[2/3] libstdc++: Implement ranges::iota from P2440R1

  Tested on x86_64-pc-linux-gnu, does this look OK for trunk?

libstdc++-v3/ChangeLog:

	* include/bits/ranges_algo.h (out_value_result): Define.
	(iota_result): Define.
	(__iota_fn, iota): Define.
	* testsuite/25_algorithms/iota/1.cc: New test.
---
 libstdc++-v3/include/bits/ranges_algo.h       | 48 +++++++++++++++++++
 .../testsuite/25_algorithms/iota/1.cc         | 29 +++++++++++
 2 files changed, 77 insertions(+)
 create mode 100644 libstdc++-v3/testsuite/25_algorithms/iota/1.cc
  

On Mon, 14 Nov 2022 at 04:52, Patrick Palka via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
>
> libstdc++-v3/ChangeLog:
>
>         * include/bits/ranges_algo.h (out_value_result): Define.
>         (iota_result): Define.
>         (__iota_fn, iota): Define.
>         * testsuite/25_algorithms/iota/1.cc: New test.
> ---
>  libstdc++-v3/include/bits/ranges_algo.h       | 48 +++++++++++++++++++
>  .../testsuite/25_algorithms/iota/1.cc         | 29 +++++++++++
>  2 files changed, 77 insertions(+)
>  create mode 100644 libstdc++-v3/testsuite/25_algorithms/iota/1.cc
>
> diff --git a/libstdc++-v3/include/bits/ranges_algo.h b/libstdc++-v3/include/bits/ranges_algo.h
> index da0ca981dc3..f003117c569 100644
> --- a/libstdc++-v3/include/bits/ranges_algo.h
> +++ b/libstdc++-v3/include/bits/ranges_algo.h
> @@ -3517,6 +3517,54 @@ namespace ranges
>    };
>
>    inline constexpr __contains_subrange_fn contains_subrange{};
> +
> +  template<typename _Out, typename _Tp>
> +    struct out_value_result
> +    {
> +      [[no_unique_address]] _Out out;
> +      [[no_unique_address]] _Tp value;
> +
> +      template<typename _Out2, typename _Tp2>
> +       requires convertible_to<const _Out&, _Out2>
> +         && convertible_to<const _Tp&, _Tp2>
> +       constexpr
> +       operator out_value_result<_Out2, _Tp2>() const &
> +       { return {out, value}; }
> +
> +      template<typename _Out2, typename _Tp2>
> +       requires convertible_to<_Out, _Out2>
> +         && convertible_to<_Tp, _Tp2>
> +       constexpr
> +       operator out_value_result<_Out2, _Tp2>() &&
> +       { return {std::move(out), std::move(value)}; }
> +    };
> +
> +  template<typename _Out, typename _Tp>
> +    using iota_result = out_value_result<_Out, _Tp>;
> +
> +  struct __iota_fn
> +  {
> +    template<input_or_output_iterator _Out, sentinel_for<_Out> _Sent, weakly_incrementable _Tp>
> +      requires indirectly_writable<_Out, const _Tp&>
> +      constexpr iota_result<_Out, _Tp>
> +      operator()(_Out __first, _Sent __last, _Tp __value) const
> +      {
> +       while (__first != __last)
> +         {
> +           *__first = static_cast<add_const_t<_Tp>&>(__value);

Is this any different to const_cast<const _Tp&>(__value) ?

We know _Tp must be the same as decay_t<_Tp> and non-void, because
it's passed by value, and therefore I think is_same_v<const _Tp,
add_const_t<_Tp>> is always true, isn't it? We don't need to care
about people saying ranges::iota.operator()<O,S,T&>(o,s,t), those
people are animals.

We would just change the function parameter to const _Tp which would
mean that *_first = __value; always uses a const lvalue, but maybe
that's a bit too subtle. The cast makes it more explicit what's
happening, especially the const_cast version.


> +           ++__first;
> +           ++__value;
> +         }
> +       return {std::move(__first), std::move(__value)};
> +      }
> +
> +    template<weakly_incrementable _Tp, output_range<const _Tp&> _Range>
> +      constexpr iota_result<borrowed_iterator_t<_Range>, _Tp>
> +      operator()(_Range&& __r, _Tp __value) const
> +      { return (*this)(ranges::begin(__r), ranges::end(__r), std::move(__value)); }
> +  };
> +
> +  inline constexpr __iota_fn iota{};
>  #endif // C++23
>  } // namespace ranges
>
> diff --git a/libstdc++-v3/testsuite/25_algorithms/iota/1.cc b/libstdc++-v3/testsuite/25_algorithms/iota/1.cc
> new file mode 100644
> index 00000000000..ad2bf08adf5
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/25_algorithms/iota/1.cc
> @@ -0,0 +1,29 @@
> +// { dg-options "-std=gnu++23" }
> +// { dg-do run { target c++23 } }
> +
> +#include <algorithm>
> +#include <testsuite_hooks.h>
> +#include <testsuite_iterators.h>
> +
> +namespace ranges = std::ranges;
> +
> +void
> +test01()
> +{
> +  int x[3] = {};
> +  __gnu_test::test_output_range<int> rx(x);
> +  auto r0 = ranges::iota(rx, 0);
> +  VERIFY( r0.out.ptr == x+3 );
> +  VERIFY( r0.value == 3 );
> +  VERIFY( ranges::equal(x, (int[]){0,1,2}) );
> +  auto r1 = ranges::iota(x, x+2, 5);
> +  VERIFY( r1.out == x+2 );
> +  VERIFY( r1.value == 7 );
> +  VERIFY( ranges::equal(x, (int[]){5,6,2}) );
> +}
> +
> +int
> +main()
> +{
> +  test01();
> +}
> --
> 2.38.1.420.g319605f8f0
>
  

Am Mo., 14. Nov. 2022 um 11:09 Uhr schrieb Jonathan Wakely via
Libstdc++ <libstdc++@gcc.gnu.org>:
>
> On Mon, 14 Nov 2022 at 04:52, Patrick Palka via Libstdc++
> <libstdc++@gcc.gnu.org> wrote:
> >
> > Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
> >
> > libstdc++-v3/ChangeLog:
> >
> >         * include/bits/ranges_algo.h (out_value_result): Define.
> >         (iota_result): Define.
> >         (__iota_fn, iota): Define.
> >         * testsuite/25_algorithms/iota/1.cc: New test.
> > ---
> >  libstdc++-v3/include/bits/ranges_algo.h       | 48 +++++++++++++++++++
> >  .../testsuite/25_algorithms/iota/1.cc         | 29 +++++++++++
> >  2 files changed, 77 insertions(+)
> >  create mode 100644 libstdc++-v3/testsuite/25_algorithms/iota/1.cc
> >
> > diff --git a/libstdc++-v3/include/bits/ranges_algo.h b/libstdc++-v3/include/bits/ranges_algo.h
> > index da0ca981dc3..f003117c569 100644
> > --- a/libstdc++-v3/include/bits/ranges_algo.h
> > +++ b/libstdc++-v3/include/bits/ranges_algo.h
> > @@ -3517,6 +3517,54 @@ namespace ranges
> >    };
> >
> >    inline constexpr __contains_subrange_fn contains_subrange{};
> > +
> > +  template<typename _Out, typename _Tp>
> > +    struct out_value_result
> > +    {
> > +      [[no_unique_address]] _Out out;
> > +      [[no_unique_address]] _Tp value;
> > +
> > +      template<typename _Out2, typename _Tp2>
> > +       requires convertible_to<const _Out&, _Out2>
> > +         && convertible_to<const _Tp&, _Tp2>
> > +       constexpr
> > +       operator out_value_result<_Out2, _Tp2>() const &
> > +       { return {out, value}; }
> > +
> > +      template<typename _Out2, typename _Tp2>
> > +       requires convertible_to<_Out, _Out2>
> > +         && convertible_to<_Tp, _Tp2>
> > +       constexpr
> > +       operator out_value_result<_Out2, _Tp2>() &&
> > +       { return {std::move(out), std::move(value)}; }
> > +    };
> > +
> > +  template<typename _Out, typename _Tp>
> > +    using iota_result = out_value_result<_Out, _Tp>;
> > +
> > +  struct __iota_fn
> > +  {
> > +    template<input_or_output_iterator _Out, sentinel_for<_Out> _Sent, weakly_incrementable _Tp>
> > +      requires indirectly_writable<_Out, const _Tp&>
> > +      constexpr iota_result<_Out, _Tp>
> > +      operator()(_Out __first, _Sent __last, _Tp __value) const
> > +      {
> > +       while (__first != __last)
> > +         {
> > +           *__first = static_cast<add_const_t<_Tp>&>(__value);
>
> Is this any different to const_cast<const _Tp&>(__value) ?

I think it is. const_cast<const _Tp&> can potentially mean the removal
of volatile, so I would always look with suspicion on const_cast<const
_Tp&>, while static_cast is clearer. Alternatively, as_const could be
used, which does add_const_t.

- Daniel
  

On Mon, 14 Nov 2022 at 10:17, Daniel Krügler <daniel.kruegler@gmail.com> wrote:
>
> Am Mo., 14. Nov. 2022 um 11:09 Uhr schrieb Jonathan Wakely via
> Libstdc++ <libstdc++@gcc.gnu.org>:
> >
> > On Mon, 14 Nov 2022 at 04:52, Patrick Palka via Libstdc++
> > <libstdc++@gcc.gnu.org> wrote:
> > >
> > > Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
> > >
> > > libstdc++-v3/ChangeLog:
> > >
> > >         * include/bits/ranges_algo.h (out_value_result): Define.
> > >         (iota_result): Define.
> > >         (__iota_fn, iota): Define.
> > >         * testsuite/25_algorithms/iota/1.cc: New test.
> > > ---
> > >  libstdc++-v3/include/bits/ranges_algo.h       | 48 +++++++++++++++++++
> > >  .../testsuite/25_algorithms/iota/1.cc         | 29 +++++++++++
> > >  2 files changed, 77 insertions(+)
> > >  create mode 100644 libstdc++-v3/testsuite/25_algorithms/iota/1.cc
> > >
> > > diff --git a/libstdc++-v3/include/bits/ranges_algo.h b/libstdc++-v3/include/bits/ranges_algo.h
> > > index da0ca981dc3..f003117c569 100644
> > > --- a/libstdc++-v3/include/bits/ranges_algo.h
> > > +++ b/libstdc++-v3/include/bits/ranges_algo.h
> > > @@ -3517,6 +3517,54 @@ namespace ranges
> > >    };
> > >
> > >    inline constexpr __contains_subrange_fn contains_subrange{};
> > > +
> > > +  template<typename _Out, typename _Tp>
> > > +    struct out_value_result
> > > +    {
> > > +      [[no_unique_address]] _Out out;
> > > +      [[no_unique_address]] _Tp value;
> > > +
> > > +      template<typename _Out2, typename _Tp2>
> > > +       requires convertible_to<const _Out&, _Out2>
> > > +         && convertible_to<const _Tp&, _Tp2>
> > > +       constexpr
> > > +       operator out_value_result<_Out2, _Tp2>() const &
> > > +       { return {out, value}; }
> > > +
> > > +      template<typename _Out2, typename _Tp2>
> > > +       requires convertible_to<_Out, _Out2>
> > > +         && convertible_to<_Tp, _Tp2>
> > > +       constexpr
> > > +       operator out_value_result<_Out2, _Tp2>() &&
> > > +       { return {std::move(out), std::move(value)}; }
> > > +    };
> > > +
> > > +  template<typename _Out, typename _Tp>
> > > +    using iota_result = out_value_result<_Out, _Tp>;
> > > +
> > > +  struct __iota_fn
> > > +  {
> > > +    template<input_or_output_iterator _Out, sentinel_for<_Out> _Sent, weakly_incrementable _Tp>
> > > +      requires indirectly_writable<_Out, const _Tp&>
> > > +      constexpr iota_result<_Out, _Tp>
> > > +      operator()(_Out __first, _Sent __last, _Tp __value) const
> > > +      {
> > > +       while (__first != __last)
> > > +         {
> > > +           *__first = static_cast<add_const_t<_Tp>&>(__value);
> >
> > Is this any different to const_cast<const _Tp&>(__value) ?
>
> I think it is. const_cast<const _Tp&> can potentially mean the removal
> of volatile,

True.

> so I would always look with suspicion on const_cast<const
> _Tp&>, while static_cast is clearer. Alternatively, as_const could be
> used, which does add_const_t.

Which means evaluating the add_const trait *and* overload resolution
for as_const* *and* a runtime function call.

Let's go with static_cast<const _Tp&>.
  

On Mon, 14 Nov 2022, Jonathan Wakely wrote:

> On Mon, 14 Nov 2022 at 10:17, Daniel Krügler <daniel.kruegler@gmail.com> wrote:
> >
> > Am Mo., 14. Nov. 2022 um 11:09 Uhr schrieb Jonathan Wakely via
> > Libstdc++ <libstdc++@gcc.gnu.org>:
> > >
> > > On Mon, 14 Nov 2022 at 04:52, Patrick Palka via Libstdc++
> > > <libstdc++@gcc.gnu.org> wrote:
> > > >
> > > > Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
> > > >
> > > > libstdc++-v3/ChangeLog:
> > > >
> > > >         * include/bits/ranges_algo.h (out_value_result): Define.
> > > >         (iota_result): Define.
> > > >         (__iota_fn, iota): Define.
> > > >         * testsuite/25_algorithms/iota/1.cc: New test.
> > > > ---
> > > >  libstdc++-v3/include/bits/ranges_algo.h       | 48 +++++++++++++++++++
> > > >  .../testsuite/25_algorithms/iota/1.cc         | 29 +++++++++++
> > > >  2 files changed, 77 insertions(+)
> > > >  create mode 100644 libstdc++-v3/testsuite/25_algorithms/iota/1.cc
> > > >
> > > > diff --git a/libstdc++-v3/include/bits/ranges_algo.h b/libstdc++-v3/include/bits/ranges_algo.h
> > > > index da0ca981dc3..f003117c569 100644
> > > > --- a/libstdc++-v3/include/bits/ranges_algo.h
> > > > +++ b/libstdc++-v3/include/bits/ranges_algo.h
> > > > @@ -3517,6 +3517,54 @@ namespace ranges
> > > >    };
> > > >
> > > >    inline constexpr __contains_subrange_fn contains_subrange{};
> > > > +
> > > > +  template<typename _Out, typename _Tp>
> > > > +    struct out_value_result
> > > > +    {
> > > > +      [[no_unique_address]] _Out out;
> > > > +      [[no_unique_address]] _Tp value;
> > > > +
> > > > +      template<typename _Out2, typename _Tp2>
> > > > +       requires convertible_to<const _Out&, _Out2>
> > > > +         && convertible_to<const _Tp&, _Tp2>
> > > > +       constexpr
> > > > +       operator out_value_result<_Out2, _Tp2>() const &
> > > > +       { return {out, value}; }
> > > > +
> > > > +      template<typename _Out2, typename _Tp2>
> > > > +       requires convertible_to<_Out, _Out2>
> > > > +         && convertible_to<_Tp, _Tp2>
> > > > +       constexpr
> > > > +       operator out_value_result<_Out2, _Tp2>() &&
> > > > +       { return {std::move(out), std::move(value)}; }
> > > > +    };
> > > > +
> > > > +  template<typename _Out, typename _Tp>
> > > > +    using iota_result = out_value_result<_Out, _Tp>;
> > > > +
> > > > +  struct __iota_fn
> > > > +  {
> > > > +    template<input_or_output_iterator _Out, sentinel_for<_Out> _Sent, weakly_incrementable _Tp>
> > > > +      requires indirectly_writable<_Out, const _Tp&>
> > > > +      constexpr iota_result<_Out, _Tp>
> > > > +      operator()(_Out __first, _Sent __last, _Tp __value) const
> > > > +      {
> > > > +       while (__first != __last)
> > > > +         {
> > > > +           *__first = static_cast<add_const_t<_Tp>&>(__value);
> > >
> > > Is this any different to const_cast<const _Tp&>(__value) ?
> >
> > I think it is. const_cast<const _Tp&> can potentially mean the removal
> > of volatile,
> 
> True.
> 
> > so I would always look with suspicion on const_cast<const
> > _Tp&>, while static_cast is clearer. Alternatively, as_const could be
> > used, which does add_const_t.
> 
> Which means evaluating the add_const trait *and* overload resolution
> for as_const* *and* a runtime function call.
> 
> Let's go with static_cast<const _Tp&>.

Sounds good, like so?

-- >8 --


Subject: [PATCH 2/3] libstdc++: Implement ranges::iota from P2440R1

libstdc++-v3/ChangeLog:

	* include/bits/ranges_algo.h (out_value_result): Define.
	(iota_result): Define.
	(__iota_fn, iota): Define.
	* testsuite/25_algorithms/iota/1.cc: New test.
---
 libstdc++-v3/include/bits/ranges_algo.h       | 48 +++++++++++++++++++
 .../testsuite/25_algorithms/iota/1.cc         | 29 +++++++++++
 2 files changed, 77 insertions(+)
 create mode 100644 libstdc++-v3/testsuite/25_algorithms/iota/1.cc

diff --git a/libstdc++-v3/include/bits/ranges_algo.h b/libstdc++-v3/include/bits/ranges_algo.h
index 11206bdbcaa..f75735f02cb 100644
--- a/libstdc++-v3/include/bits/ranges_algo.h
+++ b/libstdc++-v3/include/bits/ranges_algo.h
@@ -3517,6 +3517,54 @@ namespace ranges
   };
 
   inline constexpr __contains_subrange_fn contains_subrange{};
+
+  template<typename _Out, typename _Tp>
+    struct out_value_result
+    {
+      [[no_unique_address]] _Out out;
+      [[no_unique_address]] _Tp value;
+
+      template<typename _Out2, typename _Tp2>
+	requires convertible_to<const _Out&, _Out2>
+	  && convertible_to<const _Tp&, _Tp2>
+	constexpr
+	operator out_value_result<_Out2, _Tp2>() const &
+	{ return {out, value}; }
+
+      template<typename _Out2, typename _Tp2>
+	requires convertible_to<_Out, _Out2>
+	  && convertible_to<_Tp, _Tp2>
+	constexpr
+	operator out_value_result<_Out2, _Tp2>() &&
+	{ return {std::move(out), std::move(value)}; }
+    };
+
+  template<typename _Out, typename _Tp>
+    using iota_result = out_value_result<_Out, _Tp>;
+
+  struct __iota_fn
+  {
+    template<input_or_output_iterator _Out, sentinel_for<_Out> _Sent, weakly_incrementable _Tp>
+      requires indirectly_writable<_Out, const _Tp&>
+      constexpr iota_result<_Out, _Tp>
+      operator()(_Out __first, _Sent __last, _Tp __value) const
+      {
+	while (__first != __last)
+	  {
+	    *__first = static_cast<const _Tp&>(__value);
+	    ++__first;
+	    ++__value;
+	  }
+	return {std::move(__first), std::move(__value)};
+      }
+
+    template<weakly_incrementable _Tp, output_range<const _Tp&> _Range>
+      constexpr iota_result<borrowed_iterator_t<_Range>, _Tp>
+      operator()(_Range&& __r, _Tp __value) const
+      { return (*this)(ranges::begin(__r), ranges::end(__r), std::move(__value)); }
+  };
+
+  inline constexpr __iota_fn iota{};
 #endif // C++23
 } // namespace ranges
 
diff --git a/libstdc++-v3/testsuite/25_algorithms/iota/1.cc b/libstdc++-v3/testsuite/25_algorithms/iota/1.cc
new file mode 100644
index 00000000000..ad2bf08adf5
--- /dev/null
+++ b/libstdc++-v3/testsuite/25_algorithms/iota/1.cc
@@ -0,0 +1,29 @@
+// { dg-options "-std=gnu++23" }
+// { dg-do run { target c++23 } }
+
+#include <algorithm>
+#include <testsuite_hooks.h>
+#include <testsuite_iterators.h>
+
+namespace ranges = std::ranges;
+
+void
+test01()
+{
+  int x[3] = {};
+  __gnu_test::test_output_range<int> rx(x);
+  auto r0 = ranges::iota(rx, 0);
+  VERIFY( r0.out.ptr == x+3 );
+  VERIFY( r0.value == 3 );
+  VERIFY( ranges::equal(x, (int[]){0,1,2}) );
+  auto r1 = ranges::iota(x, x+2, 5);
+  VERIFY( r1.out == x+2 );
+  VERIFY( r1.value == 7 );
+  VERIFY( ranges::equal(x, (int[]){5,6,2}) );
+}
+
+int
+main()
+{
+  test01();
+}
  

On Mon, 14 Nov 2022 at 15:11, Patrick Palka <ppalka@redhat.com> wrote:
>
> On Mon, 14 Nov 2022, Jonathan Wakely wrote:
>
> > On Mon, 14 Nov 2022 at 10:17, Daniel Krügler <daniel.kruegler@gmail.com> wrote:
> > >
> > > Am Mo., 14. Nov. 2022 um 11:09 Uhr schrieb Jonathan Wakely via
> > > Libstdc++ <libstdc++@gcc.gnu.org>:
> > > >
> > > > On Mon, 14 Nov 2022 at 04:52, Patrick Palka via Libstdc++
> > > > <libstdc++@gcc.gnu.org> wrote:
> > > > >
> > > > > Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
> > > > >
> > > > > libstdc++-v3/ChangeLog:
> > > > >
> > > > >         * include/bits/ranges_algo.h (out_value_result): Define.
> > > > >         (iota_result): Define.
> > > > >         (__iota_fn, iota): Define.
> > > > >         * testsuite/25_algorithms/iota/1.cc: New test.
> > > > > ---
> > > > >  libstdc++-v3/include/bits/ranges_algo.h       | 48 +++++++++++++++++++
> > > > >  .../testsuite/25_algorithms/iota/1.cc         | 29 +++++++++++
> > > > >  2 files changed, 77 insertions(+)
> > > > >  create mode 100644 libstdc++-v3/testsuite/25_algorithms/iota/1.cc
> > > > >
> > > > > diff --git a/libstdc++-v3/include/bits/ranges_algo.h b/libstdc++-v3/include/bits/ranges_algo.h
> > > > > index da0ca981dc3..f003117c569 100644
> > > > > --- a/libstdc++-v3/include/bits/ranges_algo.h
> > > > > +++ b/libstdc++-v3/include/bits/ranges_algo.h
> > > > > @@ -3517,6 +3517,54 @@ namespace ranges
> > > > >    };
> > > > >
> > > > >    inline constexpr __contains_subrange_fn contains_subrange{};
> > > > > +
> > > > > +  template<typename _Out, typename _Tp>
> > > > > +    struct out_value_result
> > > > > +    {
> > > > > +      [[no_unique_address]] _Out out;
> > > > > +      [[no_unique_address]] _Tp value;
> > > > > +
> > > > > +      template<typename _Out2, typename _Tp2>
> > > > > +       requires convertible_to<const _Out&, _Out2>
> > > > > +         && convertible_to<const _Tp&, _Tp2>
> > > > > +       constexpr
> > > > > +       operator out_value_result<_Out2, _Tp2>() const &
> > > > > +       { return {out, value}; }
> > > > > +
> > > > > +      template<typename _Out2, typename _Tp2>
> > > > > +       requires convertible_to<_Out, _Out2>
> > > > > +         && convertible_to<_Tp, _Tp2>
> > > > > +       constexpr
> > > > > +       operator out_value_result<_Out2, _Tp2>() &&
> > > > > +       { return {std::move(out), std::move(value)}; }
> > > > > +    };
> > > > > +
> > > > > +  template<typename _Out, typename _Tp>
> > > > > +    using iota_result = out_value_result<_Out, _Tp>;
> > > > > +
> > > > > +  struct __iota_fn
> > > > > +  {
> > > > > +    template<input_or_output_iterator _Out, sentinel_for<_Out> _Sent, weakly_incrementable _Tp>
> > > > > +      requires indirectly_writable<_Out, const _Tp&>
> > > > > +      constexpr iota_result<_Out, _Tp>
> > > > > +      operator()(_Out __first, _Sent __last, _Tp __value) const
> > > > > +      {
> > > > > +       while (__first != __last)
> > > > > +         {
> > > > > +           *__first = static_cast<add_const_t<_Tp>&>(__value);
> > > >
> > > > Is this any different to const_cast<const _Tp&>(__value) ?
> > >
> > > I think it is. const_cast<const _Tp&> can potentially mean the removal
> > > of volatile,
> >
> > True.
> >
> > > so I would always look with suspicion on const_cast<const
> > > _Tp&>, while static_cast is clearer. Alternatively, as_const could be
> > > used, which does add_const_t.
> >
> > Which means evaluating the add_const trait *and* overload resolution
> > for as_const* *and* a runtime function call.
> >
> > Let's go with static_cast<const _Tp&>.
>
> Sounds good, like so?

OK for trunk, thanks.


>
> -- >8 --
>
>
> Subject: [PATCH 2/3] libstdc++: Implement ranges::iota from P2440R1
>
> libstdc++-v3/ChangeLog:
>
>         * include/bits/ranges_algo.h (out_value_result): Define.
>         (iota_result): Define.
>         (__iota_fn, iota): Define.
>         * testsuite/25_algorithms/iota/1.cc: New test.
> ---
>  libstdc++-v3/include/bits/ranges_algo.h       | 48 +++++++++++++++++++
>  .../testsuite/25_algorithms/iota/1.cc         | 29 +++++++++++
>  2 files changed, 77 insertions(+)
>  create mode 100644 libstdc++-v3/testsuite/25_algorithms/iota/1.cc
>
> diff --git a/libstdc++-v3/include/bits/ranges_algo.h b/libstdc++-v3/include/bits/ranges_algo.h
> index 11206bdbcaa..f75735f02cb 100644
> --- a/libstdc++-v3/include/bits/ranges_algo.h
> +++ b/libstdc++-v3/include/bits/ranges_algo.h
> @@ -3517,6 +3517,54 @@ namespace ranges
>    };
>
>    inline constexpr __contains_subrange_fn contains_subrange{};
> +
> +  template<typename _Out, typename _Tp>
> +    struct out_value_result
> +    {
> +      [[no_unique_address]] _Out out;
> +      [[no_unique_address]] _Tp value;
> +
> +      template<typename _Out2, typename _Tp2>
> +       requires convertible_to<const _Out&, _Out2>
> +         && convertible_to<const _Tp&, _Tp2>
> +       constexpr
> +       operator out_value_result<_Out2, _Tp2>() const &
> +       { return {out, value}; }
> +
> +      template<typename _Out2, typename _Tp2>
> +       requires convertible_to<_Out, _Out2>
> +         && convertible_to<_Tp, _Tp2>
> +       constexpr
> +       operator out_value_result<_Out2, _Tp2>() &&
> +       { return {std::move(out), std::move(value)}; }
> +    };
> +
> +  template<typename _Out, typename _Tp>
> +    using iota_result = out_value_result<_Out, _Tp>;
> +
> +  struct __iota_fn
> +  {
> +    template<input_or_output_iterator _Out, sentinel_for<_Out> _Sent, weakly_incrementable _Tp>
> +      requires indirectly_writable<_Out, const _Tp&>
> +      constexpr iota_result<_Out, _Tp>
> +      operator()(_Out __first, _Sent __last, _Tp __value) const
> +      {
> +       while (__first != __last)
> +         {
> +           *__first = static_cast<const _Tp&>(__value);
> +           ++__first;
> +           ++__value;
> +         }
> +       return {std::move(__first), std::move(__value)};
> +      }
> +
> +    template<weakly_incrementable _Tp, output_range<const _Tp&> _Range>
> +      constexpr iota_result<borrowed_iterator_t<_Range>, _Tp>
> +      operator()(_Range&& __r, _Tp __value) const
> +      { return (*this)(ranges::begin(__r), ranges::end(__r), std::move(__value)); }
> +  };
> +
> +  inline constexpr __iota_fn iota{};
>  #endif // C++23
>  } // namespace ranges
>
> diff --git a/libstdc++-v3/testsuite/25_algorithms/iota/1.cc b/libstdc++-v3/testsuite/25_algorithms/iota/1.cc
> new file mode 100644
> index 00000000000..ad2bf08adf5
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/25_algorithms/iota/1.cc
> @@ -0,0 +1,29 @@
> +// { dg-options "-std=gnu++23" }
> +// { dg-do run { target c++23 } }
> +
> +#include <algorithm>
> +#include <testsuite_hooks.h>
> +#include <testsuite_iterators.h>
> +
> +namespace ranges = std::ranges;
> +
> +void
> +test01()
> +{
> +  int x[3] = {};
> +  __gnu_test::test_output_range<int> rx(x);
> +  auto r0 = ranges::iota(rx, 0);
> +  VERIFY( r0.out.ptr == x+3 );
> +  VERIFY( r0.value == 3 );
> +  VERIFY( ranges::equal(x, (int[]){0,1,2}) );
> +  auto r1 = ranges::iota(x, x+2, 5);
> +  VERIFY( r1.out == x+2 );
> +  VERIFY( r1.value == 7 );
> +  VERIFY( ranges::equal(x, (int[]){5,6,2}) );
> +}
> +
> +int
> +main()
> +{
> +  test01();
> +}
> --
> 2.38.1.420.g319605f8f0