[cfe-users] Compiling C++ 20 Example from 24.6.4.1 [range.istream.overview] produces errors with libstdc++ 10

Ray Lischner via cfe-users cfe-users at lists.llvm.org
Tue May 26 12:46:32 PDT 2020


On 5/26/20 2:57 PM, Richard Smith wrote:

> Can you humor me for a second and try calling 'view.begin()' directly 
> before using std::ranges::begin? (I'm suspicious that we're somehow not 
> triggering instantiation of istream_view<int> until too late; calling 
> 'view.begin()' earlier would resolve the problem if that's the case.)
$ cat istream_begin.cpp
#include <cassert>
#include <iostream>
#include <ranges>
#include <sstream>

int main()
{
   auto ints = std::istringstream{"0 1 2 3 4"};
   auto view{ std::ranges::istream_view<int>(ints) };
   auto view_begin{ view.begin() };
   auto begin{ std::ranges::begin(view) };
   assert(&*view_begin == &*begin);
}
$ clang++ -std=c++20 istream_begin.cpp
In file included from istream_begin.cpp:2:
In file included from 
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/iostream:39:
In file included from 
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/ostream:38:
In file included from 
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/ios:40:
In file included from 
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/bits/char_traits.h:39:
In file included from 
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/bits/stl_algobase.h:65:
In file included from 
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/bits/stl_iterator_base_types.h:71:
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/bits/iterator_concepts.h:902:13: 
error: no matching function for call to '__ranges_begin'
         = decltype(__detail::__ranges_begin(std::declval<_Tp&>()));
                    ^~~~~~~~~~~~~~~~~~~~~~~~
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/bits/range_access.h:874:5: 
note: in instantiation of template type alias '__range_iter_t' requested 
here
     using iterator_t = std::__detail::__range_iter_t<_Tp>;
     ^
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/ranges:134:43: 
note: in instantiation of template type alias 'iterator_t' requested here
       data() requires contiguous_iterator<iterator_t<_Derived>>
                                           ^
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/ranges:956:14: 
note: in instantiation of template class 
'std::ranges::view_interface<std::ranges::basic_istream_view<int, char, 
std::char_traits<char> > >' requested here
     : public view_interface<basic_istream_view<_Val, _CharT, _Traits>>
              ^
istream_begin.cpp:9:14: note: in instantiation of template class 
'std::ranges::basic_istream_view<int, char, std::char_traits<char> >' 
requested here
   auto view{ std::ranges::istream_view<int>(ints) };
              ^
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/bits/iterator_concepts.h:885:7: 
note: candidate template ignored: constraints not satisfied [with _Tp = 
std::ranges::basic_istream_view<int, char, std::char_traits<char> >]
       __ranges_begin(_Tp& __t)
       ^
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/bits/iterator_concepts.h:883:16: 
note: because 'is_array_v<std::ranges::basic_istream_view<int, char, 
std::char_traits<char> > >' evaluated to false
       requires is_array_v<_Tp> || __member_begin<_Tp&> || __adl_begin<_Tp&>
                ^
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/bits/iterator_concepts.h:883:35: 
note: and 'std::ranges::basic_istream_view<int, char, 
std::char_traits<char> > &' does not satisfy '__member_begin'
       requires is_array_v<_Tp> || __member_begin<_Tp&> || __adl_begin<_Tp&>
                                   ^
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/bits/iterator_concepts.h:867:33: 
note: because '__detail::__decay_copy(__t.begin())' would be invalid: no 
member named 'begin' in 'std::ranges::basic_istream_view<int, char, 
std::char_traits<char> >'
           { __detail::__decay_copy(__t.begin()) } -> 
input_or_output_iterator;
                                        ^
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/bits/iterator_concepts.h:883:59: 
note: and 'std::ranges::basic_istream_view<int, char, 
std::char_traits<char> > &' does not satisfy '__adl_begin'
       requires is_array_v<_Tp> || __member_begin<_Tp&> || __adl_begin<_Tp&>
                                                           ^
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/bits/iterator_concepts.h:877:29: 
note: because '__detail::__decay_copy(begin(__t))' would be invalid: 
call to deleted function 'begin'
           { __detail::__decay_copy(begin(__t)) } -> 
input_or_output_iterator;
                                    ^
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/bits/iterator_concepts.h:902:13: 
error: no matching function for call to '__ranges_begin'
         = decltype(__detail::__ranges_begin(std::declval<_Tp&>()));
                    ^~~~~~~~~~~~~~~~~~~~~~~~
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/bits/range_access.h:874:5: 
note: in instantiation of template type alias '__range_iter_t' requested 
here
     using iterator_t = std::__detail::__range_iter_t<_Tp>;
     ^
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/ranges:140:25: 
note: in instantiation of template type alias 'iterator_t' requested here
         && contiguous_iterator<iterator_t<const _Derived>>
                                ^
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/ranges:956:14: 
note: in instantiation of template class 
'std::ranges::view_interface<std::ranges::basic_istream_view<int, char, 
std::char_traits<char> > >' requested here
     : public view_interface<basic_istream_view<_Val, _CharT, _Traits>>
              ^
istream_begin.cpp:9:14: note: in instantiation of template class 
'std::ranges::basic_istream_view<int, char, std::char_traits<char> >' 
requested here
   auto view{ std::ranges::istream_view<int>(ints) };
              ^
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/bits/iterator_concepts.h:885:7: 
note: candidate template ignored: constraints not satisfied [with _Tp = 
const std::ranges::basic_istream_view<int, char, std::char_traits<char> >]
       __ranges_begin(_Tp& __t)
       ^
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/bits/iterator_concepts.h:883:16: 
note: because 'is_array_v<const std::ranges::basic_istream_view<int, 
char, std::char_traits<char> > >' evaluated to false
       requires is_array_v<_Tp> || __member_begin<_Tp&> || __adl_begin<_Tp&>
                ^
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/bits/iterator_concepts.h:883:35: 
note: and 'const std::ranges::basic_istream_view<int, char, 
std::char_traits<char> > &' does not satisfy '__member_begin'
       requires is_array_v<_Tp> || __member_begin<_Tp&> || __adl_begin<_Tp&>
                                   ^
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/bits/iterator_concepts.h:867:33: 
note: because '__detail::__decay_copy(__t.begin())' would be invalid: no 
member named 'begin' in 'std::ranges::basic_istream_view<int, char, 
std::char_traits<char> >'
           { __detail::__decay_copy(__t.begin()) } -> 
input_or_output_iterator;
                                        ^
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/bits/iterator_concepts.h:883:59: 
note: and 'const std::ranges::basic_istream_view<int, char, 
std::char_traits<char> > &' does not satisfy '__adl_begin'
       requires is_array_v<_Tp> || __member_begin<_Tp&> || __adl_begin<_Tp&>
                                                           ^
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/bits/iterator_concepts.h:877:29: 
note: because '__detail::__decay_copy(begin(__t))' would be invalid: 
call to deleted function 'begin'
           { __detail::__decay_copy(begin(__t)) } -> 
input_or_output_iterator;
                                    ^
In file included from istream_begin.cpp:3:
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/ranges:146:24: 
error: constraints not satisfied for alias template 'sentinel_t' [with 
_Range = std::ranges::basic_istream_view<int, char, 
std::char_traits<char> >]
         && sized_sentinel_for<sentinel_t<_Derived>, iterator_t<_Derived>>
                               ^~~~~~~~~~~~~~~~~~~~
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/ranges:956:14: 
note: in instantiation of template class 
'std::ranges::view_interface<std::ranges::basic_istream_view<int, char, 
std::char_traits<char> > >' requested here
     : public view_interface<basic_istream_view<_Val, _CharT, _Traits>>
              ^
istream_begin.cpp:9:14: note: in instantiation of template class 
'std::ranges::basic_istream_view<int, char, std::char_traits<char> >' 
requested here
   auto view{ std::ranges::istream_view<int>(ints) };
              ^
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/bits/range_access.h:876:12: 
note: because 'std::ranges::basic_istream_view<int, char, 
std::char_traits<char> >' does not satisfy 'range'
   template<range _Range>
            ^
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/bits/range_access.h:864:2: 
note: because 'ranges::begin(__t)' would be invalid: no matching 
function for call to object of type 'const __cust_access::_Begin'
         ranges::begin(__t);
         ^
In file included from istream_begin.cpp:3:
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/ranges:152:24: 
error: constraints not satisfied for alias template 'sentinel_t' [with 
_Range = const std::ranges::basic_istream_view<int, char, 
std::char_traits<char> >]
         && sized_sentinel_for<sentinel_t<const _Derived>,
                               ^~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/bits/range_access.h:876:12: 
note: because 'const std::ranges::basic_istream_view<int, char, 
std::char_traits<char> >' does not satisfy 'range'
   template<range _Range>
            ^
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/bits/range_access.h:864:2: 
note: because 'ranges::begin(__t)' would be invalid: no matching 
function for call to object of type 'const __cust_access::_Begin'
         ranges::begin(__t);
         ^
istream_begin.cpp:11:15: error: no matching function for call to object 
of type 'const __cust_access::_Begin'
   auto begin{ std::ranges::begin(view) };
               ^~~~~~~~~~~~~~~~~~
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/bits/range_access.h:400:2: 
note: candidate template ignored: constraints not satisfied [with _Tp = 
std::ranges::basic_istream_view<int, char, std::char_traits<char> > &]
         operator()(_Tp&& __t) const noexcept(_S_noexcept<_Tp>())
         ^
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/bits/range_access.h:397:11: 
note: because 
'is_array_v<remove_reference_t<std::ranges::basic_istream_view<int, 
char, std::char_traits<char> > &> >' evaluated to false
         requires is_array_v<remove_reference_t<_Tp>> || __member_begin<_Tp>
                  ^
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/bits/range_access.h:397:50: 
note: and 'std::ranges::basic_istream_view<int, char, 
std::char_traits<char> > &' does not satisfy '__member_begin'
         requires is_array_v<remove_reference_t<_Tp>> || __member_begin<_Tp>
                                                         ^
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/bits/iterator_concepts.h:867:33: 
note: because '__detail::__decay_copy(__t.begin())' would be invalid: no 
member named 'begin' in 'std::ranges::basic_istream_view<int, char, 
std::char_traits<char> >'
           { __detail::__decay_copy(__t.begin()) } -> 
input_or_output_iterator;
                                        ^
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/bits/range_access.h:398:7: 
note: and 'std::ranges::basic_istream_view<int, char, 
std::char_traits<char> > &' does not satisfy '__adl_begin'
           || __adl_begin<_Tp>
              ^
/usr/bin/../lib64/gcc/x86_64-suse-linux/10/../../../../include/c++/10/bits/iterator_concepts.h:877:29: 
note: because '__detail::__decay_copy(begin(__t))' would be invalid: 
call to deleted function 'begin'
           { __detail::__decay_copy(begin(__t)) } -> 
input_or_output_iterator;
                                    ^
5 errors generated.




-- 
Ray Lischner


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