[libcxx] Make std::ignore constexpr

[AntonBikineev]

Summary

This addresses DR 2773.

// 20.5.2.4, tuple creation functions:
constexpr unspecified ignore;
}

[mclow.lists]

This looks good to me.

[EricWF]

@AntonBikineev Do you need somebody to commit this for you?

[AntonBikineev]

In D27199#619278, @EricWF wrote:

@AntonBikineev Do you need somebody to commit this for you?

oh, yeah, it would be great. I don't have commit privileges.

include/tuple

public:
     template <class U1, class U2>
         tuple& operator=(const pair<U1, U2>&); // iff sizeof...(T) == 2
     template <class U1, class U2>
         tuple& operator=(pair<U1, U2>&&); //iffsizeof...(T) == 2
 
     void swap(tuple&) noexcept(AND(swap(declval<T&>(), declval<T&>())...));
 };
 
-const unspecified ignore;
+constexpr unspecified ignore;
 
 template <class... T> tuple<V...>  make_tuple(T&&...); // constexpr in C++14
 template <class... T> tuple<ATypes...> forward_as_tuple(T&&...) noexcept; // constexpr in C++14
 template <class... T> tuple<T&...> tie(T&...) noexcept; // constexpr in C++14
 template <class... Tuples> tuple<CTypes...> tuple_cat(Tuples&&... tpls); // constexpr in C++14
 
 // [tuple.apply], calling a function with a tuple of arguments:
 template <class F, class Tuple>

 {
     return tuple<_Tp&...>(__t...);
 }
 
 template <class _Up>
 struct __ignore_t
 {
     template <class _Tp>
-        _LIBCPP_INLINE_VISIBILITY
+        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
         const __ignore_t& operator=(_Tp&&) const {return *this;}
 };
 
-namespace { const __ignore_t<unsigned char> ignore = __ignore_t<unsigned char>(); }
+namespace { _LIBCPP_CONSTEXPR_AFTER_CXX14 const __ignore_t<unsigned char> ignore = __ignore_t<unsigned char>(); }
 
 template <class _Tp>
 struct __make_tuple_return_impl
 {
     typedef _Tp type;
 };
 
 template <class _Tp>

test/std/utilities/tuple/tuple.tuple/tuple.creation/tie.pass.cpp

 // UNSUPPORTED: c++98, c++03
 
 #include <tuple>
 #include <string>
 #include <cassert>
 
 #include "test_macros.h"
 
+#if TEST_STD_VER > 14
+template <class T>
+constexpr auto check_ce_ignore(T ignore)
+{
+    ignore = 4;
+    return ignore;
+}
+#endif
+
 int main()
 {
     {
         int i = 0;
         std::string s;
         std::tie(i, std::ignore, s) = std::make_tuple(42, 3.14, "C++");
         assert(i == 42);
         assert(s == "C++");
     }
 #if TEST_STD_VER > 11
     {
         static constexpr int i = 42;
         static constexpr double f = 1.1;
         constexpr std::tuple<const int &, const double &> t = std::tie(i, f);
         static_assert ( std::get<0>(t) == 42, "" );
         static_assert ( std::get<1>(t) == 1.1, "" );
     }
 #endif
+#if TEST_STD_VER > 14
+    {
+        constexpr auto t = check_ce_ignore(std::ignore);
+    }
+#endif
 }

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   <p>Last Updated: 14-Nov-2016</p>
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