyx_t_5;
```

`__pyx_t_5` is of type `‘Py_ssize_t’` (signed), and `__pyx_t_4` aka
`__pyx_t_3` is `size_t` (unsigned), causing GCC to rightfully complain.

Fix the generated code by explicitly using the signed variant of the
vector's size in the loop.

This bug has been introduced in
https://github.com/cython/cython/pull/4081, which also contains some
discussion on the use of signed vs unsigned types. This patch chooses to
keep the status quo and only fixes the compiler warning.

1 files changed, 3 insertions, 2 deletions

diff --git a/Cython/Utility/CppConvert.pyx b/Cython/Utility/CppConvert.pyx
index 35f4c50ef..1c6239b2b 100644
--- a/Cython/Utility/CppConvert.pyx
+++ b/Cython/Utility/CppConvert.pyx
@@ -66,13 +66,14 @@ cdef extern from "Python.h":
 cdef object {{cname}}(const vector[X]& v):
     if v.size() > <size_t> PY_SSIZE_T_MAX:
         raise MemoryError()
+    v_size_signed = <Py_ssize_t> v.size()
 
-    o = PyList_New(<Py_ssize_t> v.size())
+    o = PyList_New(v_size_signed)
 
     cdef Py_ssize_t i
     cdef object item
 
-    for i in range(v.size()):
+    for i in range(v_size_signed):
         item = v[i]
         Py_INCREF(item)
         PyList_SET_ITEM(o, i, item)