py::stride::compute does not accept signed type

[sizmailov]

py::stride used to have size_t to store start/stop/step values. Nowadays it uses ssize_t (issue #776, PR #782).

The issues is that py::slice::compute still accepts size_t* arguments instead of ssize_t*

pybind11/include/pybind11/pytypes.h

Lines 1073 to 1079 in 086d53e

	bool compute(size_t length, size_t *start, size_t *stop, size_t *step,
	size_t *slicelength) const {
	return PySlice_GetIndicesEx((PYBIND11_SLICE_OBJECT *) m_ptr,
	(ssize_t) length, (ssize_t *) start,
	(ssize_t *) stop, (ssize_t *) step,
	(ssize_t *) slicelength) == 0;
	}

C-style pointers casts must be read here as reinterpret_cast<>.

To obtain signed strides user should have something like

ssize_t start, stop, step, slicelength;
if (!slice.compute(
            N,
            reinterpret_cast<size_t*>(&start),
            reinterpret_cast<size_t*>(&stop),
            reinterpret_cast<size_t*>(&step),
            reinterpret_cast<size_t*>(&slicelength)
)){
          throw py::error_already_set();
}
... // use extracted values

So reinterpret_cast<> appears twice: in user code and in the library (in py::slice::compute) converting back and forth ssize_t* and size_t*.

The non-breaking solution would be to add an overload for ssize_t* to py::slice.

bool compute(ssize_t length, ssize_t *start, ssize_t *stop, ssize_t *step, 
              ssize_t *slicelength) const { 
     return PySlice_GetIndicesEx((PYBIND11_SLICE_OBJECT *) m_ptr, 
                                 length, start, 
                                 stop,  step, 
                                 slicelength) == 0; 
 }