[Bug libstdc++/107871] New: _Iter_sink:: _M_overflow missing some difference type casting

[Bug libstdc++/107871] New: _Iter_sink:: _M_overflow missing some difference type casting

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107871

            Bug ID: 107871
           Summary: _Iter_sink:: _M_overflow missing some difference type
                    casting
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hewillk at gmail dot com
  Target Milestone: ---

#include <format>

struct O {
  using difference_type = std::ranges::__detail::__max_diff_type;
  O& operator=(const char&);
  O& operator*();
  O& operator++();
  O& operator++(int);
};

int main() {
  std::format_to_n(O{}, 4, "{}", " ");
}

https://godbolt.org/z/x78qjfse6

[Bug libstdc++/107871] _Iter_sink:: _M_overflow missing some difference type casting

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107871

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This isn't valid code, you can't use max diff type for your own types. It can
only be used by the library for iota_view, but that doesn't have output
iterators.

[Bug libstdc++/107871] _Iter_sink:: _M_overflow missing some difference type casting

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107871

--- Comment #2 from 康桓瑋 <hewillk at gmail dot com> ---
This is just an example. So, are users not allowed to define integer-like class
types?

[Bug libstdc++/107871] _Iter_sink:: _M_overflow missing some difference type casting

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107871

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
No. They are only allowed to be implementation-defined types, not
program-defined types.

I can add the casts to make this work, but I don't think it's a real problem
that can occur in valid programs.

[Bug libstdc++/107871] _Iter_sink:: _M_overflow missing some difference type casting

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107871

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Maybe you could legally do:

using difference_type = iterator_t<iota_view<__int128, __int128>>;

but maybe just don't do that. If things break when you do dumb things, don't do
those things.

[Bug libstdc++/107871] _Iter_sink:: _M_overflow missing some difference type casting

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107871

--- Comment #5 from 康桓瑋 <hewillk at gmail dot com> ---
(In reply to Jonathan Wakely from comment #4)
> Maybe you could legally do:
> 
> using difference_type = iterator_t<iota_view<__int128, __int128>>;
> 
> but maybe just don't do that. If things break when you do dumb things, don't
> do those things.

I would never do that.

However, I see that you seem to have considered this issue elsewhere, in
format#L2561:

            if constexpr (!is_integral_v<decltype(__n)>
                            || sizeof(__n) > sizeof(size_t))
              {
                // __int128 or __detail::__max_diff_type
                auto __m = (decltype(__n))(size_t)-1;
                if (__n > __m)
                  __n = __m;
              }

[Bug libstdc++/107871] _Iter_sink:: _M_overflow missing some difference type casting

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107871

--- Comment #6 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:9cce91a63dcc40cb39e68f6a771e891e51c51946

commit r13-4527-g9cce91a63dcc40cb39e68f6a771e891e51c51946
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Dec 5 21:38:53 2022 +0000

    libstdc++: Add casts for integer-like difference type [PR107871]

    libstdc++-v3/ChangeLog:

            PR libstdc++/107871
            * include/std/format (_Iter_sink::_M_overflow): Add cast to
            size_t.
            (_Iter_sink<CharT, contiguous_iterator auto>::_M_make_span): Use
            typedef instead of decltype.
            * testsuite/std/format/functions/107871.cc: New test.

[Bug libstdc++/107871] _Iter_sink:: _M_overflow missing some difference type casting

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107871

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

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

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Oh yes, I wonder why I bothered handling it there :-)

It should be fixed now, thanks for the report.

[Bug libstdc++/107871] _Iter_sink:: _M_overflow missing some difference type casting

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107871

--- Comment #8 from 康桓瑋 <hewillk at gmail dot com> ---
(In reply to Jonathan Wakely from comment #4)
> Maybe you could legally do:
> 
> using difference_type = iterator_t<iota_view<__int128, __int128>>;
> 
> but maybe just don't do that. If things break when you do dumb things, don't
> do those things.

A more uncontrived example would be:

  std::vector<char> v(10);
  auto r = std::views::iota((unsigned __int128)0) | 
    std::views::transform([&](auto i) -> auto& { return v[i]; });
  auto s = std::format_to_n(r.begin(), 5, "{}", "");