Add await_result.cpp example

rt/examples/await_result.cpp

+#include <typon/typon.hpp>
+
+using namespace typon;
+
+int fibo(int n) {
+  if (n < 2)
+    return n;
+  return fibo(n - 1) + fibo(n - 2);
+}
+
+Task<int> fibo_task(int n) {
+  int r = co_await fibo(n);
+  co_return r;
+}
+
+Join<int> fibo_join(int n) {
+  if (n < 8) {
+    int x = co_await fibo(n - 1);
+    int y = co_await fibo_task(n - 2);
+    co_return x + y;
+  }
+  Forked a = co_await fork(fibo_join(n - 1));
+  int b = co_await fibo_join(n - 2);
+  co_await Sync();
+  co_return a.get() + b;
+}
+
+template <typename F, typename T>
+constexpr bool await_of_same = std::is_same_v<meta::AwaitResult<F>, T>;
+
+Task<void> truc() {
+  // this first one requires being in a Task or Join,
+  // otherwise we can't co_await a 'normal' value.
+  static_assert(await_of_same<decltype(fibo(0)), decltype(co_await fibo(0))>);
+
+  static_assert(
+    await_of_same<decltype(fibo_task(0)), decltype(co_await fibo_task(0))>);
+
+  static_assert(
+    await_of_same<decltype(fibo_join(0)), decltype(co_await fibo_join(0))>);
+
+  co_return;
+}
+
+int main() {
+  truc().call();
+
+  static_assert(await_of_same<decltype(fibo(0)), int>);
+  static_assert(await_of_same<decltype(fibo_task(0)), int &>);
+  static_assert(await_of_same<decltype(fibo_join(0)), int &>);
+
+  return 0;
+}