[Bug c++/56643] Failure to match noexcept specifier of friend template function in template class

[Casey at Carter dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56643

--- Comment #2 from Casey Carter <Casey at Carter dot net> ---
For future readers, I believe the simplest workaround is to make the friend
function a static member function of a new class, friend that class instead of
the function, and introduce a simple forwarding function in the original
function's place:

template <int N>
struct Test {
    friend struct workaround_56643;
};

struct workaround_56643 {
    template <int N>
    static void test(Test<N>&) noexcept(N == 0) {}
};

template <int N>
auto test(Test<N>& arg)
  noexcept(noexcept(workaround_56643::test(arg)))
        -> decltype(workaround_56643::test(arg)) {
             return workaround_56643::test(arg);
}

int main() {
    Test<0> t;
    test(t);
}