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2275. [CD] Why is forward_as_tuple not constexpr?

Section: 22.4.5 [tuple.creation] Status: C++14 Submitter: Marshall Clow Opened: 2013-07-30 Last modified: 2017-09-07

Priority: Not Prioritized

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Discussion:

Addresses ES 11

In n3471, a bunch of routines from header <tuple> were made constexpr.

make_tuple/tuple_cat/get<>(tuple)/relational operators — all these were "constexpr-ified".

But not forward_as_tuple.

Why not?

This was discussed in Portland, and STL opined that this was "an omission" (along with tuple_cat, which was added)

In discussion on lib@lists.isocpp.org list, Pablo agreed that forward_as_tuple should be constexpr.

[2013-09 Chicago]

Moved to Immediate, this directly addresses an NB comment and the wording is non-controversial.

Accept for Working Paper

Proposed resolution:

This wording is relative to N3691.

1. Change header <tuple> synopsis, 22.4.1 [tuple.general] as indicated:

   template <class... Types>
     constexpr tuple<Types&&...> forward_as_tuple(Types&&...) noexcept;
   

2. Change 22.4.5 [tuple.creation] before p5 as indicated:

   template <class... Types>
     constexpr tuple<Types&&...> forward_as_tuple(Types&&... t) noexcept;