Compilation issue with custom state type with clang only

In the code below, I am working with a custom state-type (a `std::pair` in this case). The code compiles with g++, icc, msvc. With clang however, compilation fails. In order to get it to build successfully, I apparently need to define `operator+()` and `operator*()` for the state type before including the odeint header. Link to godbolt: https://godbolt.org/z/6eavzdxbf

```c++
#include
#include

#include // if included here, compilations fails with clang++

using state_type = std::pair;
using deriv_type = state_type;
using time_type = double;

state_type operator+(state_type a, state_type const& b);
state_type operator*(state_type a, time_type b);
state_type operator*(time_type b, state_type a);

//~ #include // if included here, no error!

state_type operator+(state_type a, state_type const& b) {
a.first += b.first;
a.second += b.second;
return a;
}

state_type operator*(state_type a, time_type b) {
a.first *= b;
a.second *= b;
return a;
}

state_type operator*(time_type b, state_type a) {
return a * b;
}

void harmonic_oscillator(const state_type& x, deriv_type& dxdt, time_type) {
static auto constexpr gam = 0.1;

dxdt.first = x.second;
dxdt.second = -x.first - gam*x.second;
}

namespace bno = boost::numeric::odeint;
using stepper_type = bno::runge_kutta_dopri5;

int main() {
state_type x0{1.0, 0.0};

// just to see if the operators work...
x0 * 1.;
2. * x0;
x0 + x0;

stepper_type stepper;
time_type const dt = 0.005;
for (time_type t = 0; t < 10; t += dt) {
stepper.do_step(harmonic_oscillator, x0, t, dt);
std::cout << t << '\t' << x0.first << '\t' << x0.second << '\n';
}

return EXIT_SUCCESS;
}
```