implicitly_convertible discards conversion error

[petersteneteg]

Hi

When a implicitly_convertible fails the error is discarded here:

pybind11/include/pybind11/pybind11.h

Lines 2668 to 2672 in 31d7c87

	PyObject *result = PyObject_Call((PyObject *) type, args.ptr(), nullptr);
	if (result == nullptr) {
	PyErr_Clear();
	}
	return result;

And a "generic" conversion error is later reported here:

pybind11/include/pybind11/cast.h

Lines 1167 to 1169 in 31d7c87

	throw cast_error("Unable to cast Python instance of type "
	+ str(type::handle_of(handle)).cast<std::string>() + " to C++ type '"
	+ type_id<T>() + "'");

Would it be possible to save the initial error and provide that information in the final message?

[petersteneteg]

I guess the main problem is that we might try a bunch of conversions and other stuff.. so we might not be sure what error to give...
Ideally we could even report a list of potential conversions that failed?
The current solution leads to error that are very hard to understand or debug from a users perspective.