Make tuple_element static_assert in pair if the index is out of range. Also, add a message to variant_alternative<> in the similar case (it already asserted). Add tests for this

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@305196 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/utilities/utility/pairs/pairs.pair/pair.tuple_element.fail.cpp

@@ -0,0 +1,25 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <utility>
+
+// template <class T1, class T2> struct pair
+
+// tuple_element<I, pair<T1, T2> >::type
+
+#include <utility>
+
+int main()
+{
+    {
+    typedef std::pair<int, double> P;
+    std::tuple_element<2, P>::type foo; // expected-note {{requested here}}
+        // expected-error@utility:* {{static_assert failed "Index out of bounds in std::tuple_element<std::pair<T1, T2>>"}}
+    }
+}