static assert Found more than one type matching the predicate

[kerim371]

Hi,

I have a simple interface consist of three abstract classes A, B, C and when trying to build it I get static assert (it is not full text but enough to describe the idea I think):

/home/kerim/Documents/tmp/pybind11/include/pybind11/detail/common.h: In instantiation of ‘struct pybind11::detail::exactly_one<pybind11::class_<B, BImpl<B>, AImpl<B> >::is_subtype, void, BImpl<B>, AImpl<B> >’:
/home/kerim/Documents/tmp/pybind11/include/pybind11/detail/common.h:716:7:   required by substitution of ‘template<template<class> class Predicate, class Default, class ... Ts> using exactly_one_t = typename pybind11::detail::exactly_one::type [with Predicate = pybind11::class_<B, BImpl<B>, AImpl<B> >::is_subtype; Default = void; Ts = {BImpl<B>, AImpl<B>}]’
/home/kerim/Documents/tmp/pybind11/include/pybind11/pybind11.h:1335:11:   required from ‘class pybind11::class_<B, BImpl<B>, AImpl<B> >’
/home/kerim/Documents/tmp/pybind11/templated_interface/main.cpp:31:12:   required from here
/home/kerim/Documents/tmp/pybind11/include/pybind11/detail/common.h:707:25: error: static assertion failed: Found more than one type matching the predicate
  707 |     static_assert(found <= 1, "Found more than one type matching the predicate");

I uploaded the minimal project with these classes. It is small cmake project and you can choose whether to build pybind module or executable (by commenting appropriate blocks in main.cpp) to check that it works.
Here is the short story:
``

main.cpp:

#include "a.h"
#include "b.h"
#include "c.h"

#include <pybind11/pybind11.h>

namespace py = pybind11;


int main(int argc, char *argv[])
{
  createA()->a();
  createB()->b();
  createC()->c();

  return 0;
}

// comment this pybind section and the project will be normally compiled
PYBIND11_MODULE(example, m) {
  auto a = py::class_<
    A,
    AImpl<A>>
    (m, "A")
    .def("A", &A::a);

  auto b = py::class_<
    B,
    BImpl<B>,
    AImpl<B>>
    (m, "B")
    .def("B", &B::b);

  auto c = py::class_<
    C,
    CImpl<C>,
    BImpl<C>,
    AImpl<C>>
    (m, "C")
    .def("C", &C::c);
}

Appreciate any help!

[kerim371]

It seems that I did wrong inheritance structure in pybind module. I should have rather inherit from interfaces:

PYBIND11_MODULE(example, m) {
  auto a = py::class_<
    A,
    AImpl<A>>
    (m, "A")
    .def("A", &A::a);

  auto b = py::class_<
    B,
    BImpl<B>,
    A>
    (m, "B")
    .def("B", &B::b);

  auto c = py::class_<
    C,
    CImpl<C>,
    B,
    A>
    (m, "C")
    .def("C", &C::c);
}