Implement LWG 2773 - std::ignore should be constexpr.

In addition to the PR for LWG 2773 this patch also ensures
that each of std::ignores constructors or assignment operators
are constexpr.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@294165 91177308-0d34-0410-b5e6-96231b3b80d8

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

@@ -22,6 +22,24 @@
 
 #include "test_macros.h"
 
+#if TEST_STD_VER > 11
+constexpr bool test_tie_constexpr() {
+    {
+        int i = 42;
+        double f = 1.1;
+        using ExpectT = std::tuple<int&, decltype(std::ignore)&, double&>;
+        auto res = std::tie(i, std::ignore, f);
+        static_assert(std::is_same<ExpectT, decltype(res)>::value, "");
+        assert(&std::get<0>(res) == &i);
+        assert(&std::get<1>(res) == &std::ignore);
+        assert(&std::get<2>(res) == &f);
+        // FIXME: If/when tuple gets constexpr assignment
+        //res = std::make_tuple(101, nullptr, -1.0);
+    }
+    return true;
+}
+#endif
+
 int main()
 {
     {
@@ -39,5 +57,8 @@ int main()
         static_assert ( std::get<0>(t) == 42, "" );
         static_assert ( std::get<1>(t) == 1.1, "" );
     }
+    {
+        static_assert(test_tie_constexpr(), "");
+    }
 #endif
 }