[Bug c++/94674] New: std::ranges::basic_istream_view::iterator is missing std::iterator_traits entries

[Bug c++/94674] New: std::ranges::basic_istream_view::iterator is missing std::iterator_traits entries

[gcc-bugs at marehr dot dialup.fu-berlin.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94674

            Bug ID: 94674
           Summary: std::ranges::basic_istream_view::iterator is missing
                    std::iterator_traits entries
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc-bugs at marehr dot dialup.fu-berlin.de
  Target Milestone: ---

Hi gcc-team,

I'm not sure if I should have named the issue `std::iterator_traits is missing
default values for an iterator` (https://eel.is/c++draft/iterator.traits#3.2).

Maybe I misunderstood this, but I thought that I can query every iterator for
it's traits.

The following code does not work:

```c++
#include <ranges>
#include <istream>
#include <string_view>

int main()
{
    // std::string_view view; // does work
    auto && view = std::ranges::basic_istream_view<int, char,
std::char_traits<char>>{}; // does not work
    // auto && view = std::ranges::take_view<std::string_view>{}; // does work

    using iterator_t = std::ranges::iterator_t<decltype(view)>;
    using iter_value1_t = std::iter_value_t<iterator_t>;
    using iter_value2_t = typename
std::iterator_traits<iterator_t>::value_type;

    static_assert(std::same_as<iter_value1_t, iter_value2_t>);
    return 0;
}
```

with `g++-10 -std=c++2a`


https://godbolt.org/z/QAfdml

[Bug libstdc++/94674] std::ranges::basic_istream_view::iterator is missing std::iterator_traits entries

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94674

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This is a known defect in the C++20 draft:
https://cplusplus.github.io/LWG/issue3394

[Bug libstdc++/94674] std::ranges::basic_istream_view::iterator is missing std::iterator_traits entries

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94674

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

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

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The current disposition of the Library Working Group is to leave it that way,
because it doesn't meet the traditional iterator requirements, so it's expected
that iterator_traits is empty for it.

As you already know, you can use range_value_t<decltype(view)> to get the value
type instead, and range_difference_t for the difference type, and use the
concepts std::input_iterator, std::forward_iterator etc. to determine which
concepts it satisfies. Code written to work with ranges should rely on those
interfaces, not iterator_traits, which is for traditional iterator types.

[Bug libstdc++/94674] std::ranges::basic_istream_view::iterator is missing std::iterator_traits entries

[gcc-bugs at marehr dot dialup.fu-berlin.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94674

--- Comment #3 from gcc-bugs at marehr dot dialup.fu-berlin.de ---
Thank you for pointing me to this.

I find this highly unexpected. There was made a change to `std::type_traits` in
C++20 that sets default values, but it does not apply to all iterators?

My use case was slightly different, I wanted to extract the `pointer` member
type and there is no equivalent like `std::ranges::range_pointer_t` for that.

This makes it particularly cumbersome to "forward" properties of an underlying
iterator which is used when wrapping it in a new iterator.

[Bug libstdc++/94674] std::ranges::basic_istream_view::iterator is missing std::iterator_traits entries

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94674

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
That iterator doesn't have a pointer type, because in the new Ranges world that
type is not useful. It is no longer required for iterators to have operator->
so what does the 'pointer' type even mean?

[Bug libstdc++/94674] std::ranges::basic_istream_view::iterator is missing std::iterator_traits entries

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94674

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Your mistake is thinking that the iterators of views are like the iterators
you're used to.

They're not.

They have different properties (e.g. they might not be copyable, they might not
have operator->, they might not define iterator_category) and
std::iterator_traits is for use with the old kind of iterators, not the new
ones.