launder

Summary: Backport C++17's `std::launder`.

Reviewed By: yfeldblum, ot

Differential Revision: D5370531

fbshipit-source-id: b7cc6701d082b6c4855fa77bb644e151ce882deb

folly/Utility.h

@@ -260,4 +260,11 @@ class MoveOnly {
 
 using MoveOnly = moveonly_::MoveOnly;
 
+/**
+ * Backport from C++17 of std::launder
+ */
+template <typename T>
+constexpr T* launder(T* in) {
+  return (in + 1) - 1;
+}
 } // namespace folly