Fix small_vector on gcc 49

Summary:
In folly::small_vector, we were (mistakenly) relying on a default constructor
for non-trivially constructible types.

The problem got worse on gcc 4.9, because for that compiler version, we set
folly::is_trivially_copyable to be std::is_trivial. This is mostly okay, as
std::is_trivial implies std::is_trivially_copyable. The problem is that
std::is_trivial also implies std::is_default_constructuble, which which was
breaking folly::small_vector

Reviewed By: yfeldblum

Differential Revision: D8346780

fbshipit-source-id: 9c8a9dab21ea2d4fd34f636458324915c30e10a2

folly/small_vector.h

@@ -102,7 +102,9 @@ namespace detail {
  * memory is initialized to type T already.
  */
 template <class T>
-typename std::enable_if<!folly::is_trivially_copyable<T>::value>::type
+typename std::enable_if<
+    std::is_default_constructible<T>::value &&
+    !folly::is_trivially_copyable<T>::value>::type
 moveObjectsRight(T* first, T* lastConstructed, T* realLast) {
   if (lastConstructed == realLast) {
     return;
@@ -139,7 +141,9 @@ moveObjectsRight(T* first, T* lastConstructed, T* realLast) {
 // std::move_backward here will just turn into a memmove.  (TODO:
 // change to std::is_trivially_copyable when that works.)
 template <class T>
-typename std::enable_if<folly::is_trivially_copyable<T>::value>::type
+typename std::enable_if<
+    !std::is_default_constructible<T>::value ||
+    folly::is_trivially_copyable<T>::value>::type
 moveObjectsRight(T* first, T* lastConstructed, T* realLast) {
   std::move_backward(first, lastConstructed, realLast);
 }