array_t constructor takes shape as size_t, not ssize_t

[anntzer]

Issue description

The array_t constructor takes its input shape as unsigned size_t (== npt_uintp), even though the numpy API (e.g. PyArray_NewFromDescr) uses signed ssize_t (== npy_intp) and pybind11 itself uses signed sizes in most places (e.g. ssize_t shape(ssize_t dim)).

Reproducible example code

Use the following snippet into the setup provided in the pybind11/python_example repo:

#include <pybind11/pybind11.h>
#include <pybind11/numpy.h>

namespace py = pybind11;

py::array_t<int> make_array(ssize_t size) {
    return py::array_t<int>{{size}};
}

PYBIND11_MODULE(python_example, m) {
    m.def("make_array", &make_array);
}

Compilation (pybind11 2.2.4, gcc 8.2.1) yields

src/main.cpp: In function ‘pybind11::array_t<int> make_array(ssize_t)’:
src/main.cpp:7:35: warning: narrowing conversion of ‘size’ from ‘ssize_t’ {aka ‘long int’} to ‘pybind11::size_t’ {aka ‘long unsigned int’} inside { } [-Wnarrowing]
     return py::array_t<int>{{size}};