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From: Ville Voutilainen <ville.voutilainen@gmail.com>
Date: Wed, 7 Apr 2021 21:26:58 +0300
Message-ID: <CAFk2RUaAC56Hu9pgnFd4XObeEO+sCvQetA2G9MNAdKVg1=pCkg@mail.gmail.com>
Subject: Re: [RFC] Deprecate non-standard constructors in std::pair
To: Jonathan Wakely <jwakely@redhat.com>
Cc: "libstdc++" <libstdc++@gcc.gnu.org>
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On Wed, 7 Apr 2021 at 21:25, Ville Voutilainen
<ville.voutilainen@gmail.com> wrote:
>
> On Wed, 7 Apr 2021 at 21:17, Jonathan Wakely <jwakely@redhat.com> wrote:
> > This fails to compile because of that bug:
> >
> > #include <utility>
> >
> > struct X {
> >    X(void* = 0) { }
> >    X(const X&) = default;
> >    X(const X&&) = delete;
> > };
> >
> > struct move_only {
> >    move_only() = default;
> >    move_only(move_only&&) = default;
> > };
> >
> > std::pair<move_only, X> p0(move_only(), 0);
> > std::pair<move_only, X> p1(move_only(), {});
> >
> > The pair(U1&&, const T2&) constructor should be viable, but it fails
> > the _MoveCopyPair constraint check because X(const X&&) is deleted.
> >
> > I'm not sure I care about this though. It would only work because of
> > those non-standard constructors which we're talking about deprecating.
> > I'm not very motivated to fix them so they accept this, when we're
> > going to deprecate them anyway.
>
> Nah, let's not bother. The type X has an explicitly deleted move
> constructor, which the library
> doesn't go to heroic extents to support.

And more than that, it's trying to be a "copy-only" type, which the
library doesn't try to support
at all, quite the contrary.