[BUG] Callbacks accepting Eigen::Array of custom types

Issue description

I have a callback method that used to (about two years ago) accept an Eigen::Array of a custom type that could be either integer or double (the independent variable in an optimization problem). Now that code no longer works, and I get errors like:

RuntimeError: NumPy type info missing for struct CEGO::numberish

I've narrowed the problem down to callbacks that take Eigen::Array of non POD types, and tried to add the necessary type definitions, but whatever I do I cannot get it to work like it used to. The library is basically toast. Oddly, the binder instance of the library still works fine, which runs older versions of pybind11/Eigen/python, so the regression has happened since then.

Maybe this should never have worked since Eigen doesn't like heterogeneous types, but it did used to work nonetheless.

Reproducible example code

// My custom double/int type (recommendations are welcomed)
struct numberish {
        enum types { EMPTY, INT, DOUBLE } type;
        numberish() { u.d = std::numeric_limits<double>::max(); type = EMPTY; }
        numberish(const int &value) { u.i = value; type = INT; }
        numberish(const long unsigned int& value) { u.i = value; type = INT; }
        numberish(const double &value) { u.d = value; type = DOUBLE; }
        union id {
            double d;
            int i;
        } u;
        ...
};

// The new DTYPE definitions I tried to add
PYBIND11_NUMPY_DTYPE(CEGO::numberish::id, i, d);
PYBIND11_NUMPY_DTYPE(CEGO::numberish, type, u);

// The exposed method
template <typename T> using EArray = Eigen::Array<T, Eigen::Dynamic, 1, 0, Eigen::Dynamic, 1>;
m.def("indexer", [](std::function<CEGO::numberish(const EArray<CEGO::numberish> &)> &f){ 
        EArray<CEGO::numberish> a(10); a = 3.9; return f(a); 
    });

Start at the m.def("indexer") callback entry point and inspect the Eigen::Array and PYBIND11_NUMPY_DTYPE usage in the reproducer. Compare the current behavior with the older pybind11, Eigen, and Python versions mentioned; done means the callback accepts an Eigen::Array of CEGO::numberish without the NumPy type-info error.