Rough edges in the when_all coroutine, part 2: Overloaded comma operator

Raymond Chen

Last time, we looked at a problematic edge case in our when_all coroutine: The empty parameter list.

There’s another edge case that can cause trouble, and that’s the case where the comma operator itself has been overloaded.¹

struct S
  void Detonate();
  S operator,(S right) { Detonate(); return right; }

struct async_s : std::experimental::suspend_never
  S await_resume() { return {}; }

when_all(async_s(), async_s()); // kaboom

We start by defining a type S that has a comma operator. When you comma two S objects together, the first one explodes.

Next, we define an awaitable object async_s: When you co_await, an S comes out.

And then we pass two of these objects to when_all. The expectation is that the when_all awaits the two objects, throws away the results, and returns.

Instead, what happens is that the S object explodes.

What went wrong is that our fold expression expanded to

IAsyncAction when_all(async_s v1, async_s v2)
  (co_await v1, co_await v2);

The intent of the comma in the fold expression was to throw away the left-hand operand, leaving the last surviving operand to be thrown away by the statement-expression. But thanks to the custom comma operator, it actually causes the left-hand operand to explode.

To suppress any custom comma operators, we can cast the result of the co_await to void. Since you cannot overload the comma operator for void, this forces the use of the default comma operator, so we just comma-combine a bunch of voids, which is harmless.

template <typename... T>
Windows::Foundation::IAsyncAction when_all(T... async)
    (void(co_await async), ...);

Here is the PR that fixes the empty parameter list and comma operator issues, and a follow-up.

Bonus chatter: We could also have used a right fold:

    (co_await async, ..., void());

which expands to

    (co_await v1, (co_await v2, void()));

But I think casting away the value is simpler.

¹ Shame on you.