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); };
| ^~~~~~~~~~~~~~~~~~~~~~~~~
