StdTuple: Add note about redundant Applicative instance.

include/boost/hana/ext/std/tuple.hpp

@@ -73,7 +73,10 @@ namespace boost { namespace hana {
         { return helper(f, tuple, std::index_sequence_for<Xs...>{}); }
     };
 
-    //! @todo Check `Applicative` laws.
+    //! @todo
+    //! Get rid of this redundant instance once
+    //! http://llvm.org/bugs/show_bug.cgi?id=19616
+    //! is fixed.
     template <>
     struct Applicative::instance<StdTuple> : Applicative::mcd {
         template <typename X>

test/ext/std/tuple/bugs/libcxx_19616.cpp

@@ -0,0 +1,34 @@
+/*
+@copyright Louis Dionne 2014
+Distributed under the Boost Software License, Version 1.0.
+(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
+ */
+
+#include <boost/hana/ext/std/tuple.hpp>
+
+#include <boost/hana/detail/static_assert.hpp>
+
+#include <tuple>
+using namespace boost::hana;
+
+
+int main() {
+    // Make sure we workaround the bug at:
+    // http://llvm.org/bugs/show_bug.cgi?id=19616
+    BOOST_HANA_STATIC_ASSERT(
+        lift<StdTuple>(std::make_tuple(1)) ==
+        std::make_tuple(std::make_tuple(1))
+    );
+
+    // The following shows the actual bug:
+#if 0
+    static_assert(
+        std::tuple_cat(
+            std::make_tuple(std::make_tuple(1)),
+            std::make_tuple()
+        )
+        ==
+        std::make_tuple(std::make_tuple(1))
+    , "");
+#endif
+}