How do I put a `std::expected` into a range?

Question

The following code compiles:

#if 0 /*
g++ -std=c++23 $0 -o exe \
  -Wall -Werror -Wextra -Wsign-conversion -pedantic-errors \
  && ./exe $@
RET=$?
rm -f exe
exit $RET
*/
#endif

#include <expected>
#include <ranges>
#include <vector>

int main() {
  auto v = std::vector{std::expected<int, int>(0)};
  auto r1 = std::views::all(std::vector{0});
  // auto r2 = std::views::all(v);
}

But if I uncomment the definition of r2, I get the following error:

In file included from ./expected-in-range.cpp:11:
/usr/include/c++/15.1.1/expected: In substitution of ‘template<class _Up>  requires !(__is_expected<_Up>) && requires(const _Tp& __t, const _Up& __u) {{__t == __u} -> decltype(auto) [requires std::convertible_to<<placeholder>, bool>];} constexpr bool std::operator==(const expected<int, int>&, const _Up&) [with _Up = __gnu_cxx::__normal_iterator<std::expected<int, int>*, std::vector<std::expected<int, int>, std::allocator<std::expected<int, int> > > >]’:
/usr/include/c++/15.1.1/expected:1175:12:   required by substitution of ‘template<class _Up>  requires !(__is_expected<_Up>) && requires(const _Tp& __t, const _Up& __u) {{__t == __u} -> decltype(auto) [requires std::convertible_to<<placeholder>, bool>];} constexpr bool std::operator==(const expected<int, int>&, const _Up&) [with _Up = __gnu_cxx::__normal_iterator<std::expected<int, int>*, std::vector<std::expected<int, int>, std::allocator<std::expected<int, int> > > >]’
 1175 |             { __t == __u } -> convertible_to<bool>;
      |               ~~~~^~~~~~
/usr/include/c++/15.1.1/concepts:306:10:   required by substitution of ‘template<class _Tp>  requires (__maybe_borrowed_range<_Tp>) && ((is_bounded_array_v<typename std::remove_reference<_Tp>::type>) || (__member_end<_Tp>) || (__adl_end<_Tp>)) constexpr auto std::ranges::__access::_End::operator()(_Tp&&) const [with _Tp = std::vector<std::expected<int, int>, std::allocator<std::expected<int, int> > >&]’
  306 |           { __t == __u } -> __boolean_testable;
      |             ~~~~^~~~~~
/usr/include/c++/15.1.1/bits/ranges_base.h:517:13:   required by substitution of ‘template<class _Range>  requires (viewable_range<_Range>) && ((view<typename std::decay<_Tp>::type>) || (__can_ref_view<_Range>) || (__can_owning_view<_Range>)) constexpr auto std::ranges::views::_All::operator()(_Range&&) const [with _Range = std::vector<std::expected<int, int>, std::allocator<std::expected<int, int> > >&]’
  517 |         ranges::end(__t);
      |         ~~~~~~~~~~~^~~~~
./expected-in-range.cpp:18:28:   required from here
   18 |   auto r2 = std::views::all(v);
      |             ~~~~~~~~~~~~~~~^~~
/usr/include/c++/15.1.1/expected:1178:2:   required by the constraints of ‘template<class _Tp, class _Er> template<class _Up>  requires !(__is_expected<_Up>) && requires(const _Tp& __t, const _Up& __u) {{__t == __u} -> decltype(auto) [requires std::convertible_to<<placeholder>, bool>];} constexpr bool std::operator==(const expected<_Tp, _Er>&, const _Up&)’
/usr/include/c++/15.1.1/expected:1174:7:   in requirements with ‘const _Tp& __t’, ‘const _Up& __u’ [with _Tp = int; _Up = __gnu_cxx::__normal_iterator<std::expected<int, int>*, std::vector<std::expected<int, int>, std::allocator<std::expected<int, int> > > >]
/usr/include/c++/15.1.1/expected:1174:14: error: satisfaction of atomic constraint ‘requires(const _Tp& __t, const _Up& __u) {{__t == __u} -> decltype(auto) [requires std::convertible_to<<placeholder>, bool>];} [with _Tp = _Tp; _Up = _Up]’ depends on itself
 1174 |           && requires (const _Tp& __t, const _Up& __u) {
      |              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1175 |             { __t == __u } -> convertible_to<bool>;
      |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1176 |           }
      |           ~

How am I meant to understand that error? What does it mean? How can I do what I'm trying to do?

The above is an SSCCE - the full thing I'm trying to do is have a function that accepts some iterable of ints, a second function that returns an expected<int, Something>, and pass the output of a bunch of invocations of the second function to the first by doing

f(
  all(std::vector{g(), g(), g()})
  | filter([](auto e){ return e.has_value(); })
  | transform([](auto e){ return e.value(); })
);

So if there's a less foolish way of doing that, I'd also accept that as an answer.

For what it's worth, when I tried your test code on Godbolt, it didn't compile with gcc 15.1 (as expected) but did compile with clang 20.1.0: godbolt.org/z/49szEP54q — jjramsey
– jjramsey, Commented Jun 20 at 17:34
Could be a bug in gcc as well. Judging from en.cppreference.com/w/cpp/ranges/all_view.html, it doesn't look like you're doing something obviously wrong. — jjramsey
– jjramsey, Commented Jun 20 at 17:41
it looks like when constructing a range from the vector, it's trying to find operator==(expected<int,int>, vector::iterator) for some reason, but I can't fathom why. — Mooing Duck
– Mooing Duck, Commented Jun 20 at 17:42
After studying the details deeper, it seems to be have something to do with checking if this is a "borrowed" range, and dangling pointer checks. But regardless of the reason, it shouldn't be comparing elements to iterators, so you've found a bug in this std::range implementation code. — Mooing Duck
– Mooing Duck, Commented Jun 20 at 17:47
No, you definitely found a bug. It should definitely not be comparing an element to an iterator. I'm just trying to work out how to bypass the bug — Mooing Duck
– Mooing Duck, Commented Jun 20 at 17:52

康桓瑋 · Accepted Answer · 2025-06-20 18:39:42Z

9

This is a GCC Bug 119714 which has been fixed in trunk.

answered Jun 20 at 18:39

康桓瑋

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