On Fri, 8 Dec 2023 at 10:00, Arthur O'Dwyer <arthur.j.odwyer_at_[hidden]>
wrote:

> On Thu, Dec 7, 2023 at 10:29 AM Edward Catmur via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
>
>> Another case that I think is ultimately related:
>>
>> #include <functional>
>> int main() {
>> std::copyable_function<void()> f;
>> std::move_only_function<void()> g = f;
>> if (g)
>> g(); // oops, throws std::bad_function_call (and crashes)
>> }
>>
>
> I agree that this is unfortunate — and probably should be fixed — because
> that's the entire point of putting these types into the STL: so that they
> can interoperate correctly!
>
> However #1: this really just shows that `function` should never have been
> nullable in the first place. The original sin here is using "g is truthy"
> as a proxy for "g is safe to call." The best approach is just to maintain a
> program invariant that you never branch on the truthiness of a `function` —
> if you're in a situation where you want a "maybe-a-function," you should
> just use `optional<function>` or some such. The bool conversion for
> `function` itself is a trap.
>

We have an empty state because `function` is semiregular, which is
necessary since it is default-constructible and movable - given that a
default-constructed state or a moved-from state exist, we may as well have
both exist and be the same state. And then given that that state inhabits
the type, we must have a way to distinguish that state and wide-contract
methods that behave in a well-defined manner on encountering it.

However #2: the copyable_function => move_only_function conversion problem
> seems closely related to the function_ref => function conversion problem:
>
> https://quuxplusone.github.io/blog/2019/05/10/function-ref-vs-string-view/#but-consider
>
> std::function<int()> convert(std::function_ref<int()> f) { return f; }
> int main() {
> std::function<int()> f = convert([]() { return 42; });
> f(); // boom, this `function` stores a `function_ref` which is now
> dangling
> }
>
> In the function_ref case there's really nothing we can say other than
> "don't do that, then." It's the same as storing any other
> callable-containing-a-dangling-reference (e.g. most lambdas) into a
> `std::function`.
>

I feel that one direction is sensible (conversions in the direction
copyable -> move_only -> ref should always be allowed), whereas the other
direction is not, so conversions should be discouraged if allowed at all.
Obviously a copyable_function cannot be constructed from a
move_only_function at all, but similarly I feel that constructing a
copyable_function or move_only_function from a function_ref should at least
be explicit, precisely because it is prone to dangling.