[committed] libstdc++: Use concepts and conditional explicit in std::optional

[Jonathan Wakely]

On Thu, 25 Jul 2024 at 19:23, Jonathan Wakely <jwakely@redhat.com> wrote:
>
> On Thu, 25 Jul 2024 at 19:22, François Dumont <frs.dumont@gmail.com> wrote:
> >
> >
> > On 25/07/2024 12:55, Jonathan Wakely wrote:
> > > Tested x86_64-linux. Pushed to trunk.
> > >
> > > -- >8 --
> > >
> > > For C++20 mode we can improve compile times by using conditional
> > > explicit to reduce the number of constructor overloads. We can also use
> > > requires-clauses instead of SFINAE to implement constraints on the
> > > constructors and assignment operators.
> > >
> > > libstdc++-v3/ChangeLog:
> > >
> > >       * include/std/optional (optional): Use C++20 features to
> > >       simplify overload sets for constructors and assignment
> > >       operators.
> > > ---
> > >   libstdc++-v3/include/std/optional | 130 +++++++++++++++++++++++++++---
> > >   1 file changed, 121 insertions(+), 9 deletions(-)
> > >
> > > diff --git a/libstdc++-v3/include/std/optional b/libstdc++-v3/include/std/optional
> > > index 700e7047aba..2cc0221865e 100644
> > > --- a/libstdc++-v3/include/std/optional
> > > +++ b/libstdc++-v3/include/std/optional
> > > @@ -768,6 +768,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > >           is_assignable<_Tp&, const optional<_Up>&&>,
> > >           is_assignable<_Tp&, optional<_Up>&&>>;
> > >
> > > +#if __cpp_concepts && __cpp_conditional_explicit && __glibcxx_remove_cvref
> > > +# define _GLIBCXX_USE_CONSTRAINTS_FOR_OPTIONAL 1
> > > +#endif
> > > +
> > >     /**
> > >       * @brief Class template for optional values.
> > >       */
> > > @@ -794,17 +798,37 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > >         using _Base = _Optional_base<_Tp>;
> > >
> > >         // SFINAE helpers
> > > -      template<typename _Up>
> > > -     using __not_self = __not_<is_same<optional, __remove_cvref_t<_Up>>>;
> > > -      template<typename _Up>
> > > -     using __not_tag = __not_<is_same<in_place_t, __remove_cvref_t<_Up>>>;
> > > -      template<typename... _Cond>
> > > -     using _Requires = enable_if_t<__and_v<_Cond...>, bool>;
> >
> > Minor but above comment is an orphan now.
>
> Everything below it is a SFINAE helper, in both branches of the #if

But if that's not clear, maybe the comment does more harm than good
and should be removed.