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Subject: Re: [std-proposals] Operator functions to fix ADL
From: Dusan Jovanovic (DBJ) (dbj_at_[hidden])
Date: 2020-10-13 09:57:21


A.O.D. has written: -- I'd be interested to see a compiler patch to warn
whenever "ADL lookup occurred, on a non-operator, and ADL found a
best-matching candidate which would *not* have been found by regular
unqualified lookup." It couldn't be used in production because
approximately every C++ program depends on ADL *somewhere*. But it could
give us a sense of the magnitude of the issue. --

Why not just going further: *-fno-adl*

That would certainly "rock the boat". In my not-a-compiler-writer naivete,
I might think, contrary to A.O.D. (gasp!) a lot of code will work unbroken
with that switch.? Especially that silent not-minority not using std lib.

DBJ

On Tue, 13 Oct 2020 at 16:34, Arthur O'Dwyer via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> On Mon, Oct 12, 2020 at 12:54 PM Григорий Шуренков via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
>
>> [Someone wrote:]
>> > As one of the "I am just a regular user, not a compiler guru" lurkers
>> on this list, I'd just like to mention that anything that involves
>> call-site annotation is a great source of confusion for regular users,
>> especially when the answer to "Why?" is "obscure reason X" as it would be
>> in this case. Shifting the responsibility to the caller is just as
>> ineffective as documenting the issue in a manual.
>>
>> Let me explain why it might be a good idea to have a call-site annotation
>> for ADL. The reason is not obscure, the annotation designates an unusual
>> function call - a customization point. Moreover, until recently it was a
>> recommended practice to annotate such calls in the following way:
>> using std::swap;
>> swap(a, b);
>>
>
> I agree with you both. :) One of the big problems with ADL -- and
> unfortunately the biggest problem with ADL is that we can't get rid of it
> ;) -- is that it's a tool that can be used in several different ways, for
> several different purposes. Sometimes the programmer wants to say "This
> function is intended for use with ADL"; sometimes the programmer wants to
> say "This call-site intends to use ADL." Either intent can be valid, at
> different places in the same program.
>
> See my blog post "What is the std::swap two-step?"
>
> https://quuxplusone.github.io/blog/2020/07/11/the-std-swap-two-step/#when-should-i-use-the-two-step
> where I compare plain old ADL to a pure virtual function, and
> ADL-with-the-two-step to a non-pure, yet virtual, function.
>
> I'd be interested to see a compiler patch to warn whenever "ADL lookup
> occurred, on a non-operator, and ADL found a best-matching candidate which
> would *not* have been found by regular unqualified lookup." It couldn't
> be used in production because approximately every C++ program depends on
> ADL *somewhere*. But it could give us a sense of the magnitude of the
> issue.
>
> Third point is that such annotation allows what is currently not possible.
>> If you look up the definition of range-based for loop in the standard you'd
>> see that it is impossible to write its implementation in plain C++. To find
>> begin and end for a container it performs only ADL w/o usual name lookup.
>> An annotated call would do exactly that.
>>
>
> IIRC, std::ranges::begin(x) and the other Ranges CPOs are also specified
> to perform only ADL without the usual unqualified name lookup. They have a
> way to approximate that, by using a detail namespace and a "poison pill";
> but it's not 100% perfect and of course it's more expensive in terms of
> compile time than a simple built-in annotation would be.
>
> my $.02,
> -Arthur
> --
> Std-Proposals mailing list
> Std-Proposals_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>



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