Multiple const members of struct breaks PYBIND11_NUMPY_DTYPE

[rprechelt]

Issue description

Attempting to PYBIND11_NUMPY_DTYPE on a struct containing more than one const double gives a ImportError: NumPy: invalid buffer descriptor! error on import. A class containing a single const double runs correctly. If you change the struct to contain two const double's or a const double and a regular double, it fails on import.

Reproducible example code

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

namespace py = pybind11;

// a struct with one enum and a double
struct StructType1 {

  const double x;

  StructType1(const double x_) : x(x_) {};
  
};


// a struct with one enum and two doubles
struct StructType2 {

  const double x;
  const double y;

  StructType2(const double x_,
              const double y_) : x(x_), y(y_) {};


};


PYBIND11_MODULE(mystruct, m) {

  // wrap our class with one double
  py::class_<StructType1>(m, "StructType1")
    .def(py::init<const double>());

  // and wrap our class with two doubles
  py::class_<StructType2>(m, "StructType2")
    .def(py::init<const double, const double>());

  // this line is fine
  PYBIND11_NUMPY_DTYPE(StructType1, x);

  // including the below line gives a
  // ImportError: NumPy: invalid buffer descriptor!
  // PYBIND11_NUMPY_DTYPE(StructType2, x, y);

}

[YannickJadoul]

Thanks for reporting! Something is indeed going wrong, there.

Fixing template <typename T> struct is_fmt_numeric<T, enable_if_t<std::is_arithmetic<T>::value>> in common.h with std::remove_cv solves the issue:

template <typename T> struct is_fmt_numeric<T, enable_if_t<std::is_arithmetic<T>::value>> {
    using U = typename std::remove_cv<T>::type;
    static constexpr bool value = true;
    static constexpr int index = std::is_same<U, bool>::value ? 0 : 1 + (
        std::is_integral<U>::value ? detail::log2(sizeof(U))*2 + std::is_unsigned<U>::value : 8 + (
        std::is_same<U, double>::value ? 1 : std::is_same<U, long double>::value ? 2 : 0));
};

I'm working on a PR, but need to add some tests before submitting!