[folly] Enable support of applyTuple for const tuples

Summary: This diff fixes the helper template ReturnValue to enable using of applyTuple with
constant refs.

Reviewed By: @ot, @ddrcoder

Differential Revision: D2284733

folly/ApplyTuple.h

@@ -91,7 +91,7 @@ struct CallTuple {
 
 // The point of this meta function is to extract the contents of the
 // tuple as a parameter pack so we can pass it into std::result_of<>.
-template<class F, class Args> struct ReturnValue {};
+template<class F, class Args> struct ReturnValue;
 template<class F, class ...Args>
 struct ReturnValue<F,std::tuple<Args...>> {
   typedef typename std::result_of<F (Args...)>::type type;
@@ -104,12 +104,12 @@ struct ReturnValue<F,std::tuple<Args...>> {
 template<class Callable, class Tuple>
 typename detail::ReturnValue<
   typename std::decay<Callable>::type,
-  typename std::remove_reference<Tuple>::type
+  typename std::decay<Tuple>::type
 >::type
 applyTuple(const Callable& c, Tuple&& t) {
   typedef typename detail::ReturnValue<
     typename std::decay<Callable>::type,
-    typename std::remove_reference<Tuple>::type
+    typename std::decay<Tuple>::type
   >::type RetT;
   return detail::CallTuple<RetT>::call(c, std::forward<Tuple>(t));
 }

folly/test/ApplyTupleTest.cpp

@@ -86,7 +86,7 @@ struct GuardObj : GuardObjBase {
     : f_(std::move(f))
     , args_(std::move(args))
   {}
-  GuardObj(GuardObj&& g)
+  GuardObj(GuardObj&& g) noexcept
     : GuardObjBase(std::move(g))
     , f_(std::move(g.f_))
     , args_(std::move(g.args_))
@@ -163,4 +163,6 @@ TEST(ApplyTuple, Test) {
   Mover m;
   folly::applyTuple(move_only_func,
                     std::forward_as_tuple(std::forward<Mover>(Mover())));
+  const auto tuple3 = std::make_tuple(1, 2, 3.0);
+  folly::applyTuple(func, tuple3);
 }