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### 1335. Insufficient requirements for `tuple::operator<()`

Section: 20.5.8 [tuple.rel] Status: C++11 Submitter: Joe Gottman Opened: 2010-05-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The requirements section for `std::tuple` says the following:

> Requires: For all `i`, where `0 <= i and i < sizeof...(Types)`, `get<i>(t) < get<i>(u)` is a valid expression returning a type that is convertible to `bool`. `sizeof...(TTypes) == sizeof...(UTypes)`.

This is necessary but not sufficient, as the algorithm for comparing `tuple`s also computes `get<i>(u) < get<i>(t)` (note the order)

[ Post-Rapperswil ]

> Moved to Tentatively Ready with updated wording correcting change-bars after 6 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

1. Change [tuple.rel] before p. 4 as indicated [Remark to the editor: This paragraph doesn't have a number yet, but it seems to me as if it should have one]:

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

   > > Requires: For all `i`, where `0 <= i` and `i < sizeof...(Types)`, `get<i>(t) < get<i>(u)` and `get<i>(u) < get<i>(t)`is a valid expression returning a type that is are valid expressions returning types that are convertible to `bool`. `sizeof...(TTypes) == sizeof...(UTypes)`.