Scratchpad for compile-time control flow

constexpr_control_flow.cpp

void foo() {
  for (auto x : tuple) {
    if (runtime-bool) // ill formed
      constexpr break;
  }
}

void foo() {
  for (auto x : tuple) {
    if constexpr (compile-time-bool) // yes!
      constexpr break;
  }
}


void foo() {
  for (auto x : tuple) {
    constexpr break; // always breaks at the first iteration of the loop
  }
}


void foo() {
  for (auto x : tuple) {
    xxx;
    if constexpr (compile-time-bool)
      constexpr break;
    yyy; // not instantiated when the `constexpr break` is hit
  }
}

void foo() {
  for (auto x : tuple) {
    xxx;
    if constexpr (compile-time-bool)
      return z;
    yyy; // not instantiated when the `return` is hit
  }
}

template<typename T>
void f() {
  if constexpr (compile-time-bool)
    constexpr return;
  xxx; // never instantiated if the `constexpr return` is hit
}