On Fri, 28 Feb 2025, Jonathan Wakely wrote:
> On 28/02/25 16:27 -0500, Patrick Palka wrote:
> > On Thu, 27 Feb 2025, Jonathan Wakely wrote:
> >
> > > The specification for std::ranges::iter_move apparently requires us to
> > > handle types which do not satisfy std::indirectly_readable, for example
> > > with overloaded operator* which behaves differently for different value
> > > categories.
> > >
> > > libstdc++-v3/ChangeLog:
> > >
> > > PR libstdc++/106612
> > > * include/bits/iterator_concepts.h (_IterMove::__iter_ref_t):
> > > New alias template.
> > > (_IterMove::__result): Use __iter_ref_t instead of
> > > std::iter_reference_t.
> > > (_IterMove::__type): Remove incorrect __dereferenceable
> > > constraint.
> > > (_IterMove::operator()): Likewise. Add correct constraints. Use
> > > __iter_ref_t instead of std::iter_reference_t. Forward parameter
> > > as correct value category.
> > > (iter_swap): Add comments.
> > > * testsuite/24_iterators/customization_points/iter_move.cc: Test
> > > that iter_move is found by ADL and that rvalue arguments are
> > > handled correctly.
> > > ---
> > >
> > > Tested x86_64-linux.
> > >
> > > I think the spec is silly to require this, but here we are.
> > >
> > > libstdc++-v3/include/bits/iterator_concepts.h | 33 +++++--
> > > .../customization_points/iter_move.cc | 95 +++++++++++++++++++
> > > 2 files changed, 119 insertions(+), 9 deletions(-)
> > >
> > > diff --git a/libstdc++-v3/include/bits/iterator_concepts.h
> > > b/libstdc++-v3/include/bits/iterator_concepts.h
> > > index 4265c475273..555af3bdb38 100644
> > > --- a/libstdc++-v3/include/bits/iterator_concepts.h
> > > +++ b/libstdc++-v3/include/bits/iterator_concepts.h
> > > @@ -103,32 +103,42 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > > namespace ranges
> > > {
> > > /// @cond undocumented
> > > + // Implementation of std::ranges::iter_move, [iterator.cust.move].
> > > namespace __imove
> > > {
> > > void iter_move() = delete;
> > >
> > > + // Satisfied if _Tp is a class or enumeration type and iter_move
> > > + // can be found by argument-dependent lookup.
> > > template<typename _Tp>
> > > concept __adl_imove
> > > = (std::__detail::__class_or_enum<remove_reference_t<_Tp>>)
> > > - && requires(_Tp&& __t) { iter_move(static_cast<_Tp&&>(__t)); };
> > > + && requires(_Tp&& __t) { iter_move(static_cast<_Tp&&>(__t)); };
> > >
> > > struct _IterMove
> > > {
> > > private:
> > > + // The type returned by dereferencing a value of type _Tp.
> > > + // Unlike iter_reference_t this preserves the value category of _Tp.
> > > + template<typename _Tp> requires requires { *std::declval<_Tp>(); }
> >
> > IIUC this requires-clause is redundant since it's checking a subset
> > of the alias definition (and alias templates are transparent so if the
> > decltype is invalid it'll induce an error in the immediate context)?
>
> Good point, I've removed the constraint.
>
> > > + using __iter_ref_t = decltype(*std::declval<_Tp>());
> > > +
> > > template<typename _Tp>
> > > struct __result
> > > - { using type = iter_reference_t<_Tp>; };
> > > + { using type = __iter_ref_t<_Tp>; };
> > >
> > > + // Use iter_move(E) if that works.
> > > template<typename _Tp>
> > > requires __adl_imove<_Tp>
> > > struct __result<_Tp>
> > > { using type = decltype(iter_move(std::declval<_Tp>())); };
> > >
> > > + // Otherwise, if *E if an lvalue, use std::move(*E).
> > > template<typename _Tp>
> > > requires (!__adl_imove<_Tp>)
> > > - && is_lvalue_reference_v<iter_reference_t<_Tp>>
> > > + && is_lvalue_reference_v<__iter_ref_t<_Tp>>
> > > struct __result<_Tp>
> > > - { using type = remove_reference_t<iter_reference_t<_Tp>>&&; };
> > > + { using type = remove_reference_t<__iter_ref_t<_Tp>>&&; };
> > >
> > > template<typename _Tp>
> > > static constexpr bool
> > > @@ -142,10 +152,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > >
> > > public:
> > > // The result type of iter_move(std::declval<_Tp>())
> > > - template<std::__detail::__dereferenceable _Tp>
> > > + template<typename _Tp>
> > > using __type = typename __result<_Tp>::type;
> > >
> > > - template<std::__detail::__dereferenceable _Tp>
> > > + template<typename _Tp>
> > > + requires __adl_imove<_Tp> || requires { *std::declval<_Tp>(); }
> >
> > Maybe requires { typename __iter_ref_t<_Tp>; } instead to make it more
> > obvious that the __iter_ref_t<_Tp> in the function body is always valid?
>
> Yes, that makes the constraint more obviously match what it does.
>
> > > [[nodiscard]]
> > > constexpr __type<_Tp>
> > > operator()(_Tp&& __e) const
> > > @@ -153,10 +164,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > > {
> > > if constexpr (__adl_imove<_Tp>)
> > > return iter_move(static_cast<_Tp&&>(__e));
> > > - else if constexpr (is_lvalue_reference_v<iter_reference_t<_Tp>>)
> >
> > With the new constraints it's not clear that iter_reference_t<_Tp> is
> > always valid here, say, if _Tp has an operator* that returns void?
>
> Using iter_reference_t here was a bug, which is why it's been replaced
> with using __iter_ref_t:
>
> > > - return static_cast<__type<_Tp>>(*__e);
> > > + else if constexpr (is_lvalue_reference_v<__iter_ref_t<_Tp>>)
> > > + return std::move(*static_cast<_Tp&&>(__e));
> > > else
> > > - return *__e;
> > > + return *static_cast<_Tp&&>(__e);
> > > }
> > > };
> > > } // namespace __imove
>
> New version attached, re-testing now...
LGTM
