[Bug libstdc++/106201] filesystem::directory_iterator is a borrowable range?

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106201

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #1)
> The third one fails but not because of anything to do with borrowable
> ranges. The problem is that the filesystem::swap(path&, path&) overload is
> found by ADL and considered by overload resolution when checking whether
> counted_iterator<filesystem::directory_iterator> is swappable. Overload
> resolution checks whether the iterator can be converted to a path, which
> causes constraint recursion.

The full explanation is that views::take creates a view whose iterator is
counted_iterator<filesystem::directory_iterator> and the constraints for
ranges::for_each check that that iterator is copyable, movable, and swappable.
That fails with libstdc++:

#include <filesystem>
#include <ranges>
using I = std::counted_iterator<std::filesystem::directory_iterator>;
static_assert( std::swappable<I> );

The swappable concept finds filesystem::swap(path&, path&) via ADL. Overload
resolution checks if counted_iterator<filesystem::directory_iterator> can be
converted to path, which considers the path constructors, which checks if
counted_iterator is an iterator, which uses std::iterator_traits, which depends
on the iterator being copyable, which depends on it being swappable, and we're
recursing.

However, I *think* there's a compiler bug here. We should not be considering
constructors of path to see if we can get a path& lvalue. That would never
work, constructing a path would give us an rvalue. Clang compiles the example
when using libstdc++ headers.

Clang and EDG both compile this, but G++ doesn't:

namespace effing
{
  template<typename T>
    concept effable = requires (T& t) { f(t); };

  template <class T>
    requires effable<T> || true
  void eff(T&&) { }
}

struct path {
  template<effing::effable T> path(const T&) { }
};

void f(path&);

struct iterator { };

int main()
{
  iterator i;
  effing::eff(i);
}

conv.C: In substitution of 'template<class T>  requires  effable<T>
path::path(const T&) [with T = iterator]':
conv.C:4:42:   recursively required by substitution of 'template<class T> 
requires  effable<T> path::path(const T&) [with T = iterator]'
conv.C:4:42:   required by substitution of 'template<class T>  requires
(effable<T>) || true void effing::eff(T&&) [with T = iterator&]'
conv.C:22:14:   required from here
conv.C:4:13:   required for the satisfaction of 'effable<T>' [with T =
iterator]
conv.C:4:23:   in requirements with 'T& t' [with T = iterator]
conv.C:4:23: error: satisfaction of atomic constraint 'requires(T& t) {f(t);}
[with T = T]' depends on itself
    4 |     concept effable = requires (T& t) { f(t); };
      |                       ^~~~~~~~~~~~~~~~~~~~~~~~~