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2934. optional<const T> doesn't compare with T

Section: 22.5.8 [optional.comp.with.t] Status: C++17 Submitter: Ville Voutilainen Opened: 2017-02-17 Last modified: 2021-06-06

Priority: Not Prioritized

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Discussion:

Consider: 

optional<const int> x = 42;
int y = 666;
x == y; // ill-formed

The comparison is ill-formed, because in [optional.comp_with_t]/1, 

template <class T> constexpr bool operator==(const optional<T>& x, const T& v);

the T is deduced to be both const int and int, which is ill-formed.

Since it became apparent that the root cause of this issue is also causing difficulties with e.g. comparing optional<string> with literals etc., here's a p/r that

  1. adds requirements for optional's comparisons with T

  2. turns optional-optional comparisons into two-template-parameter templates, allowing comparing mixed optionals

  3. turns optional-T comparisons into two-template-parameter templates, allowing comparing optionals with T and types comparable with T

[Kona 2017-02-28]

Accepted as Immediate to avoid having break ABI later.

Proposed resolution:

This wording is relative to N4640.

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

    […]

// 22.5.6 [optional.relops], relational operators
template <class T>
constexpr bool operator==(const optional<T>&, const optional<T>&);
template <class T>
constexpr bool operator!=(const optional<T>&, const optional<T>&);
template <class T>
constexpr bool operator<(const optional<T>&, const optional<T>&);
template <class T>
constexpr bool operator>(const optional<T>&, const optional<T>&);
template <class T>
constexpr bool operator<=(const optional<T>&, const optional<T>&);
template <class T>
constexpr bool operator>=(const optional<T>&, const optional<T>&);

[…]

//  [optional.comp_with_t], comparison with T
template <class T> constexpr bool operator==(const optional<T>&, const T&);
template <class T> constexpr bool operator==(const T&, const optional<T>&);
template <class T> constexpr bool operator!=(const optional<T>&, const T&);
template <class T> constexpr bool operator!=(const T&, const optional<T>&);
template <class T> constexpr bool operator<(const optional<T>&, const T&);
template <class T> constexpr bool operator<(const T&, const optional<T>&);
template <class T> constexpr bool operator<=(const optional<T>&, const T&);
template <class T> constexpr bool operator<=(const T&, const optional<T>&);
template <class T> constexpr bool operator>(const optional<T>&, const T&);
template <class T> constexpr bool operator>(const T&, const optional<T>&);
template <class T> constexpr bool operator>=(const optional<T>&, const T&);
template <class T> constexpr bool operator>=(const T&, const optional<T>&);

  2. Modify 22.5.6 [optional.relops] as indicated:

    template <class T> constexpr bool operator==(const optional<T>& x, const optional<T>& y);

    […] 

    template <class T> constexpr bool operator!=(const optional<T>& x, const optional<T>& y);

    […] 

    template <class T> constexpr bool operator<(const optional<T>& x, const optional<T>& y);

    […] 

    template <class T> constexpr bool operator>(const optional<T>& x, const optional<T>& y);

    […] 

    template <class T> constexpr bool operator<=(const optional<T>& x, const optional<T>& y);

    […] 

    template <class T> constexpr bool operator>=(const optional<T>& x, const optional<T>& y);

    […]

  3. Modify [optional.comp_with_t] as indicated:

    template <class T> constexpr bool operator==(const optional<T>& x, const T& v);

    -1- Effects: Equivalent to: return bool(x) ? *x == v : false; 

    template <class T> constexpr bool operator==(const T& v, const optional<T>& x);

    -2- Effects: Equivalent to: return bool(x) ? v == *x : false; 

    template <class T> constexpr bool operator!=(const optional<T>& x, const T& v);

    -3- Effects: Equivalent to: return bool(x) ? *x != v : true; 

    template <class T> constexpr bool operator!=(const T& v, const optional<T>& x);

    -4- Effects: Equivalent to: return bool(x) ? v != *x : true; 

    template <class T> constexpr bool operator<(const optional<T>& x, const T& v);

    -5- Effects: Equivalent to: return bool(x) ? *x < v : true; 

    template <class T> constexpr bool operator<(const T& v, const optional<T>& x);

    -6- Effects: Equivalent to: return bool(x) ? v < *x : false; 

    template <class T> constexpr bool operator<=(const optional<T>& x, const T& v);

    -7- Effects: Equivalent to: return bool(x) ? *x <= v : true; 

    template <class T> constexpr bool operator<=(const T& v, const optional<T>& x);

    -8- Effects: Equivalent to: return bool(x) ? v <= *x : false; 

    template <class T> constexpr bool operator>(const optional<T>& x, const T& v);

    -9- Effects: Equivalent to: return bool(x) ? *x > v : false; 

    template <class T> constexpr bool operator>(const T& v, const optional<T>& x);

    -10- Effects: Equivalent to: return bool(x) ? v > *x : true; 

    template <class T> constexpr bool operator>=(const optional<T>& x, const T& v);

    -11- Effects: Equivalent to: return bool(x) ? *x >= v : false; 

    template <class T> constexpr bool operator>=(const T& v, const optional<T>& x);

    -12- Effects: Equivalent to: return bool(x) ? v >= *x : true;