On Fri, 12 Jan 2024, Jonathan Wakely wrote:
> On Fri, 12 Jan 2024 at 18:33, Patrick Palka <ppa...@redhat.com> wrote:
> >
> > On Fri, 12 Jan 2024, Jonathan Wakely wrote:
> >
> > > On Fri, 12 Jan 2024 at 17:55, Patrick Palka <ppa...@redhat.com> wrote:
> > > >
> > > > On Thu, 11 Jan 2024, Jonathan Wakely wrote:
> > > >
> > > > > I'd like to commit this to trunk for GCC 14. Please take a look.
> > > > >
> > > > > -- >8 --
> > > > >
> > > > > This is the last part of PR libstdc++/108822 implementing P2255R2,
> > > > > which
> > > > > makes it ill-formed to create a std::tuple that would bind a reference
> > > > > to a temporary.
> > > > >
> > > > > The dangling checks are implemented as deleted constructors for C++20
> > > > > and higher, and as Debug Mode static assertions in the constructor
> > > > > body
> > > > > for older standards. This is similar to the r13-6084-g916ce577ad109b
> > > > > changes for std::pair.
> > > > >
> > > > > As part of this change, I've reimplemented most of std::tuple for
> > > > > C++20,
> > > > > making use of concepts to replace the enable_if constraints, and using
> > > > > conditional explicit to avoid duplicating most constructors. We could
> > > > > use conditional explicit for the C++11 implementation too (with
> > > > > pragmas
> > > > > to disables the -Wc++17-extensions warnings), but that should be done
> > > > > as
> > > > > a stage 1 change for GCC 15 rather than now.
> > > > >
> > > > > The partial specialization for std::tuple<T1, T2> is no longer used
> > > > > for
> > > > > C++20 (or more precisely, for a C++20 compiler that supports concepts
> > > > > and conditional explicit). The additional constructors and assignment
> > > > > operators that take std::pair arguments have been added to the C++20
> > > > > implementation of the primary template, with sizeof...(_Elements)==2
> > > > > constraints. This avoids reimplementing all the other constructors in
> > > > > the std::tuple<T1, T2> partial specialization to use concepts. This
> > > > > way
> > > > > we avoid four implementations of every constructor and only have
> > > > > three!
> > > > > (The primary template has an implementation of each constructor for
> > > > > C++11 and another for C++20, and the tuple<T1,T2> specialization has
> > > > > an
> > > > > implementation of each for C++11, so that's three for each
> > > > > constructor.)
> > > > >
> > > > > In order to make the constraints more efficient on the C++20 version
> > > > > of
> > > > > the default constructor I've also added a variable template for the
> > > > > __is_implicitly_default_constructible trait, implemented using
> > > > > concepts.
> > > > >
> > > > > libstdc++-v3/ChangeLog:
> > > > >
> > > > > PR libstdc++/108822
> > > > > * include/std/tuple (tuple): Add checks for dangling references.
> > > > > Reimplement constraints and constant expressions using C++20
> > > > > features.
> > > > > * include/std/type_traits [C++20]
> > > > > (__is_implicitly_default_constructible_v): Define.
> > > > > (__is_implicitly_default_constructible): Use variable template.
> > > > > * testsuite/20_util/tuple/dangling_ref.cc: New test.
> > > > > ---
> > > > > libstdc++-v3/include/std/tuple | 1021
> > > > > ++++++++++++-----
> > > > > libstdc++-v3/include/std/type_traits | 11 +
> > > > > .../testsuite/20_util/tuple/dangling_ref.cc | 105 ++
> > > > > 3 files changed, 841 insertions(+), 296 deletions(-)
> > > > > create mode 100644
> > > > > libstdc++-v3/testsuite/20_util/tuple/dangling_ref.cc
> > > > >
> > > > > diff --git a/libstdc++-v3/include/std/tuple
> > > > > b/libstdc++-v3/include/std/tuple
> > > > > index 50e11843757..cd05b638923 100644
> > > > > --- a/libstdc++-v3/include/std/tuple
> > > > > +++ b/libstdc++-v3/include/std/tuple
> > > > > @@ -752,11 +752,467 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > > > > template<typename... _Elements>
> > > > > class tuple : public _Tuple_impl<0, _Elements...>
> > > > > {
> > > > > - typedef _Tuple_impl<0, _Elements...> _Inherited;
> > > > > + using _Inherited = _Tuple_impl<0, _Elements...>;
> > > > >
> > > > > template<bool _Cond>
> > > > > using _TCC = _TupleConstraints<_Cond, _Elements...>;
> > > >
> > > > I guess this should be moved into the #else branch if it's not used in
> > > > the new impl.
> > >
> > > Ah yes, I left them there until I was sure I wouldn't need them ...
> > > then didn't move them when I didn't need them.
> > >
> > > >
> > > > >
> > > > > +#if __cpp_concepts && __cpp_conditional_explicit // >= C++20
> > > > > + template<typename... _UTypes>
> > > > > + static consteval bool
> > > > > + __constructible()
> > > > > + {
> > > > > + if constexpr (sizeof...(_UTypes) == sizeof...(_Elements))
> > > > > + return (is_constructible_v<_Elements, _UTypes> && ...);
> > > >
> > > > IIUC this (and all the other new constraints) won't short-circuit like
> > > > the old versions do :/ Not sure how much that matters?
> > >
> > > Yeah, I thought about that, but we have efficient built-ins for these
> > > traits now, so I think it's probably OK?
> >
> > Performance wise agreed, though I suppose removing the short circuiting
> > could break existing (though not necessarily valid) code that relied
> > on it to prevent an ill-formed template instantiation. It seems
> > the standard https://eel.is/c++draft/tuple uses conjunction_v in some
> > constraints, and fold-expressions in others, implying short circuiting
> > in some cases but not others?
> >
> > >
> > > If not we could go back to sharing the _TupleConstraints implementations.
> >
> > IMHO I'd be more comfortable with that.
>
> Here's an incremental diff to make that change:
>
> --- a/libstdc++-v3/include/std/tuple
> +++ b/libstdc++-v3/include/std/tuple
> @@ -763,7 +763,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> __constructible()
> {
> if constexpr (sizeof...(_UTypes) == sizeof...(_Elements))
> - return (is_constructible_v<_Elements, _UTypes> && ...);
> + return __and_v<is_constructible<_Elements, _UTypes>...>;
> else
> return false;
> }
> @@ -773,7 +773,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> __nothrow_constructible()
> {
> if constexpr (sizeof...(_UTypes) == sizeof...(_Elements))
> - return (is_nothrow_constructible_v<_Elements, _UTypes> && ...);
> + return __and_v<is_nothrow_constructible<_Elements, _UTypes>...>;
> else
> return false;
> }
> @@ -783,7 +783,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> __convertible()
> {
> if constexpr (sizeof...(_UTypes) == sizeof...(_Elements))
> - return (is_convertible_v<_UTypes, _Elements> && ...);
> + return __and_v<is_convertible<_UTypes, _Elements>...>;
> else
> return false;
> }
> @@ -1526,7 +1526,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> __assignable()
> {
> if constexpr (sizeof...(_UTypes) == sizeof...(_Elements))
> - return (is_assignable_v<_Elements&, _UTypes> && ...);
> + return __and_v<is_assignable<_Elements&, _UTypes>...>;
> else
> return false;
> }
> @@ -1536,7 +1536,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> __nothrow_assignable()
> {
> if constexpr (sizeof...(_UTypes) == sizeof...(_Elements))
> - return (is_nothrow_assignable_v<_Elements&, _UTypes> && ...);
> + return __and_v<is_nothrow_assignable<_Elements&, _UTypes>...>;
> else
> return false;
> }
> @@ -1547,7 +1547,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> __const_assignable()
> {
> if constexpr (sizeof...(_UTypes) == sizeof...(_Elements))
> - return (is_assignable_v<const _Elements&, _UTypes> && ...);
> + return __and_v<is_assignable<const _Elements&, _UTypes>...>;
> else
> return false;
> }
>
> Happier with that?
>
> It passes all the tuple tests, I'm running the full suite now.
>
>
LGTM
