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/variant/variant.variant/variant.helper/variant_alternative.fail.cpp

@@ -0,0 +1,34 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+// <array>
+
+// template <size_t I, class T> struct variant_alternative; // undefined
+// template <size_t I, class T> struct variant_alternative<I, const T>;
+// template <size_t I, class T> struct variant_alternative<I, volatile T>;
+// template <size_t I, class T> struct variant_alternative<I, const volatile T>;
+// template <size_t I, class T>
+//   using variant_alternative_t = typename variant_alternative<I, T>::type;
+//
+// template <size_t I, class... Types>
+//    struct variant_alternative<I, variant<Types...>>;
+
+
+#include <variant>
+#include <cassert>
+
+
+int main()
+{
+    {
+        typedef std::variant<int, double> T;
+        std::variant_alternative<2, T>::type foo; // expected-note {{requested here}}
+        // expected-error@variant:* {{static_assert failed "Index out of bounds in std::variant_alternative<>"}}
+    }
+}