https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109751
--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I think the error message is correct.
In the original code we have:
```
...
template<typename _Iter>
concept incrementable = regular<_Iter> && requires(_Iter __i) { { __i++ }
-> same_as<_Iter>; };
...
namespace boost { namespace stl_interfaces { inline namespace v2 {
template< typename D, typename IteratorConcept, typename
DifferenceType>
struct iterator_interface
{
friend constexpr bool operator>=(D lhs, D rhs)
requires std::equality_comparable<D> { }
};
...
template<typename T>
struct iterator : boost::stl_interfaces::iterator_interface<iterator<T>,
std::forward_iterator_tag, T>
{
bool operator==(iterator) const;
};
```
iterator<T> is not complete until after the instantiation is finished but we
have a std::equality_comparable which does the confusion
