njlr

constexpr unsigned fibonacci(const unsigned x) {
  return x <= 1 ? 
    1 : 
    fibonacci(x - 1) + fibonacci(x - 2);
}

int main() {
  return fibonacci(5);
}

njlr

template<unsigned n>
struct Fibonacci {
  static const unsigned value = Fibonacci<n - 1>::value + Fibonacci<n - 2>::value;
};

template<>
struct Fibonacci<0> {
  static const unsigned value = 0;
};

njlr

template<class L, class R>
auto fold(L l, R r) {
  using lTag = typename L::tag;
  using rTag = typename R::tag;
  if constexpr (is_base_of<rTag, BarTag>::value) {
    if constexpr (is_same<lTag, FooTag>::value) {
      return foldFB(l, r);
    } else {
      return foldBB(l, r);
    }

njlr

struct BazTag : FooTag, BarTag {};

template<class L, class R,
  enable_if_t<
   is_same<L::tag, FooTag>::value && 
   is_base_of<R::tag, BarTag>::value
  > fold(L l, R r) {
 return foldFB(l, r);
}

njlr

struct FooTag {};
struct BarTag {};

auto foldFF(...) {}
auto foldFB(...) {}
auto foldBF(...) {}
auto foldBB(...) {}

struct A {
 /* ... */

njlr

template<class T>
int compute(T x) {
 if constexpr( supportsAPI(T{}) ) {
   // only gets compiled if the condition is true
   return x.Method();
 } else {
   return 0;
 }
}

njlr

template<class T>
auto compute(T x) -> decltype( enable_if_t< supportsAPI(T{}), int>{}) {
 return x.Method();
}

template<class T>
auto compute(T x) -> decltype( enable_if_t<!supportsAPI(T{}), int>{}) {
 return 0;
}

njlr

template<class T>
constexpr auto supportsAPI(T x) -> decltype(x.Method1(), x.Method2(), true_type{}) {
 return {};
}
constexpr auto supportsAPI(...) -> false_type {
 return {};
}

njlr

template<unsigned n>
struct Get {
 template<class X, class…Xs>
 constexpr auto operator()(X x, Xs…xs) {
   if constexpr(n > sizeof…(xs) ) {
     return;
   } else if constexpr(n > 0) {
     return Get<n-1>{}(xs…);
   } else {
     return x;

njlr

template<unsigned n>
struct Arg {
 template<class X, class...Xs>
 constexpr auto operator()(X x, Xs...xs) {
   return Arg<n - 1>{}(xs...);
 }
};

template<>
struct Arg<0> {