Issue 2934: optional<const T> doesn't compare with T

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

View all other issues in [optional.comp.with.t].

View all issues with C++17 status.

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

adds requirements for optional's comparisons with T
turns optional-optional comparisons into two-template-parameter templates, allowing comparing mixed optionals
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.

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

[…]

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

[…]

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

Modify 22.5.6 [optional.relops] as indicated:

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

[…]

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

[…]

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

[…]

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

[…]

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

[…]

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

[…]

Modify [optional.comp_with_t] as indicated:
```
template <class T, class U> constexpr bool operator==(const optional<T>& x, const TU& v);
```
-?- Requires: The expression *x == v shall be well-formed and its result shall be convertible to bool. [Note: T need not be EqualityComparable. — end note]
-1- Effects: Equivalent to: return bool(x) ? *x == v : false;
```
template <class T, class U> constexpr bool operator==(const TU& v, const optional<T>& x);
```
-?- Requires: The expression v == *x shall be well-formed and its result shall be convertible to bool.
-2- Effects: Equivalent to: return bool(x) ? v == *x : false;
```
template <class T, class U> constexpr bool operator!=(const optional<T>& x, const TU& v);
```
-?- Requires: The expression *x != v shall be well-formed and its result shall be convertible to bool.
-3- Effects: Equivalent to: return bool(x) ? *x != v : true;
```
template <class T, class U> constexpr bool operator!=(const TU& v, const optional<T>& x);
```
-?- Requires: The expression v != *x shall be well-formed and its result shall be convertible to bool.
-4- Effects: Equivalent to: return bool(x) ? v != *x : true;
```
template <class T, class U> constexpr bool operator<(const optional<T>& x, const TU& v);
```
-?- Requires: The expression *x < v shall be well-formed and its result shall be convertible to bool.
-5- Effects: Equivalent to: return bool(x) ? *x < v : true;
```
template <class T, class U> constexpr bool operator<(const TU& v, const optional<T>& x);
```
-?- Requires: The expression v < *x shall be well-formed and its result shall be convertible to bool.
-6- Effects: Equivalent to: return bool(x) ? v < *x : false;
```
template <class T, class U> constexpr bool operator<=(const optional<T>& x, const TU& v);
```
-?- Requires: The expression *x <= v shall be well-formed and its result shall be convertible to bool.
-7- Effects: Equivalent to: return bool(x) ? *x <= v : true;
```
template <class T, class U> constexpr bool operator<=(const TU& v, const optional<T>& x);
```
-?- Requires: The expression v <= *x shall be well-formed and its result shall be convertible to bool.
-8- Effects: Equivalent to: return bool(x) ? v <= *x : false;
```
template <class T, class U> constexpr bool operator>(const optional<T>& x, const TU& v);
```
-?- Requires: The expression *x > v shall be well-formed and its result shall be convertible to bool.
-9- Effects: Equivalent to: return bool(x) ? *x > v : false;
```
template <class T, class U> constexpr bool operator>(const TU& v, const optional<T>& x);
```
-?- Requires: The expression v > *x shall be well-formed and its result shall be convertible to bool.
-10- Effects: Equivalent to: return bool(x) ? v > *x : true;
```
template <class T, class U> constexpr bool operator>=(const optional<T>& x, const TU& v);
```
-?- Requires: The expression *x >= v shall be well-formed and its result shall be convertible to bool.
-11- Effects: Equivalent to: return bool(x) ? *x >= v : false;
```
template <class T, class U> constexpr bool operator>=(const TU& v, const optional<T>& x);
```
-?- Requires: The expression v >= *x shall be well-formed and its result shall be convertible to bool.
-12- Effects: Equivalent to: return bool(x) ? v >= *x : true;

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

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