libstdc++: Implement P2968R2 "Making std::ignore a first-class object"

This was recently approved for C++26, but we can apply the changes for
all modes back to C++11. There's no reason not to make the assignment
usable in constant expressions for C++11 mode, and noexcept for all
modes.

Move the definitions to <bits/utility.h> so they're available in
<utility> as well as <tuple>.

libstdc++-v3/ChangeLog:

	* include/bits/utility.h (_Swallow_assign): Make assignment
	constexpr for C++11 as well, and add noexcept.
	* include/std/tuple (_Swallow_assign, ignore): Move to
	bits/utility.h.
	* testsuite/20_util/headers/utility/ignore.cc: New test.

1 files changed, 0 insertions, 31 deletions

diff --git a/libstdc++-v3/include/std/tuple b/libstdc++-v3/include/std/tuple
index df3f6e3..93b649e 100644
--- a/libstdc++-v3/include/std/tuple
+++ b/libstdc++-v3/include/std/tuple
@@ -2845,37 +2845,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     swap(tuple<_Elements...>&, tuple<_Elements...>&) = delete;
 #endif
 
-  // A class (and instance) which can be used in 'tie' when an element
-  // of a tuple is not required.
-  // _GLIBCXX14_CONSTEXPR
-  // 2933. PR for LWG 2773 could be clearer
-  struct _Swallow_assign
-  {
-    template<class _Tp>
-      _GLIBCXX14_CONSTEXPR const _Swallow_assign&
-      operator=(const _Tp&) const
-      { return *this; }
-  };
-
-  // _GLIBCXX_RESOLVE_LIB_DEFECTS
-  // 2773. Making std::ignore constexpr
-  /** Used with `std::tie` to ignore an element of a tuple
-   *
-   * When using `std::tie` to assign the elements of a tuple to variables,
-   * unwanted elements can be ignored by using `std::ignore`. For example:
-   *
-   * ```
-   * int x, y;
-   * std::tie(x, std::ignore, y) = std::make_tuple(1, 2, 3);
-   * ```
-   *
-   * This assignment will perform `x=1; std::ignore=2; y=3;` which results
-   * in the second element being ignored.
-   *
-   * @since C++11
-   */
-  _GLIBCXX17_INLINE constexpr _Swallow_assign ignore{};
-
   /// Partial specialization for tuples
   template<typename... _Types, typename _Alloc>
     struct uses_allocator<tuple<_Types...>, _Alloc> : true_type { };