https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95606
Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|--- |INVALID
Status|UNCONFIRMED |RESOLVED
--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I think this is not a bug in GCC. A specialization of std::swap still depends
on the signature of the primary template, and that primary template requires
the argument types to be swappable. The is_swappable trait requires
move_constructible, and that requires complete types. Checking
is_move_constructible<json> will perform overload resolution which considers
the basic_json(json_ref) constructor, which tries to see if json_ref can be
constructed from a basic_json rvalue, which checks the constraint on the
json_ref constructor, which depends on basic_json, which is incomplete at this
point.
The json code is incorrect and should be fixed.
The std::swap<json> specialization is wrong and should be replaced by a normal
(non-template) overload in the same namespace as the json type:
void swap(json&, json&) noexcept;
The json_ref constructor should be constrained to avoid recursive
instantiations with incomplete types, e.g.
template<typename T, typename U>
using is_not = std::is_same<std::remove_cv_t<std::remove_reference_t<T>>, U>;
class json_ref
{
public:
template <typename T,
typename = std::enable_if_t<!is_not<T, json_ref>::value>,
typename = std::enable_if_t<!is_not<T, json>::value>,
bool = std::is_constructible<json, T>::value>
json_ref(T &&){}
};
