Compatibility with VS2019 (#1182)

Summary:
Some changes to get Poly compiling with VS2019 (and correcting an oversight from https://github.com/facebook/folly/pull/1174)

The change to `detail/TypeList.h` is straightforward I think.

The introduction of `getOpsImpl` I find a bit distasteful...I will remark that as of Visual Studio 16.2.0 Preview 3.0 it is not necessary, but it is still needed on Visual Studio 16.1.5 (latest release, non preview).

Orvid in your comment [here](https://github.com/facebook/folly/pull/1178#issuecomment-506864383)  you mentioned the question of supporting VS 2017 or VS 2019 given that Poly had no MSVC support at all beforehand. I re-downloaded VS 2017 on my machine to test against the fixes added here and the situation seemed discouraging with lots more errors and perhaps less obvious workarounds.

I have not made any changes to the CMake stuff for Poly on MSVC: https://github.com/facebook/folly/blob/master/CMakeLists.txt#L235

I'm not sure if once we resolve the issue above we might want to add some logic maybe [here](https://github.com/facebook/folly/blob/master/CMakeLists.txt#L63) to handle which version of MSVC is approved for poly; will wait for further direction on that.
Pull Request resolved: https://github.com/facebook/folly/pull/1182

Reviewed By: Orvid

Differential Revision: D16109504

Pulled By: yfeldblum

fbshipit-source-id: d20b242494b075ce91452215c2025584fe488d65

folly/Poly.h

@@ -50,6 +50,12 @@ namespace folly {
 template <class I>
 struct Poly;
 
+// MSVC workaround
+template <class Node, class Tfx, class Access>
+struct PolySelf_ {
+  using type = decltype(Access::template self_<Node, Tfx>());
+};
+
 /**
  * Within the definition of interface `I`, `PolySelf<Base>` is an alias for
  * the instance of `Poly` that is currently being instantiated. It is
@@ -101,7 +107,7 @@ template <
     class Node,
     class Tfx = detail::MetaIdentity,
     class Access = detail::PolyAccess>
-using PolySelf = decltype(Access::template self_<Node, Tfx>());
+using PolySelf = _t<PolySelf_<Node, Tfx, Access>>;
 
 /**
  * When used in conjunction with `PolySelf`, controls how to construct `Poly`
@@ -111,7 +117,7 @@ using PolySelf = decltype(Access::template self_<Node, Tfx>());
  */
 using PolyDecay = detail::MetaQuote<std::decay_t>;
 
-#if !defined(__cpp_template_auto)
+#if !FOLLY_POLY_NTTP_AUTO
 
 /**
  * Use `FOLLY_POLY_MEMBERS(MEMS...)` on pre-C++17 compilers to specify a

folly/detail/PolyDetail.h

@@ -695,16 +695,21 @@ struct BasePtr {
   VTable<I> const* vptr_;
 };
 
-template <class I, class T, std::enable_if_t<inSitu<T>(), int> = 0>
-constexpr void* (*getOps() noexcept)(Op, Data*, void*) {
+template <class I, class T>
+constexpr void* (*getOpsImpl(std::true_type) noexcept)(Op, Data*, void*) {
   return &execInSitu<I, T>;
 }
 
-template <class I, class T, std::enable_if_t<!inSitu<T>(), int> = 0>
-constexpr void* (*getOps() noexcept)(Op, Data*, void*) {
+template <class I, class T>
+constexpr void* (*getOpsImpl(std::false_type) noexcept)(Op, Data*, void*) {
   return &execOnHeap<I, T>;
 }
 
+template <class I, class T>
+constexpr void* (*getOps() noexcept)(Op, Data*, void*) {
+  return getOpsImpl<I, T>(std::integral_constant<bool, inSitu<T>()>{});
+}
+
 template <class I, FOLLY_AUTO... Arch, class... S>
 struct VTable<I, PolyMembers<Arch...>, TypeList<S...>>
     : BasePtr<S>..., std::tuple<SignatureOf<Arch, I>...> {

folly/detail/TypeList.h

@@ -168,8 +168,10 @@ using If = MetaApply<impl::If_<If_>, Then, Else>;
  */
 template <template <class...> class C, class... Ts>
 class MetaDefer {
-  template <template <class...> class D = C, class = D<Ts...>>
+  template <template <class...> class D, class = D<Ts...>>
   static char (&try_(int))[1];
+
+  template <template <class...> class D, class = void>
   static char (&try_(long))[2];
   struct Result {
     using type = C<Ts...>;
@@ -177,7 +179,7 @@ class MetaDefer {
 
  public:
   template <class... Us>
-  using apply = _t<If<sizeof(try_(0)) - 1 || sizeof...(Us), Empty, Result>>;
+  using apply = _t<If<sizeof(try_<C>(0)) - 1 || sizeof...(Us), Empty, Result>>;
 };
 
 /**