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Subject: Re: [std-proposals] This deduction and access control
From: Arthur O'Dwyer (arthur.j.odwyer_at_[hidden])
Date: 2020-07-20 18:01:47


On Mon, Jul 20, 2020 at 6:08 PM Gašper Ažman via Std-Proposals <
std-proposals_at_[hidden]> wrote:
> On Mon, Jul 20, 2020, 21:53 Marcin Jaczewski via Std-Proposals <
std-proposals_at_[hidden]> wrote:
>> pon., 20 lip 2020 o 22:42 Barry Revzin <barry.revzin_at_[hidden]>
napisał(a):
>>> If by "both call conventions", you mean to also Y::g(Y{}, 1, 2), then
yes
>>> that already is valid as stated in the paper (because they're like
static member functions)
>>> in the same way that Y{}.g(1, 2) is valid (because they're like
non-static member functions).
>>>
>>> If what you mean is to allow g(Y{}, 1, 2), then... no.
>>
>> That was my point, allow `g(Y{}, 3, 2)`, because what difference is to
>> the definition of `friend int g(Y const&, int, int);`?
>> Of course paper do not suggest this, but for me this is a logical
>> conclusion. As this is op-in we do not have problems like other
>> solutions had for that functionality.
>
> That is separable functionality that we will revisit in a future paper
with explicit syntax to enable it.

Gašper: That's not separable functionality; it's absolutely integral to the
proposal, in the sense that it cannot be added later.
I think Barry is right that WG21 shouldn't *want* to add it later, but
anyway, if they ship the ability to say Y::g(x, y), they can't later go
back and *add *the ability to also say `g(x, y)` unqualified.

Under the current proposal (AIUI, which is to say, not well),

namespace N {
    struct S {
        void swap(S& this lhs, S& rhs);
    };
}
S a, b;
a.swap(b); // OK
S::swap(a, b); // OK
N::swap(a, b); // Won't compile
swap(a, b); // Won't compile
std::ranges::swap(a, b); // Will compile and call std::swap(a, b)

Under the proposal "plus ADL" as Marcin was talking about (AIUI),

a.swap(b); // OK
S::swap(a, b); // OK but maybe not preferred?
N::swap(a, b); // Won't compile
swap(a, b); // *OK*
std::ranges::swap(a, b); // Will compile *and call S::swap(a, b)*

Whenever you add a thing, you have to get it right on the first try, or
else design it very carefully for extensibility.
The default is *never* "oh, we can just add that wrinkle later."

my $.02,
–Arthur



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