Fix incorrect forward_tuple test

Summary:
[Folly] Fix incorrect `forward_tuple` test which only passes with libc++ < 3.8 and libstdc++ < 8.

The test would fail with libc++ >= 3.8 and with libstdc++ >= 8.

Spec fix in: https://cplusplus.github.io/LWG/issue2485.
Upstream libc++ fix in: https://github.com/llvm-mirror/libcxx/commit/199bee0ea7750b8e96a25414c3c2a7b465336e36.
Upstream libstdc++ fix in: https://github.com/gcc-mirror/gcc/commit/6d8efcd444067c046d72ac8e11737230f031a031.

Closes #1078.

Reviewed By: pixelb

Differential Revision: D14590323

fbshipit-source-id: ff023ab844d5da97e83f09576897304ac06678b8

folly/functional/test/ApplyTupleTest.cpp

@@ -575,10 +575,18 @@ TEST(ForwardTuple, Basic) {
                decltype(folly::forward_tuple(std::move(tuple))),
                std::tuple<int&&, double&&>>::value));
   EXPECT_EQ(folly::forward_tuple(std::move(tuple)), tuple);
+#if defined(__GLIBCXX__) && (!defined(_GLIBCXX_RELEASE) || _GLIBCXX_RELEASE < 8)
+  constexpr bool before_lwg2485 = true;
+#else
+  constexpr bool before_lwg2485 = false;
+#endif
   EXPECT_TRUE(
       (std::is_same<
           decltype(folly::forward_tuple(std::move(folly::as_const(tuple)))),
-          std::tuple<const int&, const double&>>::value));
+          std::conditional_t<
+              before_lwg2485,
+              std::tuple<const int&, const double&>,
+              std::tuple<const int&&, const double&&>>>::value));
   EXPECT_EQ(folly::forward_tuple(std::move(folly::as_const(tuple))), tuple);
 
   auto integer = 1;