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### 928. Wrong concepts used for `tuple`'s comparison operators

Section: 20.5.8 [tuple.rel] Status: NAD Concepts Submitter: Joe Gottman Opened: 2008-10-28 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In the latest working draft for C++0x, `tuple`'s `operator==` and `operator<` are declared as

>     template<class... TTypes, class... UTypes>
>       requires EqualityComparable<TTypes, UTypes>...
>       bool operator==(const tuple<TTypes...>& t, const tuple<UTypes...>& u);

and

>     template<class... TTypes, class... UTypes>
>       requires LessThanComparable<TTypes, UTypes>...
>       bool operator<(const tuple<TTypes...>& t, const tuple<UTypes...>& u);

But the concepts `EqualityComparable` and `LessThanComparable` only take one parameter, not two. Also, even if `LessThanComparable` could take two parameters, the definition of `tuple::operator<()` should also require

>     LessThanComparable<UTypes, TTypes>... // (note the order)

since the algorithm for `tuple::operator<` is the following (pseudo-code)

>     for (size_t N = 0; N < sizeof...(TTypes); ++N) {
>         if (get<N>(t) < get<N>(u) return true;
>         else if ((get<N>(u) < get<N>(t)) return false;
>     }

>     return false;

Similar problems hold for `tuples`'s other comparison operators.

[ Post Summit: ]

> Recommend Tentatively Ready.

Proposed resolution:

In 20.5.1 [tuple.general] and 20.5.8 [tuple.rel] change:

>     template<class... TTypes, class... UTypes>
>       requires EqualityComparableHasEqualTo<TTypes, UTypes>...
>       bool operator==(const tuple<TTypes...>&, const tuple<UTypes...>&);

>     template<class... TTypes, class... UTypes>
>       requires LessThanComparableHasLess<TTypes, UTypes>... && HasLess<UTypes, TTypes>...
>       bool operator<(const tuple<TTypes...>&, const tuple<UTypes...>&);

>     template<class... TTypes, class... UTypes>
>       requires EqualityComparableHasEqualTo<TTypes, UTypes>...
>       bool operator!=(const tuple<TTypes...>&, const tuple<UTypes...>&);

>     template<class... TTypes, class... UTypes>
>       requires LessThanComparableHasLess<UTTypes, TUTypes>... && HasLess<UTypes, TTypes>...
>       bool operator>(const tuple<TTypes...>&, const tuple<UTypes...>&);

>     template<class... TTypes, class... UTypes>
>       requires LessThanComparableHasLess<UTTypes, TUTypes>... && HasLess<UTypes, TTypes>...
>       bool operator<=(const tuple<TTypes...>&, const tuple<UTypes...>&);

>     template<class... TTypes, class... UTypes>
>       requires LessThanComparableHasLess<TTypes, UTypes>... && HasLess<UTypes, TTypes>...
>       bool operator>=(const tuple<TTypes...>&, const tuple<UTypes...>&);