[1/2] libstdc++: Implement ranges::adjacent_view from P2321R2

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

libstdc++-v3/ChangeLog:

	* include/std/ranges (adjacent_view): Define.
	(enable_borrowed_range<adjacent_view>): Define.
	(__detail::__repeated_tuple): Define.
	(adjacent_view::_Iterator): Define.
	(adjacent_view::_Sentinel): Define.
	(views::__detail::__can_adjacent_view): Define.
	(views::_Adjacent): Define.
	(views::adjacent): Define.
	(views::pairwise): Define.
	* testsuite/std/ranges/adaptors/adjacent/1.cc: New test.
---
 libstdc++-v3/include/std/ranges               | 359 ++++++++++++++++++
 .../std/ranges/adaptors/adjacent/1.cc         | 110 ++++++
 2 files changed, 469 insertions(+)
 create mode 100644 libstdc++-v3/testsuite/std/ranges/adaptors/adjacent/1.cc
  

On Tue, 30 Aug 2022 at 18:14, Patrick Palka via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk?


> +    constexpr
> +    _Iterator(__as_sentinel, iterator_t<_Base> __first, iterator_t<_Base> __last)
> +    {
> +      if constexpr (!bidirectional_range<_Base>)
> +       for (auto& __it : _M_current)
> +         __it = __last;
> +      else
> +       for (ssize_t __i = _Nm-1; __i >= 0; --__i)

ssize_t is defined by POSIX in <sys/types.h> but isn't in ISO C or
C++. It looks like MinGW defines it to ... something ... sometimes,
but I don't think we can rely on it for non-POSIX targets.


> +    template<size_t _Nm>
> +      struct _Adjacent : __adaptor::_RangeAdaptorClosure
> +      {
> +       template<viewable_range _Range>
> +         requires (_Nm == 0) || __detail::__can_adjacent_view<_Nm, _Range>
> +         [[nodiscard]]
> +         constexpr auto
> +         operator()(_Range&& __r) const

Does this attribute actually work here?

I thought we needed to use `operator() [[nodiscard]]` for functions
with a requires-clause, because otherwise the attribute gives a parse
error. Maybe I've misremembered the problem, and it just doesn't give
a -Wunused-result warning. The decl above is setting off my spidey
sense for some reason though.
  

On Wed, 31 Aug 2022, Jonathan Wakely wrote:

> On Tue, 30 Aug 2022 at 18:14, Patrick Palka via Libstdc++
> <libstdc++@gcc.gnu.org> wrote:
> >
> > Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
> 
> 
> > +    constexpr
> > +    _Iterator(__as_sentinel, iterator_t<_Base> __first, iterator_t<_Base> __last)
> > +    {
> > +      if constexpr (!bidirectional_range<_Base>)
> > +       for (auto& __it : _M_current)
> > +         __it = __last;
> > +      else
> > +       for (ssize_t __i = _Nm-1; __i >= 0; --__i)
> 
> ssize_t is defined by POSIX in <sys/types.h> but isn't in ISO C or
> C++. It looks like MinGW defines it to ... something ... sometimes,
> but I don't think we can rely on it for non-POSIX targets.

I see.  Using ssize_t makes the loop a little cleaner but it's hardly
necessary, so consider it rewritten into:

  for (size_t __i = 0; __i < _Nm; ++__i)
    {
      _M_current[_Nm - 1 - __i] = __last;
      ranges::advance(__last, -1, __first);
    }

> 
> 
> > +    template<size_t _Nm>
> > +      struct _Adjacent : __adaptor::_RangeAdaptorClosure
> > +      {
> > +       template<viewable_range _Range>
> > +         requires (_Nm == 0) || __detail::__can_adjacent_view<_Nm, _Range>
> > +         [[nodiscard]]
> > +         constexpr auto
> > +         operator()(_Range&& __r) const
> 
> Does this attribute actually work here?
> 
> I thought we needed to use `operator() [[nodiscard]]` for functions
> with a requires-clause, because otherwise the attribute gives a parse
> error. Maybe I've misremembered the problem, and it just doesn't give
> a -Wunused-result warning. The decl above is setting off my spidey
> sense for some reason though.

Oops yeah, it looks like this position of [[nodiscard]] works with
standard concepts, but it's a parse error with -fconcepts-ts due to the
different requires-clause grammar.

Here's v2 which avoids using ssize_t, and puts the [[nodiscard]] after
'operator()' (_Zip and _ZipTransform have the same issue which I can
fix separately):

-- >8 --

libstdc++-v3/ChangeLog:

	* include/std/ranges (adjacent_view): Define.
	(enable_borrowed_range<adjacent_view>): Define.
	(__detail::__repeated_tuple): Define.
	(adjacent_view::_Iterator): Define.
	(adjacent_view::_Sentinel): Define.
	(views::__detail::__can_adjacent_view): Define.
	(views::_Adjacent): Define.
	(views::adjacent): Define.
	(views::pairwise): Define.
	* testsuite/std/ranges/adaptors/adjacent/1.cc: New test.
---
 libstdc++-v3/include/std/ranges               | 358 ++++++++++++++++++
 .../std/ranges/adaptors/adjacent/1.cc         | 110 ++++++
 2 files changed, 468 insertions(+)
 create mode 100644 libstdc++-v3/testsuite/std/ranges/adaptors/adjacent/1.cc

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 6e2e561ed12..2352aad76fc 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -5081,6 +5081,364 @@ namespace views::__adaptor
 
     inline constexpr _ZipTransform zip_transform;
   }
+
+  template<forward_range _Vp, size_t _Nm>
+    requires view<_Vp> && (_Nm > 0)
+  class adjacent_view : public view_interface<adjacent_view<_Vp, _Nm>>
+  {
+    _Vp _M_base = _Vp();
+
+    template<bool> class _Iterator;
+    template<bool> class _Sentinel;
+
+    struct __as_sentinel
+    { };
+
+  public:
+    adjacent_view() requires default_initializable<_Vp> = default;
+
+    constexpr explicit
+    adjacent_view(_Vp __base)
+      : _M_base(std::move(__base))
+    { }
+
+    constexpr auto
+    begin() requires (!__detail::__simple_view<_Vp>)
+    { return _Iterator<false>(ranges::begin(_M_base), ranges::end(_M_base)); }
+
+    constexpr auto
+    begin() const requires range<const _Vp>
+    { return _Iterator<true>(ranges::begin(_M_base), ranges::end(_M_base)); }
+
+    constexpr auto
+    end() requires (!__detail::__simple_view<_Vp>)
+    {
+      if constexpr (common_range<_Vp>)
+	return _Iterator<false>(__as_sentinel{}, ranges::begin(_M_base), ranges::end(_M_base));
+      else
+	return _Sentinel<false>(ranges::end(_M_base));
+    }
+
+    constexpr auto
+    end() const requires range<const _Vp>
+    {
+      if constexpr (common_range<const _Vp>)
+	return _Iterator<true>(__as_sentinel{}, ranges::begin(_M_base), ranges::end(_M_base));
+      else
+	return _Sentinel<true>(ranges::end(_M_base));
+    }
+
+    constexpr auto
+    size() requires sized_range<_Vp>
+    {
+      using _ST = decltype(ranges::size(_M_base));
+      using _CT = common_type_t<_ST, size_t>;
+      auto __sz = static_cast<_CT>(ranges::size(_M_base));
+      __sz -= std::min<_CT>(__sz, _Nm - 1);
+      return static_cast<_ST>(__sz);
+    }
+
+    constexpr auto
+    size() const requires sized_range<const _Vp>
+    {
+      using _ST = decltype(ranges::size(_M_base));
+      using _CT = common_type_t<_ST, size_t>;
+      auto __sz = static_cast<_CT>(ranges::size(_M_base));
+      __sz -= std::min<_CT>(__sz, _Nm - 1);
+      return static_cast<_ST>(__sz);
+    }
+  };
+
+  template<typename _Vp, size_t _Nm>
+    inline constexpr bool enable_borrowed_range<adjacent_view<_Vp, _Nm>>
+      = enable_borrowed_range<_Vp>;
+
+  namespace __detail
+  {
+    // Yields tuple<_Tp, ..., _Tp> with _Nm elements.
+    template<typename _Tp, size_t _Nm>
+      using __repeated_tuple = decltype(std::tuple_cat(std::declval<array<_Tp, _Nm>>()));
+  }
+
+  template<forward_range _Vp, size_t _Nm>
+    requires view<_Vp> && (_Nm > 0)
+  template<bool _Const>
+  class adjacent_view<_Vp, _Nm>::_Iterator
+  {
+    using _Base = __detail::__maybe_const_t<_Const, _Vp>;
+    array<iterator_t<_Base>, _Nm> _M_current = array<iterator_t<_Base>, _Nm>();
+
+    constexpr
+    _Iterator(iterator_t<_Base> __first, sentinel_t<_Base> __last)
+    {
+      for (auto& __i : _M_current)
+	{
+	  __i = __first;
+	  ranges::advance(__first, 1, __last);
+	}
+    }
+
+    constexpr
+    _Iterator(__as_sentinel, iterator_t<_Base> __first, iterator_t<_Base> __last)
+    {
+      if constexpr (!bidirectional_range<_Base>)
+	for (auto& __it : _M_current)
+	  __it = __last;
+      else
+	for (size_t __i = 0; __i < _Nm; ++__i)
+	  {
+	    _M_current[_Nm - 1 - __i] = __last;
+	    ranges::advance(__last, -1, __first);
+	  }
+    }
+
+    static auto
+    _S_iter_concept()
+    {
+      if constexpr (random_access_range<_Base>)
+	return random_access_iterator_tag{};
+      else if constexpr (bidirectional_range<_Base>)
+	return bidirectional_iterator_tag{};
+      else
+	return forward_iterator_tag{};
+    }
+
+    friend class adjacent_view;
+
+  public:
+    using iterator_category = input_iterator_tag;
+    using iterator_concept = decltype(_S_iter_concept());
+    using value_type = conditional_t<_Nm == 2,
+				     pair<range_value_t<_Base>, range_value_t<_Base>>,
+				     __detail::__repeated_tuple<range_value_t<_Base>, _Nm>>;
+    using difference_type = range_difference_t<_Base>;
+
+    _Iterator() = default;
+
+    constexpr
+    _Iterator(_Iterator<!_Const> __i)
+      requires _Const && convertible_to<iterator_t<_Vp>, iterator_t<_Base>>
+    {
+      for (size_t __j = 0; __j < _Nm; ++__j)
+	_M_current[__j] = std::move(__i[__j]);
+    }
+
+    constexpr auto
+    operator*() const
+    {
+      auto __f = [](auto& __i) -> decltype(auto) { return *__i; };
+      return __detail::__tuple_transform(__f, _M_current);
+    }
+
+    constexpr _Iterator&
+    operator++()
+    {
+      for (auto& __i : _M_current)
+	++__i;
+      return *this;
+    }
+
+    constexpr _Iterator
+    operator++(int)
+    {
+      auto __tmp = *this;
+      ++*this;
+      return __tmp;
+    }
+
+    constexpr _Iterator&
+    operator--() requires bidirectional_range<_Base>
+    {
+      for (auto& __i : _M_current)
+	--__i;
+      return *this;
+    }
+
+    constexpr _Iterator
+    operator--(int) requires bidirectional_range<_Base>
+    {
+      auto __tmp = *this;
+      --*this;
+      return __tmp;
+    }
+
+    constexpr _Iterator&
+    operator+=(difference_type __x)
+      requires random_access_range<_Base>
+    {
+      for (auto& __i : _M_current)
+	__i += __x;
+      return *this;
+    }
+
+    constexpr _Iterator&
+    operator-=(difference_type __x)
+      requires random_access_range<_Base>
+    {
+      for (auto& __i : _M_current)
+	__i -= __x;
+      return *this;
+    }
+
+    constexpr auto
+    operator[](difference_type __n) const
+      requires random_access_range<_Base>
+    {
+      auto __f = [&](auto& __i) -> decltype(auto) { return __i[__n]; };
+      return __detail::__tuple_transform(__f, _M_current);
+    }
+
+    friend constexpr bool
+    operator==(const _Iterator& __x, const _Iterator& __y)
+    { return __x._M_current.back() == __y._M_current.back(); }
+
+    friend constexpr bool
+    operator<(const _Iterator& __x, const _Iterator& __y)
+      requires random_access_range<_Base>
+    { return __x._M_current.back() < __y._M_current.back(); }
+
+    friend constexpr bool
+    operator>(const _Iterator& __x, const _Iterator& __y)
+      requires random_access_range<_Base>
+    { return __y < __x; }
+
+    friend constexpr bool
+    operator<=(const _Iterator& __x, const _Iterator& __y)
+      requires random_access_range<_Base>
+    { return !(__y < __x); }
+
+    friend constexpr bool
+    operator>=(const _Iterator& __x, const _Iterator& __y)
+      requires random_access_range<_Base>
+    { return !(__x < __y); }
+
+    friend constexpr auto
+    operator<=>(const _Iterator& __x, const _Iterator& __y)
+      requires random_access_range<_Base>
+	&& three_way_comparable<iterator_t<_Base>>
+    { return __x._M_current.back() <=> __y._M_current.back(); }
+
+    friend constexpr _Iterator
+    operator+(const _Iterator& __i, difference_type __n)
+      requires random_access_range<_Base>
+    {
+      auto __r = __i;
+      __r += __n;
+      return __r;
+    }
+
+    friend constexpr _Iterator
+    operator+(difference_type __n, const _Iterator& __i)
+      requires random_access_range<_Base>
+    {
+      auto __r = __i;
+      __r += __n;
+      return __r;
+    }
+
+    friend constexpr _Iterator
+    operator-(const _Iterator& __i, difference_type __n)
+      requires random_access_range<_Base>
+    {
+      auto __r = __i;
+      __r -= __n;
+      return __r;
+    }
+
+    friend constexpr difference_type
+    operator-(const _Iterator& __x, const _Iterator& __y)
+      requires sized_sentinel_for<iterator_t<_Base>, iterator_t<_Base>>
+    { return __x._M_current.back() - __y._M_current.back(); }
+
+    friend constexpr auto
+    iter_move(const _Iterator& __i)
+    { return __detail::__tuple_transform(ranges::iter_move, __i._M_current); }
+
+    friend constexpr void
+    iter_swap(const _Iterator& __l, const _Iterator& __r)
+      requires indirectly_swappable<iterator_t<_Base>>
+    {
+      for (size_t __i = 0; __i < _Nm; __i++)
+	ranges::iter_swap(__l._M_current[__i], __r._M_current[__i]);
+    }
+  };
+
+  template<forward_range _Vp, size_t _Nm>
+    requires view<_Vp> && (_Nm > 0)
+  template<bool _Const>
+  class adjacent_view<_Vp, _Nm>::_Sentinel
+  {
+    using _Base = __detail::__maybe_const_t<_Const, _Vp>;
+
+    sentinel_t<_Base> _M_end = sentinel_t<_Base>();
+
+    constexpr explicit
+    _Sentinel(sentinel_t<_Base> __end)
+      : _M_end(__end)
+    { }
+
+    friend class adjacent_view;
+
+  public:
+    _Sentinel() = default;
+
+    constexpr
+    _Sentinel(_Sentinel<!_Const> __i)
+      requires _Const && convertible_to<sentinel_t<_Vp>, sentinel_t<_Base>>
+      : _M_end(std::move(__i._M_end))
+    { }
+
+    template<bool _OtherConst>
+      requires sentinel_for<sentinel_t<_Base>,
+			    iterator_t<__detail::__maybe_const_t<_OtherConst, _Vp>>>
+    friend constexpr bool
+    operator==(const _Iterator<_OtherConst>& __x, const _Sentinel& __y)
+    { return __x._M_current.back() == __y._M_end; }
+
+    template<bool _OtherConst>
+      requires sized_sentinel_for<sentinel_t<_Base>,
+				  iterator_t<__detail::__maybe_const_t<_OtherConst, _Vp>>>
+    friend constexpr range_difference_t<__detail::__maybe_const_t<_OtherConst, _Vp>>
+    operator-(const _Iterator<_OtherConst>& __x, const _Sentinel& __y)
+    { return __x._M_current.back() - __y._M_end; }
+
+    template<bool _OtherConst>
+      requires sized_sentinel_for<sentinel_t<_Base>,
+				  iterator_t<__detail::__maybe_const_t<_OtherConst, _Vp>>>
+    friend constexpr range_difference_t<__detail::__maybe_const_t<_OtherConst, _Vp>>
+    operator-(const _Sentinel& __y, const _Iterator<_OtherConst>& __x)
+    { return __y._M_end - __x._M_current.back(); }
+  };
+
+  namespace views
+  {
+    namespace __detail
+    {
+      template<size_t _Nm, typename _Range>
+	concept __can_adjacent_view
+	  = requires { adjacent_view<all_t<_Range>, _Nm>(std::declval<_Range>()); };
+    }
+
+    template<size_t _Nm>
+      struct _Adjacent : __adaptor::_RangeAdaptorClosure
+      {
+	template<viewable_range _Range>
+	  requires (_Nm == 0) || __detail::__can_adjacent_view<_Nm, _Range>
+	  constexpr auto
+	  operator() [[nodiscard]] (_Range&& __r) const
+	  {
+	    if constexpr (_Nm == 0)
+	      return views::empty<tuple<>>;
+	    else
+	      return adjacent_view<all_t<_Range>, _Nm>(std::forward<_Range>(__r));
+	  }
+      };
+
+    template<size_t _Nm>
+      inline constexpr _Adjacent<_Nm> adjacent;
+
+    inline constexpr auto pairwise = adjacent<2>;
+  }
 #endif // C++23
 } // namespace ranges
 
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent/1.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent/1.cc
new file mode 100644
index 00000000000..9829f79364f
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent/1.cc
@@ -0,0 +1,110 @@
+// { dg-options "-std=gnu++23" }
+// { dg-do run { target c++23 } }
+
+#include <ranges>
+#include <algorithm>
+#include <utility>
+#include <testsuite_hooks.h>
+#include <testsuite_iterators.h>
+
+namespace ranges = std::ranges;
+namespace views = std::views;
+
+constexpr bool
+test01()
+{
+  static_assert(ranges::empty(std::array{1, 2, 3} | views::adjacent<0>));
+
+  auto v1 = std::array{1, 2} | views::adjacent<1>;
+  const auto i0 = v1.begin(), i1 = v1.begin() + 1;
+  VERIFY( i0 + 1 - 1 == i0 );
+  VERIFY( i0 < i1 );
+  VERIFY( i1 < v1.end() );
+  VERIFY( i1 - i0 == 1 );
+  VERIFY( i0 - i1 == -1 );
+  VERIFY( v1.end() - i1 == 1 );
+  VERIFY( i1 - v1.end() == -1 );
+  ranges::iter_swap(i0, i1);
+  VERIFY( ranges::equal(std::move(v1) | views::keys, (int[]){2, 1}) );
+
+  int x[] = {1, 2, 3, 4};
+  auto v2 = x | views::pairwise;
+  auto i2 = v2.begin();
+  i2 += 2;
+  i2 -= -1;
+  VERIFY( i2 == v2.end() );
+  VERIFY( ranges::size(v2) == 3 );
+  VERIFY( ranges::size(std::as_const(v2)) == 3 );
+  VERIFY( ranges::equal(v2 | views::keys, (int[]){1, 2, 3}) );
+  VERIFY( ranges::equal(v2 | views::values, (int[]){2, 3, 4}) );
+
+  int y[] = {1, 2, 3, 4, 5};
+  const auto v3 = y | views::adjacent<3>;
+  VERIFY( ranges::size(v3) == 3 );
+  for (unsigned i = 0; i < ranges::size(x); i++)
+    {
+      VERIFY( &std::get<0>(v3[i]) == &y[i] + 0 );
+      VERIFY( &std::get<1>(v3[i]) == &y[i] + 1 );
+      VERIFY( &std::get<2>(v3[i]) == &y[i] + 2 );
+    }
+
+  const auto v5 = y | views::adjacent<5>;
+  VERIFY( ranges::equal(v5, views::single(std::make_tuple(1, 2, 3, 4, 5))) );
+
+  const auto v6 = y | views::adjacent<6>;
+  VERIFY( ranges::empty(v6) );
+
+  const auto v0 = y | views::adjacent<0>;
+  VERIFY( ranges::empty(v0) );
+
+  return true;
+}
+
+constexpr bool
+test02()
+{
+  using __gnu_test::test_input_range;
+  using __gnu_test::test_forward_range;
+  using __gnu_test::test_random_access_range;
+
+  using ty1 = ranges::adjacent_view<views::all_t<test_forward_range<int>>, 2>;
+  static_assert(ranges::forward_range<ty1>);
+  static_assert(!ranges::bidirectional_range<ty1>);
+  static_assert(!ranges::sized_range<ty1>);
+
+  using ty2 = ranges::adjacent_view<views::all_t<test_random_access_range<int>>, 3>;
+  static_assert(ranges::random_access_range<ty2>);
+  static_assert(ranges::sized_range<ty2>);
+
+  return true;
+}
+
+constexpr bool
+test03()
+{
+  auto v = views::iota(0, 4) | views::filter([](auto) { return true; }) | views::pairwise;
+  using ty = decltype(v);
+  static_assert(ranges::forward_range<ty>);
+  static_assert(ranges::common_range<ty>);
+  static_assert(!ranges::sized_range<ty>);
+  VERIFY( v.begin() == v.begin() );
+  VERIFY( v.begin() != v.end() );
+  VERIFY( ranges::next(v.begin(), 3) == v.end() );
+  auto it = v.begin();
+  ++it;
+  it++;
+  VERIFY( ranges::next(it) == v.end() );
+  it--;
+  --it;
+  VERIFY( it == v.begin() );
+
+  return true;
+}
+
+int
+main()
+{
+  static_assert(test01());
+  static_assert(test02());
+  static_assert(test03());
+}
  

On Wed, 31 Aug 2022 at 14:30, Patrick Palka <ppalka@redhat.com> wrote:
>
> On Wed, 31 Aug 2022, Jonathan Wakely wrote:
>
> > On Tue, 30 Aug 2022 at 18:14, Patrick Palka via Libstdc++
> > <libstdc++@gcc.gnu.org> wrote:
> > >
> > > Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
> >
> >
> > > +    constexpr
> > > +    _Iterator(__as_sentinel, iterator_t<_Base> __first, iterator_t<_Base> __last)
> > > +    {
> > > +      if constexpr (!bidirectional_range<_Base>)
> > > +       for (auto& __it : _M_current)
> > > +         __it = __last;
> > > +      else
> > > +       for (ssize_t __i = _Nm-1; __i >= 0; --__i)
> >
> > ssize_t is defined by POSIX in <sys/types.h> but isn't in ISO C or
> > C++. It looks like MinGW defines it to ... something ... sometimes,
> > but I don't think we can rely on it for non-POSIX targets.
>
> I see.  Using ssize_t makes the loop a little cleaner but it's hardly
> necessary, so consider it rewritten into:
>
>   for (size_t __i = 0; __i < _Nm; ++__i)
>     {
>       _M_current[_Nm - 1 - __i] = __last;
>       ranges::advance(__last, -1, __first);
>     }
>
> >
> >
> > > +    template<size_t _Nm>
> > > +      struct _Adjacent : __adaptor::_RangeAdaptorClosure
> > > +      {
> > > +       template<viewable_range _Range>
> > > +         requires (_Nm == 0) || __detail::__can_adjacent_view<_Nm, _Range>
> > > +         [[nodiscard]]
> > > +         constexpr auto
> > > +         operator()(_Range&& __r) const
> >
> > Does this attribute actually work here?
> >
> > I thought we needed to use `operator() [[nodiscard]]` for functions
> > with a requires-clause, because otherwise the attribute gives a parse
> > error. Maybe I've misremembered the problem, and it just doesn't give
> > a -Wunused-result warning. The decl above is setting off my spidey
> > sense for some reason though.
>
> Oops yeah, it looks like this position of [[nodiscard]] works with
> standard concepts, but it's a parse error with -fconcepts-ts due to the
> different requires-clause grammar.
>
> Here's v2 which avoids using ssize_t, and puts the [[nodiscard]] after
> 'operator()' (_Zip and _ZipTransform have the same issue which I can
> fix separately):

OK for trunk with those changes, thanks.