What are the rules for creating your own (pipeable) ranges ::views and ::actions?

Question

In this answer, we can see that we can create our own views. I have tried that:

template <typename Range>
struct squared_view : std::ranges::view_interface<squared_view<Range>> {
    struct iterator;

    constexpr squared_view() = default;
    constexpr squared_view(Range t): t(t) { }

    auto begin() const { return iterator(std::ranges::begin(t)); }
    auto end() const { return iterator(std::ranges::end(t)); }

    Range t;
};

template <typename Range>
struct squared_view<Range>::iterator {
    using value_type = typename std::ranges::iterator_t<Range>::value_type;

    constexpr iterator() = default;
    constexpr iterator(std::ranges::iterator_t<Range> it): it_{it} { }

    iterator& operator++() {
        ++it_;
        return *this;
    }

    iterator operator++(int) {
        const iterator current{*this};
        ++it_;
        return current;
    }

    value_type operator*() const {
        const auto value = *it_;
        return value * value;
    }

    bool operator==(iterator const& rhs) const { return it_ == rhs.it_; }

private:
    std::ranges::iterator_t<Range> it_;
};

template <std::ranges::range Range>
squared_view(Range&&) -> squared_view<std::ranges::views::all_t<Range>>;

struct squared_fn {
    template <typename Rng>
    auto operator()(Rng&& rng) const {
        return squared_view{std::forward<Rng>(rng)};
    }

    template <typename Rng>
    friend decltype(auto) operator|(Rng&& rng, squared_fn fun) {
        return fun(std::forward<Rng>(rng));
    }
};

constexpr squared_fn squared;

and when I thought I have succeeded:

int main() {
    std::vector<int> vec = {1, 2, 3};
    auto rng = vec | squared;

    for (const auto e : rng) {
        std::cout << e << ' ';  // prints 1 4 9
    }
}

it turned out that I actually failed, because std::ranges::range<decltype(rng)> is false.

Trying to figure out why, I tried to take std::ranges::begin(rng), which resulted in a few dozen lines of errors, which, more or less, boiled down to:

note: no operand of the disjunction is satisfied
requires is_array_v<remove_reference_t<_Tp>> || __member_begin<_Tp> || __adl_begin<_Tp>

which is weird, because I did provide member begin() and end() (unless __member_begin<_Tp> doesn't do what it looks like it's doing).

The full error message is as follows:

C:\Users\Lenovo\CLionProjects\EasyNamespace\main.cpp: In function 'int main()':
C:\Users\Lenovo\CLionProjects\EasyNamespace\main.cpp:72:27: error: no match for call to '(const std::ranges::__cust_access::_Begin) (squared_view<std::ranges::ref_view<std::vector<int> > >&)'
   72 |     std::ranges::begin(rng);
      |                           ^
In file included from C:/msys64/mingw64/include/c++/10.1.0/string:54,
                 from C:/msys64/mingw64/include/c++/10.1.0/bits/locale_classes.h:40,
                 from C:/msys64/mingw64/include/c++/10.1.0/bits/ios_base.h:41,
                 from C:/msys64/mingw64/include/c++/10.1.0/ios:42,
                 from C:/msys64/mingw64/include/c++/10.1.0/ostream:38,
                 from C:/msys64/mingw64/include/c++/10.1.0/iostream:39,
                 from C:\Users\Lenovo\CLionProjects\EasyNamespace\main.cpp:1:
C:/msys64/mingw64/include/c++/10.1.0/bits/range_access.h:400:2: note: candidate: 'constexpr auto std::ranges::__cust_access::_Begin::operator()(_Tp&&) const [with _Tp = squared_view<std::ranges::ref_view<std::vector<int> > >&]'
  400 |  operator()(_Tp&& __t) const noexcept(_S_noexcept<_Tp>())
      |  ^~~~~~~~
C:/msys64/mingw64/include/c++/10.1.0/bits/range_access.h:400:2: note: constraints not satisfied
C:\Users\Lenovo\CLionProjects\EasyNamespace\main.cpp: In instantiation of 'constexpr auto std::ranges::__cust_access::_Begin::operator()(_Tp&&) const [with _Tp = squared_view<std::ranges::ref_view<std::vector<int> > >&]':
C:\Users\Lenovo\CLionProjects\EasyNamespace\main.cpp:72:27:   required from here
C:/msys64/mingw64/include/c++/10.1.0/bits/range_access.h:400:2:   required by the constraints of 'template<class _Tp>  requires (__maybe_borrowed_range<_Tp>) && ((is_array_v<typename std::remove_reference<_Tp>::type>) || (__member_begin<_Tp>) || (__adl_begin<_Tp>)) constexpr auto std::ranges::__cust_access::_Begin::operator()(_Tp&&) const'
C:/msys64/mingw64/include/c++/10.1.0/bits/range_access.h:398:4: note: no operand of the disjunction is satisfied
  397 |  requires is_array_v<remove_reference_t<_Tp>> || __member_begin<_Tp>
      |           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  398 |    || __adl_begin<_Tp>
      |    ^~~~~~~~~~~~~~~~~~~
C:/msys64/mingw64/include/c++/10.1.0/bits/range_access.h:397:11: note: the operand 'is_array_v<std::remove_reference_t<_Tp> >' is unsatisfied because
  397 |  requires is_array_v<remove_reference_t<_Tp>> || __member_begin<_Tp>
      |           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  398 |    || __adl_begin<_Tp>
      |    ~~~~~~~~~~~~~~~~~~~
C:/msys64/mingw64/include/c++/10.1.0/bits/range_access.h:400:2:   required by the constraints of 'template<class _Tp>  requires (__maybe_borrowed_range<_Tp>) && ((is_array_v<typename std::remove_reference<_Tp>::type>) || (__member_begin<_Tp>) || (__adl_begin<_Tp>)) constexpr auto std::ranges::__cust_access::_Begin::operator()(_Tp&&) const'
C:/msys64/mingw64/include/c++/10.1.0/bits/range_access.h:397:11: note: the expression 'is_array_v<typename std::remove_reference<_Tp>::type> [with _Tp = squared_view<std::ranges::ref_view<std::vector<int, std::allocator<int> > > >&; _Tp = squared_view<std::ranges::ref_view<std::vector<int, std::allocator<int> > > >&]' evaluated to 'false'
  397 |  requires is_array_v<remove_reference_t<_Tp>> || __member_begin<_Tp>
      |           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
C:/msys64/mingw64/include/c++/10.1.0/bits/range_access.h:397:50: note: the operand '__member_begin<_Tp>' is unsatisfied because
  397 |  requires is_array_v<remove_reference_t<_Tp>> || __member_begin<_Tp>
      |           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~
  398 |    || __adl_begin<_Tp>
      |    ~~~~~~~~~~~~~~~~~~~                            
In file included from C:/msys64/mingw64/include/c++/10.1.0/bits/stl_iterator_base_types.h:71,
                 from C:/msys64/mingw64/include/c++/10.1.0/bits/stl_algobase.h:65,
                 from C:/msys64/mingw64/include/c++/10.1.0/bits/char_traits.h:39,
                 from C:/msys64/mingw64/include/c++/10.1.0/ios:40,
                 from C:/msys64/mingw64/include/c++/10.1.0/ostream:38,
                 from C:/msys64/mingw64/include/c++/10.1.0/iostream:39,
                 from C:\Users\Lenovo\CLionProjects\EasyNamespace\main.cpp:1:
C:/msys64/mingw64/include/c++/10.1.0/bits/iterator_concepts.h:847:15:   required for the satisfaction of '__member_begin<_Tp>' [with _Tp = squared_view<std::ranges::ref_view<std::vector<int, std::allocator<int> > > >&]
C:/msys64/mingw64/include/c++/10.1.0/bits/iterator_concepts.h:847:32:   in requirements with '_Tp& __t' [with _Tp = squared_view<std::ranges::ref_view<std::vector<int, std::allocator<int> > > >&]
C:/msys64/mingw64/include/c++/10.1.0/bits/iterator_concepts.h:849:28: note: 'std::__detail::__decay_copy(__t.begin())' does not satisfy return-type-requirement, because
  849 |    { __detail::__decay_copy(__t.begin()) } -> input_or_output_iterator;
      |      ~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~
C:/msys64/mingw64/include/c++/10.1.0/bits/iterator_concepts.h:849:6: error: deduced expression type does not satisfy placeholder constraints
  849 |    { __detail::__decay_copy(__t.begin()) } -> input_or_output_iterator;
      |    ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
C:/msys64/mingw64/include/c++/10.1.0/bits/iterator_concepts.h:849:6: note: constraints not satisfied
C:/msys64/mingw64/include/c++/10.1.0/bits/iterator_concepts.h:507:13:   required for the satisfaction of 'weakly_incrementable<_Iter>' [with _Iter = squared_view<std::ranges::ref_view<std::vector<int, std::allocator<int> > > >::iterator<std::ranges::ref_view<std::vector<int, std::allocator<int> > > >]
C:/msys64/mingw64/include/c++/10.1.0/bits/iterator_concepts.h:522:13:   required for the satisfaction of 'input_or_output_iterator<squared_view<std::ranges::ref_view<std::vector<int> > >::iterator>'
C:/msys64/mingw64/include/c++/10.1.0/bits/iterator_concepts.h:509:10:   in requirements with '_Iter __i' [with _Tp = squared_view<std::ranges::ref_view<std::vector<int, std::allocator<int> > > >::iterator<std::ranges::ref_view<std::vector<int, std::allocator<int> > > >; _Iter = squared_view<std::ranges::ref_view<std::vector<int, std::allocator<int> > > >::iterator<std::ranges::ref_view<std::vector<int, std::allocator<int> > > >]
C:/msys64/mingw64/include/c++/10.1.0/bits/iterator_concepts.h:511:11: note: the required type 'std::iter_difference_t<_Iter>' is invalid, because
  511 |  typename iter_difference_t<_Iter>;
      |  ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
C:/msys64/mingw64/include/c++/10.1.0/bits/iterator_concepts.h: In substitution of 'template<class _Tp> using __iter_diff_t = typename std::__detail::__iter_traits_impl<_Tp, std::incrementable_traits<_Iter> >::type::difference_type [with _Tp = squared_view<std::ranges::ref_view<std::vector<int> > >::iterator]':
C:/msys64/mingw64/include/c++/10.1.0/bits/iterator_concepts.h:185:11:   required by substitution of 'template<class _Tp> using iter_difference_t = std::__detail::__iter_diff_t<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type> [with _Tp = squared_view<std::ranges::ref_view<std::vector<int> > >::iterator]'
C:/msys64/mingw64/include/c++/10.1.0/bits/iterator_concepts.h:511:11:   required from 'constexpr auto std::ranges::__cust_access::_Begin::operator()(_Tp&&) const [with _Tp = squared_view<std::ranges::ref_view<std::vector<int> > >&]'
C:\Users\Lenovo\CLionProjects\EasyNamespace\main.cpp:72:27:   required from here
C:/msys64/mingw64/include/c++/10.1.0/bits/iterator_concepts.h:180:13: error: no type named 'difference_type' in 'std::__detail::__iter_traits<squared_view<std::ranges::ref_view<std::vector<int> > >::iterator, std::incrementable_traits<squared_view<std::ranges::ref_view<std::vector<int> > >::iterator> >' {aka 'struct std::incrementable_traits<squared_view<std::ranges::ref_view<std::vector<int> > >::iterator>'}
  180 |       using __iter_diff_t = typename
      |             ^~~~~~~~~~~~~
C:/msys64/mingw64/include/c++/10.1.0/bits/iterator_concepts.h: In instantiation of 'constexpr auto std::ranges::__cust_access::_Begin::operator()(_Tp&&) const [with _Tp = squared_view<std::ranges::ref_view<std::vector<int> > >&]':
C:\Users\Lenovo\CLionProjects\EasyNamespace\main.cpp:72:27:   required from here
C:/msys64/mingw64/include/c++/10.1.0/bits/iterator_concepts.h:512:21: note: nested requirement '__is_signed_integer_like<typename std::__detail::__iter_traits_impl<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type, std::incrementable_traits<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type> >::type::difference_type>' is not satisfied, because
  512 |  requires __detail::__is_signed_integer_like<iter_difference_t<_Iter>>;
      |           ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
C:/msys64/mingw64/include/c++/10.1.0/bits/iterator_concepts.h:498:15:   required for the satisfaction of '__is_signed_integer_like<typename std::__detail::__iter_traits_impl<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type, std::incrementable_traits<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type> >::type::difference_type>'
C:/msys64/mingw64/include/c++/10.1.0/bits/iterator_concepts.h:499:2: note: no operand of the disjunction is satisfied
  498 |       concept __is_signed_integer_like = signed_integral<_Tp>
      |                                          ~~~~~~~~~~~~~~~~~~~~
  499 |  || same_as<_Tp, __max_diff_type>;
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
C:/msys64/mingw64/include/c++/10.1.0/bits/iterator_concepts.h:498:42: note: the operand 'signed_integral<_Tp>' is unsatisfied because
  498 |       concept __is_signed_integer_like = signed_integral<_Tp>
      |                                          ^~~~~~~~~~~~~~~~~~~~
  499 |  || same_as<_Tp, __max_diff_type>;
      |  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~         
C:/msys64/mingw64/include/c++/10.1.0/concepts:102:13:   required for the satisfaction of 'integral<_Tp>' [with _Tp = typename std::__detail::__iter_traits_impl<typename std::remove_cv<typename std::remove_reference<typename std::__detail::__iter_traits_impl<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type, std::incrementable_traits<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type> >::type::difference_type>::type>::type, std::incrementable_traits<typename std::remove_cv<typename std::remove_reference<typename std::__detail::__iter_traits_impl<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type, std::incrementable_traits<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type> >::type::difference_type>::type>::type> >::type::difference_type]
C:/msys64/mingw64/include/c++/10.1.0/concepts:105:13:   required for the satisfaction of 'signed_integral<_Tp>' [with _Tp = typename std::__detail::__iter_traits_impl<typename std::remove_cv<typename std::remove_reference<typename std::__detail::__iter_traits_impl<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type, std::incrementable_traits<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type> >::type::difference_type>::type>::type, std::incrementable_traits<typename std::remove_cv<typename std::remove_reference<typename std::__detail::__iter_traits_impl<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type, std::incrementable_traits<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type> >::type::difference_type>::type>::type> >::type::difference_type]
C:/msys64/mingw64/include/c++/10.1.0/bits/iterator_concepts.h:498:15:   required for the satisfaction of '__is_signed_integer_like<typename std::__detail::__iter_traits_impl<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type, std::incrementable_traits<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type> >::type::difference_type>'
C:/msys64/mingw64/include/c++/10.1.0/bits/iterator_concepts.h:512:11: error: no type named 'difference_type' in 'using type = struct std::incrementable_traits<squared_view<std::ranges::ref_view<std::vector<int> > >::iterator>' {aka 'struct std::incrementable_traits<squared_view<std::ranges::ref_view<std::vector<int> > >::iterator>'}
  512 |  requires __detail::__is_signed_integer_like<iter_difference_t<_Iter>>;
      |  ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
C:/msys64/mingw64/include/c++/10.1.0/bits/iterator_concepts.h:499:5: note: the operand 'same_as<_Tp, __int128>' is unsatisfied because
  498 |       concept __is_signed_integer_like = signed_integral<_Tp>
      |                                          ~~~~~~~~~~~~~~~~~~~~
  499 |  || same_as<_Tp, __max_diff_type>;
      |  ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
C:/msys64/mingw64/include/c++/10.1.0/concepts:57:15:   required for the satisfaction of '__same_as<_Tp, _Up>' [with _Tp = typename std::__detail::__iter_traits_impl<typename std::remove_cv<typename std::remove_reference<typename std::__detail::__iter_traits_impl<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type, std::incrementable_traits<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type> >::type::difference_type>::type>::type, std::incrementable_traits<typename std::remove_cv<typename std::remove_reference<typename std::__detail::__iter_traits_impl<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type, std::incrementable_traits<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type> >::type::difference_type>::type>::type> >::type::difference_type; _Up = __int128]
C:/msys64/mingw64/include/c++/10.1.0/concepts:62:13:   required for the satisfaction of 'same_as<_Tp, __int128>' [with _Tp = typename std::__detail::__iter_traits_impl<typename std::remove_cv<typename std::remove_reference<typename std::__detail::__iter_traits_impl<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type, std::incrementable_traits<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type> >::type::difference_type>::type>::type, std::incrementable_traits<typename std::remove_cv<typename std::remove_reference<typename std::__detail::__iter_traits_impl<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type, std::incrementable_traits<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type> >::type::difference_type>::type>::type> >::type::difference_type]
C:/msys64/mingw64/include/c++/10.1.0/bits/iterator_concepts.h:498:15:   required for the satisfaction of '__is_signed_integer_like<typename std::__detail::__iter_traits_impl<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type, std::incrementable_traits<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type> >::type::difference_type>'
C:/msys64/mingw64/include/c++/10.1.0/bits/iterator_concepts.h:512:11: error: no type named 'difference_type' in 'using type = struct std::incrementable_traits<squared_view<std::ranges::ref_view<std::vector<int> > >::iterator>' {aka 'struct std::incrementable_traits<squared_view<std::ranges::ref_view<std::vector<int> > >::iterator>'}
  512 |  requires __detail::__is_signed_integer_like<iter_difference_t<_Iter>>;
      |  ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from C:/msys64/mingw64/include/c++/10.1.0/string:54,
                 from C:/msys64/mingw64/include/c++/10.1.0/bits/locale_classes.h:40,
                 from C:/msys64/mingw64/include/c++/10.1.0/bits/ios_base.h:41,
                 from C:/msys64/mingw64/include/c++/10.1.0/ios:42,
                 from C:/msys64/mingw64/include/c++/10.1.0/ostream:38,
                 from C:/msys64/mingw64/include/c++/10.1.0/iostream:39,
                 from C:\Users\Lenovo\CLionProjects\EasyNamespace\main.cpp:1:
C:/msys64/mingw64/include/c++/10.1.0/bits/range_access.h:398:7: note: the operand '__adl_begin<_Tp>' is unsatisfied because
  397 |  requires is_array_v<remove_reference_t<_Tp>> || __member_begin<_Tp>
      |           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  398 |    || __adl_begin<_Tp>
      |    ~~~^~~~~~~~~~~~~~~~
In file included from C:/msys64/mingw64/include/c++/10.1.0/bits/stl_iterator_base_types.h:71,
                 from C:/msys64/mingw64/include/c++/10.1.0/bits/stl_algobase.h:65,
                 from C:/msys64/mingw64/include/c++/10.1.0/bits/char_traits.h:39,
                 from C:/msys64/mingw64/include/c++/10.1.0/ios:40,
                 from C:/msys64/mingw64/include/c++/10.1.0/ostream:38,
                 from C:/msys64/mingw64/include/c++/10.1.0/iostream:39,
                 from C:\Users\Lenovo\CLionProjects\EasyNamespace\main.cpp:1:
C:/msys64/mingw64/include/c++/10.1.0/bits/iterator_concepts.h:856:15:   required for the satisfaction of '__adl_begin<_Tp>' [with _Tp = squared_view<std::ranges::ref_view<std::vector<int, std::allocator<int> > > >&]
C:/msys64/mingw64/include/c++/10.1.0/bits/iterator_concepts.h:857:5:   in requirements with '_Tp& __t' [with _Tp = squared_view<std::ranges::ref_view<std::vector<int, std::allocator<int> > > >&]
C:/msys64/mingw64/include/c++/10.1.0/bits/iterator_concepts.h:859:28: note: the required expression 'std::__detail::__decay_copy(std::__detail::begin(__t))' is invalid, because
  859 |    { __detail::__decay_copy(begin(__t)) } -> input_or_output_iterator;
      |      ~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~
C:/msys64/mingw64/include/c++/10.1.0/bits/iterator_concepts.h:859:34: error: call of overloaded 'begin(squared_view<std::ranges::ref_view<std::vector<int> > >&)' is ambiguous
  859 |    { __detail::__decay_copy(begin(__t)) } -> input_or_output_iterator;
      |                             ~~~~~^~~~~
C:/msys64/mingw64/include/c++/10.1.0/bits/iterator_concepts.h:852:10: note: candidate: 'void std::__detail::begin(auto:1&) [with auto:1 = squared_view<std::ranges::ref_view<std::vector<int> > >]' (deleted)
  852 |     void begin(auto&) = delete;
      |          ^~~~~
C:/msys64/mingw64/include/c++/10.1.0/bits/iterator_concepts.h:853:10: note: candidate: 'void std::__detail::begin(const auto:2&) [with auto:2 = squared_view<std::ranges::ref_view<std::vector<int> > >]' (deleted)
  853 |     void begin(const auto&) = delete;
      |          ^~~~~
In file included from C:/msys64/mingw64/include/c++/10.1.0/string:54,
                 from C:/msys64/mingw64/include/c++/10.1.0/bits/locale_classes.h:40,
                 from C:/msys64/mingw64/include/c++/10.1.0/bits/ios_base.h:41,
                 from C:/msys64/mingw64/include/c++/10.1.0/ios:42,
                 from C:/msys64/mingw64/include/c++/10.1.0/ostream:38,
                 from C:/msys64/mingw64/include/c++/10.1.0/iostream:39,
                 from C:\Users\Lenovo\CLionProjects\EasyNamespace\main.cpp:1:
C:/msys64/mingw64/include/c++/10.1.0/bits/range_access.h:51:5: note: candidate: 'constexpr decltype (__cont.begin()) std::begin(_Container&) [with _Container = squared_view<std::ranges::ref_view<std::vector<int> > >; decltype (__cont.begin()) = squared_view<std::ranges::ref_view<std::vector<int> > >::iterator]'
   51 |     begin(_Container& __cont) -> decltype(__cont.begin())
      |     ^~~~~
C:/msys64/mingw64/include/c++/10.1.0/bits/range_access.h:61:5: note: candidate: 'constexpr decltype (__cont.begin()) std::begin(const _Container&) [with _Container = squared_view<std::ranges::ref_view<std::vector<int> > >; decltype (__cont.begin()) = squared_view<std::ranges::ref_view<std::vector<int> > >::iterator]'
   61 |     begin(const _Container& __cont) -> decltype(__cont.begin())
      |     ^~~~~
mingw32-make.exe[3]: *** [CMakeFiles\EasyNamespace.dir\build.make:82: CMakeFiles/EasyNamespace.dir/main.cpp.obj] Error 1
mingw32-make.exe[2]: *** [CMakeFiles\Makefile2:95: CMakeFiles/EasyNamespace.dir/all] Error 2
mingw32-make.exe[1]: *** [CMakeFiles\Makefile2:102: CMakeFiles/EasyNamespace.dir/rule] Error 2
mingw32-make.exe: *** [Makefile:137: EasyNamespace] Error 2

What exactly did I miss? How do I properly create my own <ranges> extensions?

Answer 1

Your iterator doesn't satisfy the std::input_or_output_iterator concept.

It needs to be std::weakly_incrementable, which requires std::iter_difference_t to be valid. Currently, since your type has no difference_type member type or specialiase std::iterator_traits, std::iter_difference_t is invalid.

So just add an alias:

template <typename Range>
struct squared_view<Range>::iterator {
    using difference_type = std::iter_difference_t<std::ranges::iterator_t<Range>>;

    // ...
};