https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100233
Bug ID: 100233
Summary: [10/11/12] std::views::elements only accepts types
that are defined on std::get
Product: gcc
Version: 10.3.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: gcc-bugs at marehr dot dialup.fu-berlin.de
Target Milestone: ---
Hi gcc-team,
the following code will not work on custom tuples that don't add a get overload
within the std namespace. These tuples work in structured bindings, but not
within std::views::elements.
```cpp
#include <utility>
#include <vector>
namespace my_namespace
{
template <typename ...types>
struct S
{
int x{};
};
} // namespace my_namespace
namespace std
{
template <template <typename ...> typename t, typename ...types>
requires std::is_base_of_v<my_namespace::S<types...>, t<types...>>
struct tuple_size<t<types...>> : public std::integral_constant<std::size_t,
sizeof...(types)>
{};
template <std::size_t i, template <typename ...> typename t, typename ...types>
requires (i < sizeof...(types)) &&
std::is_base_of_v<my_namespace::S<types...>, t<types...>>
struct tuple_element<i, t<types...>>
{
using type = int;
};
} // namespace std
namespace my_namespace
{
template <std::size_t i, typename ...types>
int & get(my_namespace::S<types...> & e) noexcept
{
return e.x;
}
} // my_namespace
#if !DO_FAIL
namespace std
{
using my_namespace::get;
} // namespace std
#endif
#include <ranges>
int main()
{
// does work with / without defining get within std
using tripplet_t = my_namespace::S<int, unsigned, char>;
tripplet_t tuple{};
auto & [a, b, c] = tuple;
// only works when defining within std
std::vector<tripplet_t> vec(10);
// std::views::elements<0>(vec);
using elements_view_t =
std::ranges::elements_view<std::views::all_t<decltype(vec) &>, 0>;
}
```
https://godbolt.org/z/n315fednc
```
> g++-10 --std=c++2a -DDO_FAIL=1
<source>:56:93: error: template constraint failure for 'template<class _Vp,
long unsigned int _Nm> requires (input_range<_Vp>) && ((view<_Vp>) &&
(__has_tuple_element<typename std::__detail::__iter_traits_impl<typename
std::remove_cv<typename
std::remove_reference<decltype(std::__detail::__ranges_begin((declval<_Container&>)()))>::type>::type,
std::indirectly_readable_traits<typename std::remove_cv<typename
std::remove_reference<decltype(std::__detail::__ranges_begin((declval<_Container&>)()))>::type>::type>
>::__iter_traits<typename std::remove_cv<typename
std::remove_reference<decltype(std::__detail::__ranges_begin((declval<_Container&>)()))>::type>::type,
std::indirectly_readable_traits<typename std::remove_cv<typename
std::remove_reference<decltype(std::__detail::__ranges_begin((declval<_Container&>)()))>::type>::type>
>::value_type, _Nm>) && (__has_tuple_element<typename
std::remove_reference<decltype(*(declval<decltype(std::__detail::__ranges_begin((declval<_Container&>)()))&>)())>::type,
_Nm>) &&
(__returnable_element<decltype(*(declval<decltype(std::__detail::__ranges_begin((declval<_Container&>)()))&>)()),
_Nm>)) class std::ranges::elements_view'
56 | using elements_view_t =
std::ranges::elements_view<std::views::all_t<decltype(vec) &>, 0>;
|
^
<source>:56:93: note: constraints not satisfied
In file included from <source>:44:
/opt/compiler-explorer/gcc-trunk-20210423/include/c++/12.0.0/ranges: In
substitution of 'template<class _Vp, long unsigned int _Nm> requires
(input_range<_Vp>) && ((view<_Vp>) && (__has_tuple_element<typename
std::__detail::__iter_traits_impl<typename std::remove_cv<typename
std::remove_reference<decltype(std::__detail::__ranges_begin((declval<_Container&>)()))>::type>::type,
std::indirectly_readable_traits<typename std::remove_cv<typename
std::remove_reference<decltype(std::__detail::__ranges_begin((declval<_Container&>)()))>::type>::type>
>::__iter_traits<typename std::remove_cv<typename
std::remove_reference<decltype(std::__detail::__ranges_begin((declval<_Container&>)()))>::type>::type,
std::indirectly_readable_traits<typename std::remove_cv<typename
std::remove_reference<decltype(std::__detail::__ranges_begin((declval<_Container&>)()))>::type>::type>
>::value_type, _Nm>) && (__has_tuple_element<typename
std::remove_reference<decltype(*(declval<decltype(std::__detail::__ranges_begin((declval<_Container&>)()))&>)())>::type,
_Nm>) &&
(__returnable_element<decltype(*(declval<decltype(std::__detail::__ranges_begin((declval<_Container&>)()))&>)()),
_Nm>)) class std::ranges::elements_view [with _Vp =
std::ranges::ref_view<std::vector<my_namespace::S<int, unsigned int, char> > >;
long unsigned int _Nm = 0]':
<source>:56:93: required from here
/opt/compiler-explorer/gcc-trunk-20210423/include/c++/12.0.0/ranges:3306:13:
required for the satisfaction of
'__has_tuple_element<std::ranges::range_value_t<_Vp>, _Nm>' [with _Vp =
std::ranges::ref_view<std::vector<my_namespace::S<int, unsigned int, char>,
std::allocator<my_namespace::S<int, unsigned int, char> > > >; _Nm = 0]
/opt/compiler-explorer/gcc-trunk-20210423/include/c++/12.0.0/ranges:3306:35:
in requirements with '_Tp __t' [with _Nm = 0; _Tp = my_namespace::S<int,
unsigned int, char>]
/opt/compiler-explorer/gcc-trunk-20210423/include/c++/12.0.0/ranges:3311:24:
note: the required expression 'get<_Nm>(__t)' is invalid
3311 | { std::get<_Nm>(__t) }
| ~~~~~~~~~~~~~^~~~~
```
The standard defines the `get-element` call as
```cpp
if constexpr (is_reference_v<range_reference_t<Base>>) {
return get<N>(*i);
} else {
using E = remove_cv_t<tuple_element_t<N, range_reference_t<Base>>>;
return static_cast<E>(get<N>(*i));
}
```
https://eel.is/c++draft/range.elements#iterator-3
With some good-will you could say that `get` should be called unqualified :)
I know that it isn't stated explicitly, but with how the whole ADL thing with
range adaptors work, it is unexpected that it does not work here too.
Thank you!
