https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120934
Bug ID: 120934
Summary: views::concat is ill-formed depending on argument
order
Product: gcc
Version: 15.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: francois.hamonic at gmail dot com
Target Milestone: ---
The following example will fail to compile due to `std::views::concat(r2, r1)`
iterators not satisfying `equality_comparable` while `std::views::concat(r1,
r2)` is fine.
The same behavior has been observed on all recent GCC implementations I tested
(local GCC 15.1.0 and Godbolt GCC 15.1 and 16.0 trunk).
The following error message is produced by https://godbolt.org/z/ac5jKe4Gr (GCC
16.0.0 20250702 (experimental) with ` -Wall -Wextra -std=c++26
-fconcepts-diagnostics-depth=5`).
Example :
#include <ranges>
#include <vector>
int main() {
auto r1 = std::views::single(1);
std::vector<int> vec = {2, 3};
auto r2 = std::views::join(std::views::transform(vec, std::views::single));
auto c1 = std::views::all(std::views::concat(r1, r2)); // OK
auto c2 = std::views::all(std::views::concat(r2, r1)); // not OK
}
Error message :
<source>: In function 'int main()':
<source>:10:30: error: no match for call to '(const std::ranges::views::_All)
(std::ranges::concat_view<std::ranges::join_view<std::ranges::transform_view<std::ranges::ref_view<std::vector<int,
std::allocator<int> > >, std::ranges::views::_Single> >,
std::ranges::single_view<int> >)'
10 | auto c2 = std::views::all(std::views::concat(r2, r1)); // not OK
| ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from <source>:1:
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/ranges:1429:12:
note: there is 1 candidate
1429 | struct _All : __adaptor::_RangeAdaptorClosure<_All>
| ^~~~
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/ranges:1448:9:
note: candidate 1: 'template<class _Range> requires (viewable_range<_Range>)
&& ((view<typename std::decay<_Tp>::type>) || (__can_ref_view<_Range>) ||
(__can_owning_view<_Range>)) constexpr auto
std::ranges::views::_All::operator()(_Range&&) const'
1448 | operator() [[nodiscard]] (_Range&& __r) const
| ^~~~~~~~
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/ranges:1448:9:
note: template argument deduction/substitution failed:
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/ranges:1448:9:
note: constraints not satisfied
In file included from
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/string_view:58,
from
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/basic_string.h:51,
from
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/string:56,
from
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/locale_classes.h:42,
from
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/ios_base.h:43,
from
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/streambuf:45,
from
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/streambuf_iterator.h:37,
from
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/iterator:68,
from
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/ranges:45:
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/ranges_base.h:
In substitution of 'template<class _Range> requires (viewable_range<_Range>)
&& ((view<typename std::decay<_Tp>::type>) || (__can_ref_view<_Range>) ||
(__can_owning_view<_Range>)) constexpr auto
std::ranges::views::_All::operator()(_Range&&) const [with _Range =
std::ranges::concat_view<std::ranges::join_view<std::ranges::transform_view<std::ranges::ref_view<std::vector<int,
std::allocator<int> > >, std::ranges::views::_Single> >,
std::ranges::single_view<int> >]':
<source>:10:30: required from here
10 | auto c2 = std::views::all(std::views::concat(r2, r1)); // not OK
| ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/ranges_base.h:514:13:
required for the satisfaction of 'range<_Tp>' [with _Tp =
std::ranges::concat_view<std::ranges::join_view<std::ranges::transform_view<std::ranges::ref_view<std::vector<int,
std::allocator<int> > >, std::ranges::views::_Single> >,
std::ranges::single_view<int> >]
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/ranges_base.h:831:13:
required for the satisfaction of 'viewable_range<_Range>' [with _Range =
std::ranges::concat_view<std::ranges::join_view<std::ranges::transform_view<std::ranges::ref_view<std::vector<int,
std::allocator<int> > >, std::ranges::views::_Single> >,
std::ranges::single_view<int> >]
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/ranges_base.h:514:21:
in requirements with '_Tp& __t' [with _Tp =
std::ranges::concat_view<std::ranges::join_view<std::ranges::transform_view<std::ranges::ref_view<std::vector<int,
std::allocator<int> > >, std::ranges::views::_Single> >,
std::ranges::single_view<int> >]
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/ranges_base.h:517:20:
note: the required expression 'std::ranges::_Cpo::end(__t)' is invalid, because
517 | ranges::end(__t);
| ~~~~~~~~~~~^~~~~
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/ranges_base.h:517:20:
error: no match for call to '(const std::ranges::__access::_End)
(std::ranges::concat_view<std::ranges::join_view<std::ranges::transform_view<std::ranges::ref_view<std::vector<int,
std::allocator<int> > >, std::ranges::views::_Single> >,
std::ranges::single_view<int> >&)'
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/ranges_base.h:162:12:
note: there is 1 candidate
162 | struct _End
| ^~~~
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/ranges_base.h:182:9:
note: candidate 1: 'template<class _Tp> requires (__maybe_borrowed_range<_Tp>)
&& ((is_bounded_array_v<typename std::remove_reference<_Tp>::type>) ||
(__member_end<_Tp>) || (__adl_end<_Tp>)) constexpr auto
std::ranges::__access::_End::operator()(_Tp&&) const'
182 | operator()[[nodiscard]](_Tp&& __t) const
noexcept(_S_noexcept<_Tp&>())
| ^~~~~~~~
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/ranges_base.h:182:9:
note: template argument deduction/substitution failed:
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/ranges_base.h:182:9:
note: constraints not satisfied
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/ranges_base.h:
In substitution of 'template<class _Tp> requires (__maybe_borrowed_range<_Tp>)
&& ((is_bounded_array_v<typename std::remove_reference<_Tp>::type>) ||
(__member_end<_Tp>) || (__adl_end<_Tp>)) constexpr auto
std::ranges::__access::_End::operator()(_Tp&&) const [with _Tp =
std::ranges::concat_view<std::ranges::join_view<std::ranges::transform_view<std::ranges::ref_view<std::vector<int,
std::allocator<int> > >, std::ranges::views::_Single> >,
std::ranges::single_view<int> >&]':
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/ranges_base.h:517:13:
required by substitution of 'template<class _Range> requires
(viewable_range<_Range>) && ((view<typename std::decay<_Tp>::type>) ||
(__can_ref_view<_Range>) || (__can_owning_view<_Range>)) constexpr auto
std::ranges::views::_All::operator()(_Range&&) const [with _Range =
std::ranges::concat_view<std::ranges::join_view<std::ranges::transform_view<std::ranges::ref_view<std::vector<int,
std::allocator<int> > >, std::ranges::views::_Single> >,
std::ranges::single_view<int> >]'
517 | ranges::end(__t);
| ~~~~~~~~~~~^~~~~
<source>:10:30: required from here
10 | auto c2 = std::views::all(std::views::concat(r2, r1)); // not OK
| ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/ranges_base.h:182:2:
required by the constraints of 'template<class _Tp> requires
(__maybe_borrowed_range<_Tp>) && ((is_bounded_array_v<typename
std::remove_reference<_Tp>::type>) || (__member_end<_Tp>) || (__adl_end<_Tp>))
constexpr auto std::ranges::__access::_End::operator()(_Tp&&) const'
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/ranges_base.h:180:32:
note: no operand of the disjunction is satisfied
179 | requires is_bounded_array_v<remove_reference_t<_Tp>>
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
180 | || __member_end<_Tp> || __adl_end<_Tp>
| ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/ranges_base.h:179:18:
note: the operand 'is_bounded_array_v<typename
std::remove_reference<_Tp>::type>' is unsatisfied because
179 | requires is_bounded_array_v<remove_reference_t<_Tp>>
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
180 | || __member_end<_Tp> || __adl_end<_Tp>
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/ranges_base.h:182:2:
required by the constraints of 'template<class _Tp> requires
(__maybe_borrowed_range<_Tp>) && ((is_bounded_array_v<typename
std::remove_reference<_Tp>::type>) || (__member_end<_Tp>) || (__adl_end<_Tp>))
constexpr auto std::ranges::__access::_End::operator()(_Tp&&) const'
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/ranges_base.h:179:18:
note: the expression 'is_bounded_array_v<typename
std::remove_reference<_Tp>::type> [with _Tp =
std::ranges::concat_view<std::ranges::join_view<std::ranges::transform_view<std::ranges::ref_view<std::vector<int,
std::allocator<int> > >, std::ranges::views::_Single> >,
std::ranges::single_view<int> >&]' evaluated to 'false'
179 | requires is_bounded_array_v<remove_reference_t<_Tp>>
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/ranges_base.h:180:14:
note: the operand '__member_end<_Tp>' is unsatisfied because
179 | requires is_bounded_array_v<remove_reference_t<_Tp>>
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
180 | || __member_end<_Tp> || __adl_end<_Tp>
| ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/version.h:51,
from
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/concepts:38,
from
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/ranges:39:
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/ranges_base.h:147:15:
required for the satisfaction of '__member_end<_Tp>' [with _Tp =
std::ranges::concat_view<std::ranges::join_view<std::ranges::transform_view<std::ranges::ref_view<std::vector<int,
std::allocator<int> > >, std::ranges::views::_Single> >,
std::ranges::single_view<int> >&]
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/ranges_base.h:147:30:
in requirements with '_Tp& __t' [with _Tp =
std::ranges::concat_view<std::ranges::join_view<std::ranges::transform_view<std::ranges::ref_view<std::vector<int,
std::allocator<int> > >, std::ranges::views::_Single> >,
std::ranges::single_view<int> >&]
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/ranges_base.h:149:13:
note: '(auto)(__t.end())' does not satisfy return-type-requirement, because
149 | { _GLIBCXX_AUTO_CAST(__t.end()) } ->
sentinel_for<__range_iter_t<_Tp>>;
| ^~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/ranges_base.h:149:13:
error: deduced expression type does not satisfy placeholder constraints
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/ranges_base.h:149:13:
note: constraints not satisfied
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/concepts:304:15:
required for the satisfaction of '__weakly_eq_cmp_with<_Sent, _Iter>' [with
_Sent =
std::ranges::concat_view<std::ranges::join_view<std::ranges::transform_view<std::ranges::ref_view<std::vector<int,
std::allocator<int> > >, std::ranges::views::_Single> >,
std::ranges::single_view<int> >::_Iterator<false>; _Iter =
std::ranges::concat_view<std::ranges::join_view<std::ranges::transform_view<std::ranges::ref_view<std::vector<int,
std::allocator<int> > >, std::ranges::views::_Single> >,
std::ranges::single_view<int> >::_Iterator<false>]
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/iterator_concepts.h:676:13:
required for the satisfaction of 'sentinel_for<decltype(auto) [requires
std::sentinel_for<<placeholder>,
decltype(std::ranges::__access::__begin((declval<_Container&>)()))>], decltype
(std::ranges::__access::__begin(declval<_Container&>()))>' [with decltype(auto)
[requires std::sentinel_for<<placeholder>,
decltype(std::ranges::__access::__begin((declval<_Container&>)()))>] =
std::ranges::concat_view<std::ranges::join_view<std::ranges::transform_view<std::ranges::ref_view<std::vector<int,
std::allocator<int> > >, std::ranges::views::_Single> >,
std::ranges::single_view<int> >::_Iterator<false>; _Tp =
std::ranges::concat_view<std::ranges::join_view<std::ranges::transform_view<std::ranges::ref_view<std::vector<int,
std::allocator<int> > >, std::ranges::views::_Single> >,
std::ranges::single_view<int> >&]
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/concepts:305:4:
in requirements with 'std::remove_reference_t<_Tp>& __t',
'std::remove_reference_t<_Rhs>& __u' [with _Tp =
std::ranges::concat_view<std::ranges::join_view<std::ranges::transform_view<std::ranges::ref_view<std::vector<int,
std::allocator<int> > >, std::ranges::views::_Single> >,
std::ranges::single_view<int> >::_Iterator<false>; _Up =
std::ranges::concat_view<std::ranges::join_view<std::ranges::transform_view<std::ranges::ref_view<std::vector<int,
std::allocator<int> > >, std::ranges::views::_Single> >,
std::ranges::single_view<int> >::_Iterator<false>]
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/concepts:306:17:
note: the required expression '(__t == __u)' is invalid, because
306 | { __t == __u } -> __boolean_testable;
| ~~~~^~~~~~
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/concepts:306:17:
error: no match for 'operator==' (operand types are
'std::remove_reference_t<std::ranges::concat_view<std::ranges::join_view<std::ranges::transform_view<std::ranges::ref_view<std::vector<int,
std::allocator<int> > >, std::ranges::views::_Single> >,
std::ranges::single_view<int> >::_Iterator<false> >' {aka 'const
std::remove_reference<std::ranges::concat_view<std::ranges::join_view<std::ranges::transform_view<std::ranges::ref_view<std::vector<int,
std::allocator<int> > >, std::ranges::views::_Single> >,
std::ranges::single_view<int> >::_Iterator<false> >::type'} and
'std::remove_reference_t<std::ranges::concat_view<std::ranges::join_view<std::ranges::transform_view<std::ranges::ref_view<std::vector<int,
std::allocator<int> > >, std::ranges::views::_Single> >,
std::ranges::single_view<int> >::_Iterator<false> >' {aka 'const
std::remove_reference<std::ranges::concat_view<std::ranges::join_view<std::ranges::transform_view<std::ranges::ref_view<std::vector<int,
std::allocator<int> > >, std::ranges::views::_Single> >,
std::ranges::single_view<int> >::_Iterator<false> >::type'})
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/concepts:306:17:
note: there are 2 candidates
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/ranges:10052:5:
note: candidate 1: 'constexpr bool std::ranges::operator==(const
concat_view<join_view<transform_view<ref_view<std::vector<int,
std::allocator<int> > >, views::_Single> >, single_view<int>
>::_Iterator<false>&, std::default_sentinel_t)' (reversed)
10052 | operator==(const _Iterator& __it, default_sentinel_t)
| ^~~~~~~~
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/ranges:10052:39:
note: no known conversion for argument 2 from
'std::remove_reference_t<std::ranges::concat_view<std::ranges::join_view<std::ranges::transform_view<std::ranges::ref_view<std::vector<int,
std::allocator<int> > >, std::ranges::views::_Single> >,
std::ranges::single_view<int> >::_Iterator<false> >' {aka 'const
std::remove_reference<std::ranges::concat_view<std::ranges::join_view<std::ranges::transform_view<std::ranges::ref_view<std::vector<int,
std::allocator<int> > >, std::ranges::views::_Single> >,
std::ranges::single_view<int> >::_Iterator<false> >::type'} to
'std::default_sentinel_t'
10052 | operator==(const _Iterator& __it, default_sentinel_t)
| ^~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/ranges:10043:5:
note: candidate 2: 'constexpr bool std::ranges::operator==(const
concat_view<join_view<transform_view<ref_view<std::vector<int,
std::allocator<int> > >, views::_Single> >, single_view<int>
>::_Iterator<false>&, const
concat_view<join_view<transform_view<ref_view<std::vector<int,
std::allocator<int> > >, views::_Single> >, single_view<int>
>::_Iterator<false>&) requires
(equality_comparable<decltype(std::ranges::__access::__begin((declval<typename
std::__conditional<_Const>::type<const _Vs, _Vs>&>)()))> && ...)'
10043 | operator==(const _Iterator& __x, const _Iterator& __y)
| ^~~~~~~~
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/ranges:10043:5:
note: constraints not satisfied
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/ranges: In
instantiation of 'constexpr bool std::ranges::operator==(const
concat_view<join_view<transform_view<ref_view<std::vector<int,
std::allocator<int> > >, views::_Single> >, single_view<int>
>::_Iterator<false>&, const
concat_view<join_view<transform_view<ref_view<std::vector<int,
std::allocator<int> > >, views::_Single> >, single_view<int>
>::_Iterator<false>&) requires
(equality_comparable<decltype(std::ranges::__access::__begin((declval<typename
std::__conditional<_Const>::type<const _Vs, _Vs>&>)()))> && ...)':
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/concepts:306:10:
required by substitution of 'template<class _Tp> requires
(__maybe_borrowed_range<_Tp>) && ((is_bounded_array_v<typename
std::remove_reference<_Tp>::type>) || (__member_end<_Tp>) || (__adl_end<_Tp>))
constexpr auto std::ranges::__access::_End::operator()(_Tp&&) const [with _Tp =
std::ranges::concat_view<std::ranges::join_view<std::ranges::transform_view<std::ranges::ref_view<std::vector<int,
std::allocator<int> > >, std::ranges::views::_Single> >,
std::ranges::single_view<int> >&]'
306 | { __t == __u } -> __boolean_testable;
| ~~~~^~~~~~
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/bits/ranges_base.h:517:13:
required by substitution of 'template<class _Range> requires
(viewable_range<_Range>) && ((view<typename std::decay<_Tp>::type>) ||
(__can_ref_view<_Range>) || (__can_owning_view<_Range>)) constexpr auto
std::ranges::views::_All::operator()(_Range&&) const [with _Range =
std::ranges::concat_view<std::ranges::join_view<std::ranges::transform_view<std::ranges::ref_view<std::vector<int,
std::allocator<int> > >, std::ranges::views::_Single> >,
std::ranges::single_view<int> >]'
517 | ranges::end(__t);
| ~~~~~~~~~~~^~~~~
<source>:10:30: required from here
10 | auto c2 = std::views::all(std::views::concat(r2, r1)); // not OK
| ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/ranges:10043:5:
required by the constraints of 'template<class ... _Vs> template<bool _Const>
constexpr bool std::ranges::operator==(const
concat_view<_Vs>::_Iterator<_Const>&, const
concat_view<_Vs>::_Iterator<_Const>&) requires
(equality_comparable<decltype(std::ranges::__access::__begin((declval<typename
std::__conditional<_Const>::type<const _Vs, _Vs>&>)()))> && ...)'
/opt/compiler-explorer/gcc-trunk-20250702/include/c++/16.0.0/ranges:10044:79:
note: the expression
'(equality_comparable<decltype(std::ranges::__access::__begin((declval<typename
std::__conditional<_Const>::type<const _Vs, _Vs>&>)()))> && ...) [with _Const =
false; _Vs =
{std::ranges::join_view<std::ranges::transform_view<std::ranges::ref_view<std::vector<int,
std::allocator<int> > >, std::ranges::views::_Single> >,
std::ranges::single_view<int>}]' evaluated to 'false'
10044 | requires (equality_comparable<iterator_t<__maybe_const_t<_Const,
_Vs>>> && ...)
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~
(cropped since the messages repeat from there)
