Avoid LNK2001 in c++20 when /GL (Whole program optimization) is ON with MSVC

[hellokenlee]

The following c++20 code would not link when using /GL in MSVC:

class __declspec(dllexport) Animal {
public:
	virtual ~Animal() = default;
	virtual void go(int n_times) {}
};

class __declspec(dllexport) PyAnimal : public Animal, public pybind11::trampoline_self_life_support {
public:
	void go(int n_times) override {
		PYBIND11_OVERRIDE_PURE(
			void,
			Animal,
			go,
			n_times
		);
	}
};

PYBIND11_MODULE(example, m, pybind11::mod_gil_not_used()) {
	pybind11::class_<Animal, PyAnimal, pybind11::smart_holder>(m, "Animal")
		.def(pybind11::init<>())
		.def("go", &Animal::go)
	;
}

The error is:

Error LNK2001 : unresolved external symbol "struct pybind11::detail::override_unused `public: virtual void __cdecl PyAnimal::go(int)'::`12'::caster" (?caster@?M@??go@PyAnimal@@UEAAXH@Z@4Uoverride_unused@detail@pybind11@@A)

We can use constexpr if to avoid this link error.

[rwgk]

Do we need the else here? The other if branch ends with a return, the else seems redundant.

I think it'd be slightly better to leave the code down here as-is, in part because the PYBIND11_WARNING_POP is then before the return below.

[hellokenlee]

The else is added here because the CI checks complained “warning C4702: unreachable code”( in which case the constexpr if evals true, and the code outside that constexpr if becomes unreachable ). Since the CI checks turn on "warning as error", the CI checks will failed.

[hellokenlee]

Sorry for the late reply.

[rwgk]

Thanks for the explanation!

The warning will be annoying, seems like a second C4127, not keeping up with the times (templates), so I think we'll probably wind up suppressing C4702 as we make more use of constexpr in the future ... but your solution is good for now.