[Bug libstdc++/102181] New: std::advance and std::views::iota<std::int64_t> don't work

[Bug libstdc++/102181] New: std::advance and std::views::iota<std::int64_t> don't work

[raffael at casagrande dot ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102181

            Bug ID: 102181
           Summary: std::advance and std::views::iota<std::int64_t> don't
                    work
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: raffael at casagrande dot ch
  Target Milestone: ---

The following simple program doesn't compile:

#include <ranges>

int main() {
  using type = std::int64_t; // using type = int works!
  auto v = std::ranges::iota_view(static_cast<type>(0),
static_cast<type>(100));
  auto b = v.begin();
  std::advance(b, static_cast<type>(1));
}


gcc version: 12.0.0 20210901 (experimental)

Error messages:

/opt/compiler-explorer/gcc-trunk-20210902/include/c++/12.0.0/bits/stl_iterator_base_funcs.h:
In instantiation of 'constexpr void std::advance(_InputIterator&, _Distance)
[with _InputIterator = std::ranges::iota_view<long int, long int>::_Iterator;
_Distance = long int]':
<source>:7:15:   required from here
/opt/compiler-explorer/gcc-trunk-20210902/include/c++/12.0.0/bits/stl_iterator_base_funcs.h:207:21:
error: no matching function for call to '__advance(std::ranges::iota_view<long
int, long int>::_Iterator&, std::__iterator_traits<std::ranges::iota_view<long
int, long int>::_Iterator, void>::difference_type&,
std::__iterator_traits<std::ranges::iota_view<long int, long int>::_Iterator,
void>::iterator_category)'
  207 |       std::__advance(__i, __d, std::__iterator_category(__i));
      |       ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-trunk-20210902/include/c++/12.0.0/bits/stl_iterator_base_funcs.h:148:5:
note: candidate: 'template<class _InputIterator, class _Distance> constexpr
void std::__advance(_InputIterator&, _Distance, std::input_iterator_tag)'
  148 |     __advance(_InputIterator& __i, _Distance __n, input_iterator_tag)
      |     ^~~~~~~~~
/opt/compiler-explorer/gcc-trunk-20210902/include/c++/12.0.0/bits/stl_iterator_base_funcs.h:148:5:
note:   template argument deduction/substitution failed:
/opt/compiler-explorer/gcc-trunk-20210902/include/c++/12.0.0/bits/stl_iterator_base_funcs.h:207:56:
note:   cannot convert 'std::__iterator_category<std::ranges::iota_view<long
int, long int>::_Iterator>((*(const std::ranges::iota_view<long int, long
int>::_Iterator*)(& __i)))' (type
'std::__iterator_traits<std::ranges::iota_view<long int, long int>::_Iterator,
void>::iterator_category' {aka 'std::output_iterator_tag'}) to type
'std::input_iterator_tag'
  207 |       std::__advance(__i, __d, std::__iterator_category(__i));
      |                                ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
/opt/compiler-explorer/gcc-trunk-20210902/include/c++/12.0.0/bits/stl_iterator_base_funcs.h:159:5:
note: candidate: 'template<class _BidirectionalIterator, class _Distance>
constexpr void std::__advance(_BidirectionalIterator&, _Distance,
std::bidirectional_iterator_tag)'
  159 |     __advance(_BidirectionalIterator& __i, _Distance __n,
      |     ^~~~~~~~~
/opt/compiler-explorer/gcc-trunk-20210902/include/c++/12.0.0/bits/stl_iterator_base_funcs.h:159:5:
note:   template argument deduction/substitution failed:
/opt/compiler-explorer/gcc-trunk-20210902/include/c++/12.0.0/bits/stl_iterator_base_funcs.h:207:56:
note:   cannot convert 'std::__iterator_category<std::ranges::iota_view<long
int, long int>::_Iterator>((*(const std::ranges::iota_view<long int, long
int>::_Iterator*)(& __i)))' (type
'std::__iterator_traits<std::ranges::iota_view<long int, long int>::_Iterator,
void>::iterator_category' {aka 'std::output_iterator_tag'}) to type
'std::bidirectional_iterator_tag'
  207 |       std::__advance(__i, __d, std::__iterator_category(__i));
      |                                ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
/opt/compiler-explorer/gcc-trunk-20210902/include/c++/12.0.0/bits/stl_iterator_base_funcs.h:175:5:
note: candidate: 'template<class _RandomAccessIterator, class _Distance>
constexpr void std::__advance(_RandomAccessIterator&, _Distance,
std::random_access_iterator_tag)'
  175 |     __advance(_RandomAccessIterator& __i, _Distance __n,
      |     ^~~~~~~~~
/opt/compiler-explorer/gcc-trunk-20210902/include/c++/12.0.0/bits/stl_iterator_base_funcs.h:175:5:
note:   template argument deduction/substitution failed:
/opt/compiler-explorer/gcc-trunk-20210902/include/c++/12.0.0/bits/stl_iterator_base_funcs.h:207:56:
note:   cannot convert 'std::__iterator_category<std::ranges::iota_view<long
int, long int>::_Iterator>((*(const std::ranges::iota_view<long int, long
int>::_Iterator*)(& __i)))' (type
'std::__iterator_traits<std::ranges::iota_view<long int, long int>::_Iterator,
void>::iterator_category' {aka 'std::output_iterator_tag'}) to type
'std::random_access_iterator_tag'
  207 |       std::__advance(__i, __d, std::__iterator_category(__i));
      |                                ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~

[Bug libstdc++/102181] std::advance and std::views::iota<std::int64_t> don't work

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102181

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It compiles with -std=gnu++20 but not -std=c++20

[Bug libstdc++/102181] std::advance and std::views::iota<std::int64_t> don't work

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102181

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
You need to use std::ranges::advance here, because iota_view is "special".

The reason std::advance doesn't work is that the type of v.begin() does not
meet the Cpp17InputIterator requirements, because its difference_type is
__int128, which is not a signed integer type when using -std=c++17 (but it is
when using -std=gnu++20). Failing to meet the Cpp17InputIterator requirements
means that std::iterator_traits<decltype(b)>::iterator_category defaults to
output_iterator_tag. And you can't advance an output iterator.

[Bug libstdc++/102181] std::advance and std::views::iota<std::int64_t> don't work

[raffael at casagrande dot ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102181

--- Comment #3 from Raffael Casagrande <raffael at casagrande dot ch> ---
thanks for the fast response. I can switch over to std::ranges::advance.

MSVC compiles the snippet without problems...

[Bug libstdc++/102181] std::advance and std::views::iota<std::int64_t> don't work

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102181

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
https://en.cppreference.com/w/cpp/iterator/advance says "InputIt must meet the
requirements of LegacyInputIterator."

https://en.cppreference.com/w/cpp/named_req/InputIterator gives a sample
concept you can use to test whether something meets those requirements. The
iterator_t<iota_view<int64_t, int64_t>> type does not satisfy that concept.

I think GCC is behaving according to the standard.

You can use std::ranges::advance because that requires
std::input_or_output_iterator, which is satisfied by your iota_view's iterator.


#include <ranges>

int main() {
  using type = std::int64_t; // using type = int works!
  auto v = std::views::iota(static_cast<type>(0), static_cast<type>(100));
  auto b = v.begin();

  static_assert( ! __LegacyInputIterator<decltype(b)> );
  // So this can't work:
  // std::advance(b, 1);

  static_assert( std::input_iterator<decltype(b)> );
  // So this can work:
  std::ranges::advance(b, static_cast<type>(1));
}

[Bug libstdc++/102181] std::advance and std::views::iota<std::int64_t> don't work

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102181

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Raffael Casagrande from comment #3)
> MSVC compiles the snippet without problems...

It looks like they do not correctly implement iota_view. This C++20 proposal,
specifically:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1522r1.pdf

This program fails with MSVC:

#include <ranges>

int main() {
  using type = uint64_t; // using type = int works!
  auto v = std::views::iota(0ull, ~0ull);
  auto b = v.end() - v.begin();
  static_assert(!std::same_as<decltype(b), int64_t>);
}

It is required to compile. You cannot use int64_t to represent the difference
type of an iota_view with that many elements.

I think GCC is correct according to the standard.

[Bug libstdc++/102181] std::advance and std::views::iota<std::int64_t> don't work

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102181

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |redi at gcc dot gnu.org
     Ever confirmed|0                           |1
         Resolution|INVALID                     |---
   Last reconfirmed|                            |2021-09-02
             Status|RESOLVED                    |ASSIGNED

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Reopening, because we can make this Just Work, even though we're not required
to.

--- a/libstdc++-v3/include/bits/stl_iterator_base_funcs.h
+++ b/libstdc++-v3/include/bits/stl_iterator_base_funcs.h
@@ -211,7 +222,11 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
     {
       // concept requirements -- taken care of in __advance
       typename iterator_traits<_InputIterator>::difference_type __d = __n;
+#if __cpp_lib_concepts
+      std::__advance(__i, __d, __detail::__iter_concept<_InputIterator>{});
+#else
       std::__advance(__i, __d, std::__iterator_category(__i));
+#endif
     }

 #if __cplusplus >= 201103L

[Bug libstdc++/102181] std::advance and std::views::iota<std::int64_t> don't work

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102181

--- Comment #7 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:d9dfd7ad3e0196f60a3fc6df6d65a40fb905409f

commit r12-4355-gd9dfd7ad3e0196f60a3fc6df6d65a40fb905409f
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Sep 29 21:19:36 2021 +0100

    libstdc++: Improve diagnostics for misuses of output iterators

    This adds deleted overloads so that the errors for invalid uses of
    std::advance and std::distance are easier to understand (see for example
    PR 102181).

    libstdc++-v3/ChangeLog:

            * include/bits/stl_iterator_base_funcs.h (__advance): Add
            deleted overload to improve diagnostics.
            (__distance): Likewise.

[Bug libstdc++/102181] [DR 3197] std::advance and std::views::iota<std::int64_t> don't work

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102181

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |SUSPENDED
            Summary|std::advance and            |[DR 3197] std::advance and
                   |std::views::iota<std::int64 |std::views::iota<std::int64
                   |_t> don't work              |_t> don't work

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Suspending until https://wg21.link/lwg3197 is resolved.