[Bug libstdc++/103951] New: [C++2b] string_view range constructor, "exception specification ... depends on itself"

[Bug libstdc++/103951] New: [C++2b] string_view range constructor, "exception specification ... depends on itself"

[iamsupermouse at mail dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103951

            Bug ID: 103951
           Summary: [C++2b] string_view range constructor, "exception
                    specification ... depends on itself"
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: iamsupermouse at mail dot ru
  Target Milestone: ---

Following code compiles with -std=c++20 and earlier, but fails with -std=c++2b,
in both GCC and Clang with libstdc++, with similar errors.

I've tested both on GCC 11.2 and on trunk 12.0.0 20220109 at gcc.godbolt.org.

    #include <string_view>

    struct Iter
    {
        Iter() {}
        Iter(std::string_view) {} // Adding `explicit` fixes the error

        Iter begin() const
        {
            return *this;
        }
    };

It's supposed to be used as `for (auto x : Iter(...))`, exactly like
`fs::directory_iterator`, but I've removed `end()` and overloaded operators for
brevity.

The error message boils down to `return *this` considering the `string_view`
constructor, which checks `Iter` against `contiguous_range`, which circularly
checks `begin()`.

I'm not sure if the code is legal or not, but the error is very unintuitive, so
I decided to report it. I would expect this to be a soft SFINAE failure.

[Bug libstdc++/103951] [C++2b] string_view range constructor, "exception specification ... depends on itself"

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103951

康桓瑋 <hewillk at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |hewillk at gmail dot com

--- Comment #1 from 康桓瑋 <hewillk at gmail dot com> ---
Reduced a bit:

#include <ranges>

struct string_view {
  string_view(std::ranges::range auto&&);
};

struct Iter {
  Iter(string_view);
  Iter begin() const { return *this; }
};

https://godbolt.org/z/n93o9f9nK

[Bug libstdc++/103951] [C++2b] string_view range constructor, "exception specification ... depends on itself"

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103951

--- Comment #2 from 康桓瑋 <hewillk at gmail dot com> ---
Here is the reduced version without the header:

template<class T> 
T&& declval() noexcept;

struct true_type {
  static constexpr bool value = true;
};

template<typename _From, typename _To>
class is_convertible {
  template<typename _To1>
  static void __test_aux(_To1) noexcept;

  template<typename _From1, typename _To1,
           typename = decltype(__test_aux<_To1>(declval<_From1>()))>
  static true_type __test(int);

 public:
  static constexpr bool value = decltype(__test<_From, _To>(0))::value;
};

template<typename _Tp>
void __decay_copy(_Tp) noexcept(is_convertible<_Tp, _Tp>::value);

template<class _Tp>
concept range = requires(_Tp& __t) {
  __decay_copy(__t.begin());
};

struct string_view {
  template<range R>
  string_view(R&&);
};

struct Iter {
  Iter(string_view);
  Iter begin() const { return *this; }
};

https://godbolt.org/z/nrcq84dno

[Bug libstdc++/103951] [C++2b] string_view range constructor, "exception specification ... depends on itself"

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103951

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This would be solved by
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p2499r0.html but I
think the std::lib code is behaving exactly as the C++23 draft requires.