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3746. optional's spaceship with U with a type derived from optional causes infinite constraint meta-recursion

Section: 22.5.8 [optional.comp.with.t] Status: WP Submitter: Ville Voutilainen Opened: 2022-07-25 Last modified: 2022-11-17

Priority: Not Prioritized

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Discussion:

What ends up happening is that the constraints of operator<=>(const optional<T>&, const U&) end up in three_way_comparable_with, and then in partially-ordered-with, and the expressions there end up performing a conversion from U to an optional, and we end up instantiating the same operator<=> again, evaluating its constraints again, until the compiler bails out.

See an online example here.

All implementations end up with infinite meta-recursion.

The solution to the problem is to stop the meta-recursion by constraining the spaceship with U so that U is not publicly and unambiguously derived from a specialization of optional, SFINAEing that candidate out, and letting 22.5.6 [optional.relops]/20 perform the comparison instead.

[2022-08-23; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4910.

[Drafting note: This proposed wording removes the only use of is-optional. ]

1. Modify 22.5.2 [optional.syn], header <optional> synopsis, as indicated:

   […]
   namespace std {
     // 22.5.3 [optional.optional], class template optional
     template<class T>
       class optional;
   
     template<class T>
       constexpr bool is-optional = false; // exposition only
     template<class T>
       constexpr bool is-optional<optional<T>> = true; // exposition only
     template<class T>
       concept is-derived-from-optional = requires(const T& t) { // exposition only
         []<class U>(const optional<U>&){ }(t);
       };
     […]
     // 22.5.8 [optional.comp.with.t], comparison with T
     […]
     template<class T, class U> constexpr bool operator>=(const T&, const optional<U>&);
     template<class T, class U> requires (!is-derived-from-optional<U>) && three_way_comparable_with<T, U>
       constexpr compare_three_way_result_t<T, U>
         operator<=>(const optional<T>&, const U&);
     […]
   }
   

2. Modify 22.5.8 [optional.comp.with.t] as indicated:

   template<class T, class U> requires (!is-derived-from-optional<U>) && three_way_comparable_with<T, U>
     constexpr compare_three_way_result_t<T, U>
       operator<=>(const optional<T>&, const U&);
   

   -25- Effects: Equivalent to: return x.has_value() ? *x <=> v : strong_ordering::less;