Fix co_nothrow in now_task, add getErrorHandle support to TaskWrapper

Summary:
Previously, `TaskWrapper` didn't support the `ExtendedCoroutineHandle` protocol called `getErrorHandle`, so `co_nothrow` had no effect in those coros. That is, `co_await co_nothrow()` would still throw.

This adds a test for the broken behavior, fixes `TaskWrapper` to implement the protocol, which causes the test to pass.

The new `if constexpr` branches in `await_transform` are covered by existing `NoexceptTest.cpp` coverage of `co_nothrow(noexceptAwaitable())`.

Reviewed By: ispeters

Differential Revision: D82927064

fbshipit-source-id: f5eb73d6cc97bcaadf2b1811f5d5d59d9af68ff6

Author: snarkmaster

third-party/folly/src/folly/coro/TaskWrapper.h

@@ -221,6 +221,16 @@ class TaskPromiseWrapperBase {
   auto& executorRef(TaskPromisePrivate tag) {
     return promise_.executorRef(tag);
   }
+
+  // These next two definitions forward `getErrorHandle` behavior to the
+  // innermost promise object.  See `ExtendedCoroutineHandle` for docs.
+
+  using use_extended_handle_concept = ExtendedCoroutineHandle::PrivateTag;
+
+  static ExtendedCoroutineHandle::PromiseBase* getPromiseBase(
+      ExtendedCoroutineHandle::PrivateTag priv, TaskPromiseWrapperBase* me) {
+    return Promise::getPromiseBase(priv, &me->promise_);
+  }
 };
 
 template <typename T, typename WrapperTask, typename Promise>

third-party/folly/src/folly/coro/safe/test/NowTaskTest.cpp

@@ -208,6 +208,18 @@ CO_TEST(NowTaskTest, nothrow) {
   auto t = co_nothrow(demoNowTask(37));
   co_await std::move(t);
 #endif
+
+  // Check that the "nothrow" functionality still works
+  struct MyErr : std::exception {};
+  auto errTask = []() -> now_task<> { co_yield co_error{MyErr{}}; };
+  auto outerTask = [&]() -> now_task<> {
+    try {
+      co_await co_nothrow(errTask());
+    } catch (...) {
+      ADD_FAILURE() << "Not reached";
+    }
+  };
+  EXPECT_THROW(co_await outerTask(), MyErr);
 }
 
 // `TryAwaitable` has a custom `operator co_await`, unlike `simple` and