[PATCH] libstdc++: Work around C++20 tuple<tuple<any>> constraint recursion [PR116440]

[Ville Voutilainen]

On Thu, 13 Mar 2025 at 23:16, Ville Voutilainen
<ville.voutilainen@gmail.com> wrote:
>
> On Thu, 13 Mar 2025 at 23:03, Patrick Palka <ppalka@redhat.com> wrote:
> > +      // Defined as a template to work around PR libstdc++/116440.
> > +      template<class...>
> > +       constexpr explicit(!__convertible<const _Elements&...>())
> > +       tuple(const _Elements&... __elements)
>
> I don't understand how a constructor template declared like this can
> ever be called. The template parameter pack
> can't be provided or deduced, and can't have a default. So we're
> effectively making this signature always lose
> overload resolution to the one that takes a pack of _UElements&&.
>
> Which may be fine. I can't head-compile a test that would fail in that
> case. If any of the incoming argument isn't one
> of _Elements, that constructor wins overload resolution anyway. If the
> incoming arguments are exactly _Elements, that
> constructor does the same thing as this one. I think.

Oh, never mind. The pack is just deduced as an empty pack.