On Tue, 6 Jun 2023 at 12:42, Marcin Jaczewski via Std-Proposals <std-proposals@lists.isocpp.org> wrote:
wt., 6 cze 2023 o 19:25 Jason McKesson via Std-Proposals
<std-proposals@lists.isocpp.org> napisał(a):
>
> On Tue, Jun 6, 2023 at 1:02 PM Nikl Kelbon via Std-Proposals
> <std-proposals@lists.isocpp.org> wrote:
> >
> > really no one want describe why visit([](...) {}, value) just compiles into undefined behavior? We want to not change THIS behavior in standard?
>
> ... did you read what he said? He's saying that there are a lot of
> possible complications from doing what you're doing, and that you need
> to figure out what most of those are before any such change can be
> seriously considered. It's not a defense of the status quo; it's a
> statement that your suggestion is undercooked.
>
> Your response isn't actually responding to the point being made.
>
> In any case, I'm not sure I understand your point. The circumstance
> you're talking about lies at an intersection between a primarily C
> feature and a C++ construct. Those intersections tend to be where a
> lot of UB lives.
>
> Here's the thing: pretty much nobody uses C-style variadics with
> variant visitation *on purpose*. If it happens, it happens purely by
> accident. The user probably meant to do `auto&&...` but made a
> mistake.
>

This is not true, some metaprogramming tricks work thanks to `...` and
overload ordering. It's possible to that in every big project at least one time
this was used on purpose.

True, but that's less necessary now that we can use constraints to adjust overload resolution order (not that such tricks are necessarily any more readable).

> But this mistake is actually pretty rare. Why? Because in most cases,
> users want to actually *do something* in that function with the
> value(s) it is given. So most of them will catch it themselves when
> they try to fill in `{}` with actual code, since they have no variable
> name to look at (remember: they did not mean to use C-style
> variadics).
>
> So the only reason this might survive to an actual usage scenario is
> if this is intended to be a default case. This would commonly be used
> in something like "overload", a type that is built from a bunch of
> lambdas to do overload resolution between their `operator()`
> overloads. Basically, the code would look like this:
>
> ```
> visit(
>   overload{
>   [](int i) {/*do stuff with i*/},
>   [](std::string const& s) {/*do stuff with s*/},
>   [](...) {/*default case. Needs no variable*/}
>   }
>   , value)
> ```
> Since it doesn't need a variable name, it's easy to accidentally use
> `...` instead of `auto&&...`.
>
> But this is still *wrong*. We don't want people to use C-style
> variadics. And the person who wrote this *also* didn't want to use
> C-style variadics. We don't *want* C++ objects to be slipped into
> C-style variadics.
>
> So the only good solution here is to provoke a compile error on such calls.
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