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Subject: Re: [std-proposals] Template qualifiers
From: Arthur O'Dwyer (arthur.j.odwyer_at_[hidden])
Date: 2019-09-29 13:36:32

On Sun, Sep 29, 2019 at 2:15 PM Gašper Ažman via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> struct A {
> int i;
> template <std::convertible_to<A> Other> A& operator=(Other&& other) {
> using A_cvref = copy_cvref_t<Other&&, A>;
> i = static_cast<A_cvref>(other).i;
> return *this;
> }
> };

This isn't equivalent to the first thing Phil wrote; it'll never generate

    A volatile & operator = (A const & a) volatile { i = a.i; return *
this; }

On the other hand, the second thing Phil wrote is *also* completely
different from the first thing Phil wrote. Phil's second thing—

    template <qualifier Q, qualifier P>
    A Q & operator = (A P a) Q { i = std::move_or_copy(a.i); return * this;

—seems like it would happily generate signatures such as `A const&
operator=(A volatile a) const`. And no substitution for `P` and `Q` could
ever generate the signature `A& operator= (A const& a)`, unless `P` is
allowed to be `const&` (but then how would deduction know to make it
`const&` instead of simply `const` or nothing-at-all)?
Phil seems to maybe be treating "&" and "&&" as possible values for a
`qualifier`, I'm not sure.
Phil also seems to be treating "the-absence-of-anything" as a possible
value for a `qualifier`.

I agree that any proposal which has been technically fleshed out (such as
P0847) is in some sense preferable to a vague, non-technical, and
unimplementable musing. However, it's also comparing apples to oranges.


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