Use structs rather than type aliases for Unit::Lift and Unit::Drop

Summary:
VS 2017 RC brings with it fixes for some things and unfortunately, a few known regressions in expression SFINAE. In this case, alias templates with dependent `decltype()` calls are broken, so switch to a templated struct instead.
It's the very last issue mentioned in: https://blogs.msdn.microsoft.com/vcblog/2016/06/07/expression-sfinae-improvements-in-vs-2015-update-3/

Reviewed By: yfeldblum

Differential Revision: D4199676

fbshipit-source-id: 9ce157cc891182509a30b000de1b509063387244

folly/Unit.h

@@ -34,10 +34,16 @@ namespace folly {
 /// possible to construct a value of this type, but it is always the same value
 /// every time, so it is uninteresting.
 struct Unit {
+  // These are structs rather than type aliases because MSVC 2017 RC has
+  // trouble correctly resolving dependent expressions in type aliases
+  // in certain very specific contexts, including a couple where this is
+  // used. See the known issues section here for more info:
+  // https://blogs.msdn.microsoft.com/vcblog/2016/06/07/expression-sfinae-improvements-in-vs-2015-update-3/
+
   template <typename T>
-  using Lift = std::conditional<std::is_same<T, void>::value, Unit, T>;
+  struct Lift : std::conditional<std::is_same<T, void>::value, Unit, T> {};
   template <typename T>
-  using Drop = std::conditional<std::is_same<T, Unit>::value, void, T>;
+  struct Drop : std::conditional<std::is_same<T, Unit>::value, void, T> {};
 
   bool operator==(const Unit& /*other*/) const { return true; }
   bool operator!=(const Unit& /*other*/) const { return false; }