absl::AnyInvocable is not guaranteed to be empty after move

[jiawen]

As the documentation in the header clearly states, when move constructing an absl::AnyInvocable(AnyInvocable&& /*f*/), fis not guaranteed to be empty although it might be. Of course, this is perfectly valid C++, but is there any rationale for why we don't guarantee this?

It would make using the trailing && variant to express "invoke-only-once" semantics less error prone to use as this issue bit me recently. Example:

struct Foo {
  absl::AnyInvocable<void() &&> call_once_;

  void DoThingAndNotifyAtMostOnce() {
    if (call_once_) {
      std::move(call_once_)();
      call_once_ = nullptr;  // <------ ahh! I forgot this!
    }
  }
};

[jiawen]

I suppose I'm actually entangling two things. I didn't dig into absl/functional/internal/any_invocable.h too deeply as it has a lot of deep magic. But I guess operator(Args...) with trailing && need not be implemented using the move constructor.

[derekmauro]

Guaranteeing something that doesn't need to be guaranteed constrains the implementation and, for example, may prevent future optimizations.

[jiawen]

Makes sense from the perspective of "APIs are forever" and Hyrum's law.