test.pass.cpp

#include <tuple>
#include <array>

using namespace std;

template <bool Enable, class ...Class>
constexpr std::enable_if_t<Enable, bool> BlowUp() {
  static_assert(Enable && sizeof...(Class) != sizeof...(Class));
  return true;
}

struct NoAssign {
  NoAssign() = default;
  NoAssign(NoAssign const&) = default;
  template <class T, class = std::enable_if_t<sizeof(T) != sizeof(T)>>
  NoAssign& operator=(T) { return *this; }
};

template <int>
struct DieOnAssign {
  DieOnAssign() = default;
  template <class T, bool = BlowUp<!is_same_v<T, DieOnAssign>, T>()>
  DieOnAssign& operator=(T) {
    return *this;
  }
};

void test_arity_checks() {
  {
    using T = tuple<int, DieOnAssign<0>, int>;
    using P = pair<int, int>;
   static_assert(!is_assignable_v<T&, P const&>); // Blows up!
  }
  {
    using T = tuple<int, int, DieOnAssign<1>>;
    using A = array<int, 1>;
    static_assert(!is_assignable_v<T&, A const&>);
  }
}

void test_lazy_eval() {
  {
    using T1 = tuple<int, NoAssign, DieOnAssign<2>>;
    using T2 = tuple<long, long, long>;
    static_assert(!is_assignable_v<T1&, T2 const&>); // OK
  }
  {
    using T1 = tuple<NoAssign, DieOnAssign<3>>;
    using T2 = pair<long, double>;
    static_assert(!is_assignable_v<T1&, T2 const&>); // Blows up!
  }
}

int main() {
  test_arity_checks();
  test_lazy_eval();
}