Don't name a member small

Summary:
Because, if you aren't compiling with `WIN32_LEAN_AND_MEAN`, including `WinSock2.h` will result in `small` being `#define`'d as `char`. The best way to work around the issue is to simply change the name of the member to something else.
It would also have been possible to solve the problem by adding a section to `folly/portability/Windows.h`, but that would have required including that header in `folly/Function.h`, which there really is no reason to do.

Reviewed By: yfeldblum

Differential Revision: D3666417

fbshipit-source-id: 21c0363a4b77bb01e4ec20b2fc625c40729879c3

folly/Function.h

@@ -242,13 +242,13 @@ enum class Op { MOVE, NUKE, FULL, HEAP };
 
 union Data {
   void* big;
-  std::aligned_storage<6 * sizeof(void*)>::type small;
+  std::aligned_storage<6 * sizeof(void*)>::type tiny;
 };
 
 template <typename Fun, typename FunT = typename std::decay<Fun>::type>
 using IsSmall = std::integral_constant<
     bool,
-    (sizeof(FunT) <= sizeof(Data::small) &&
+    (sizeof(FunT) <= sizeof(Data::tiny) &&
      // Same as is_nothrow_move_constructible, but w/ no template instantiation.
      noexcept(FunT(std::declval<FunT&&>()))
      )>;
@@ -288,7 +288,7 @@ struct FunctionTraits<ReturnType(Args...)> {
   template <typename Fun>
   static ReturnType callSmall(Data& p, Args&&... args) {
     return static_cast<ReturnType>((*static_cast<Fun*>(
-        static_cast<void*>(&p.small)))(static_cast<Args&&>(args)...));
+        static_cast<void*>(&p.tiny)))(static_cast<Args&&>(args)...));
   }
 
   template <typename Fun>
@@ -329,7 +329,7 @@ struct FunctionTraits<ReturnType(Args...) const> {
   template <typename Fun>
   static ReturnType callSmall(Data& p, Args&&... args) {
     return static_cast<ReturnType>((*static_cast<const Fun*>(
-        static_cast<void*>(&p.small)))(static_cast<Args&&>(args)...));
+        static_cast<void*>(&p.tiny)))(static_cast<Args&&>(args)...));
   }
 
   template <typename Fun>
@@ -359,11 +359,11 @@ template <typename Fun>
 bool execSmall(Op o, Data* src, Data* dst) {
   switch (o) {
     case Op::MOVE:
-      ::new (static_cast<void*>(&dst->small))
-          Fun(std::move(*static_cast<Fun*>(static_cast<void*>(&src->small))));
+      ::new (static_cast<void*>(&dst->tiny))
+          Fun(std::move(*static_cast<Fun*>(static_cast<void*>(&src->tiny))));
       FOLLY_FALLTHROUGH;
     case Op::NUKE:
-      static_cast<Fun*>(static_cast<void*>(&src->small))->~Fun();
+      static_cast<Fun*>(static_cast<void*>(&src->tiny))->~Fun();
       break;
     case Op::FULL:
       return true;
@@ -433,7 +433,7 @@ class Function final : private detail::function::FunctionTraits<FunctionType> {
   Function(Fun&& fun, SmallTag) noexcept {
     using FunT = typename std::decay<Fun>::type;
     if (!detail::function::isNullPtrFn(fun)) {
-      ::new (static_cast<void*>(&data_.small)) FunT(static_cast<Fun&&>(fun));
+      ::new (static_cast<void*>(&data_.tiny)) FunT(static_cast<Fun&&>(fun));
       call_ = &Traits::template callSmall<FunT>;
       exec_ = &detail::function::execSmall<FunT>;
     }