Fix exception ignored by coro::Future<void>

Summary: If `coro::Promise<void>` completes with exception, `coro::Future<void>` should throw this exception, but currently it just ignores it.

Reviewed By: iahs

Differential Revision: D34211495

fbshipit-source-id: 99e2855646b907ac2a87c800bc8b9cb1eff82087

folly/experimental/coro/Promise.h

@@ -143,6 +143,8 @@ class Future {
     T await_resume() {
       if constexpr (!std::is_void_v<T>) {
         return std::move(future_.state_.result.value());
+      } else {
+        future_.state_.result.throwIfFailed();
       }
     }
 

folly/experimental/coro/test/PromiseTest.cpp

@@ -49,6 +49,12 @@ CO_TEST(PromiseTest, ImmediateException) {
   EXPECT_TRUE(res.hasException<std::runtime_error>());
 }
 
+CO_TEST(PromiseTest, ImmediateExceptionVoid) {
+  auto [promise, future] = coro::makePromiseContract<void>();
+  promise.setException(std::runtime_error(""));
+  EXPECT_THROW(co_await std::move(future), std::runtime_error);
+}
+
 CO_TEST(PromiseTest, SuspendValue) {
   auto [promise, future] = coro::makePromiseContract<int>();
   auto waiter = [](auto future) -> coro::Task<int> {