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?