Linking python extension modules

Question

I have two extension modules, imaginatively named foo and bar. bar consists of a C++ function and a simple wrapper around it

#include <pybind11/pybind11.h>

int add_numbers(int a, int b)
{
    return a + b;
}

int bar_add_numbers(int  a, int b)
{
    return add_numbers(a, b);
}

PYBIND11_MODULE(_bar, m)
{
    m.def("add_numbers", &bar_add_numbers);
}

foo has an identical wrapper, but without the definition.

#include <pybind11/pybind11.h>

int add_numbers(int,  int);

int foo_add_numbers(int a, int  b)
{
    return add_numbers(a,  b);
}

PYBIND11_MODULE(_foo, m)
{
    m.def("add_numbers", &foo_add_numbers);
}

After building these with setuptools

import os
from setuptools import setup, Extension
import pybind11

os.environ['CC'] = 'clang++'
os.environ['CXX'] = 'clang++'

libbar = Extension('foo._bar',
    sources = ['bar.cpp'],
    extra_compile_args=['-std=c++14'],
    include_dirs = [pybind11.get_include()]
)

libfoo = Extension('foo._foo',
    sources = ['foo.cpp'],
    extra_compile_args=['-std=c++14'],
    include_dirs = [pybind11.get_include()]
)


setup(name='foo',
    version='1.0.0',
    ext_modules = [libbar, libfoo]
)

my expectation (hope) was that, provided foo._foo was imported after foo._bar, then add_numbers should be resolved correctly from either wrapper. This does seem to be the case on OSX.

>>> from foo import _bar
>>> from foo import _foo
>>>  _foo.add_numbers(40, 2)
42

However on an ubuntu box this fails with undefined symbol: _Z11add_numbersii. And just when I thought I might almost have a working knowledge of dynamic linking.

What do I need to do differently on linux (or more specifically debian-based distros) to get this to work?

EDIT: output from nm for the two .so files is

$ nm _bar.cpython-37m-x86_64-linux-gnu.so | grep "add_numbers"
0000000000004a50 T _Z11add_numbersii
0000000000004a60 T _Z15bar_add_numbersii

and

$ nm _foo.cpython-37m-x86_64-linux-gnu.so | grep "add_numbers"
                 U _Z11add_numbersii
00000000000048c0 T _Z15foo_add_numbersii

And fwiw the compiler flags are

clang++ -pthread -shared -Wl,-O1 -Wl,-Bsymbolic-functions -Wl,-Bsymbolic-functions -Wl,-z,relro -Wl,-Bsymbolic-functions -Wl,-z,relro -g -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2