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### 2773. Making `std::ignore constexpr`

Section: 20.5.1 [tuple.general] Status: C++17 Submitter: Vincent Reverdy Opened: 2016-09-10 Last modified: 2017-07-30

Priority: 0

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Discussion:

Currently `std::ignore` is not specified `constexpr` according to the C++ draft N4606 in the paragraph 20.5.1 [tuple.general]. It prevents some use in `constexpr` context: for example declaring a `constexpr` variable equals to the result of a function to which `std::ignore` has been passed as a parameter:

>     constexpr int i = f(std::ignore); // Won't compile

If there is no fundamental reason preventing `std::ignore` to be `constexpr`, then we propose to declare it as `constexpr` instead of as `const`.

[Issues processing Telecon 2016-10-7]

P0; set to Tentatively Ready

Proposed resolution:

This wording is relative to N4606.

1. Modify 20.5.1 [tuple.general] as indicated:

   >     // 20.5.2.4, tuple creation functions:
   >     constconstexpr unspecified ignore;