On Mon, 15 Jan 2024 at 16:27, Patrick Palka <[email protected]> wrote:
>
> On Sat, 13 Jan 2024, Jonathan Wakely wrote:
>
> > On Sat, 13 Jan 2024 at 00:06, Patrick Palka <[email protected]> wrote:
> > >
> > > On Fri, 12 Jan 2024, Jonathan Wakely wrote:
> > >
> > > > On Fri, 12 Jan 2024 at 18:33, Patrick Palka <[email protected]> wrote:
> > > > >
> > > > > On Fri, 12 Jan 2024, Jonathan Wakely wrote:
> > > > >
> > > > > > On Fri, 12 Jan 2024 at 17:55, Patrick Palka <[email protected]>
> > > > > > 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
> >
> > Pushed to trunk - thanks for the reviews.
>
> I'm seeing a redefinition error when compiling <tuple> with
> -std=c++20 -U__cpp_conditional_explicit (which IIUC is intended
> to work?):
Yes ... well, a compiler that doesn't define that is supposed to work.
Manually undef'ing predefined macros yourself is UB of course :-)
>
> /home/ppalka/gcc-build/x86_64-pc-linux-gnu/libstdc++-v3/include/tuple:1536:9:
> error: ‘template<class ... _Elements> template<class ... _UTypes> static
> consteval bool std::tuple< <template-parameter-1-1>
> >::__nothrow_assignable()’ cannot be overloaded with ‘template<class ...
> _Elements> template<class ... _UElements> static constexpr bool std::tuple<
> <template-parameter-1-1> >::__nothrow_assignable()’
> 1536 | __nothrow_assignable()
> | ^~~~~~~~~~~~~~~~~~~~
> /home/ppalka/gcc-build/x86_64-pc-linux-gnu/libstdc++-v3/include/tuple:1248:31:
> note: previous declaration ‘template<class ... _Elements> template<class ...
> _UElements> static constexpr bool std::tuple< <template-parameter-1-1>
> >::__nothrow_assignable()’
> 1248 | static constexpr bool __nothrow_assignable()
> | ^~~~~~~~~~~~~~~~~~~~
It needs this patch:
--- a/libstdc++-v3/include/std/tuple
+++ b/libstdc++-v3/include/std/tuple
@@ -1237,20 +1237,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
_TCC<_Cond>::template __is_explicitly_constructible<_Args...>(),
bool>;
- template<typename... _UElements>
- static constexpr
- __enable_if_t<sizeof...(_UElements) == sizeof...(_Elements), bool>
- __assignable()
- { return __and_<is_assignable<_Elements&, _UElements>...>::value; }
-
- // Condition for noexcept-specifier of an assignment operator.
- template<typename... _UElements>
- static constexpr bool __nothrow_assignable()
- {
- return
- __and_<is_nothrow_assignable<_Elements&, _UElements>...>::value;
- }
-
// Condition for noexcept-specifier of a constructor.
template<typename... _UElements>
static constexpr bool __nothrow_constructible()
@@ -1687,6 +1673,23 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
#else // ! concepts
+ private:
+ template<typename... _UElements>
+ static constexpr
+ __enable_if_t<sizeof...(_UElements) == sizeof...(_Elements), bool>
+ __assignable()
+ { return __and_<is_assignable<_Elements&, _UElements>...>::value; }
+
+ // Condition for noexcept-specifier of an assignment operator.
+ template<typename... _UElements>
+ static constexpr bool __nothrow_assignable()
+ {
+ return
+ __and_<is_nothrow_assignable<_Elements&, _UElements>...>::value;
+ }
+
+ public:
+
_GLIBCXX20_CONSTEXPR
tuple&
operator=(__conditional_t<__assignable<const _Elements&...>(),
