Make kj::defer work correctly when the input is a lambda with non-copyable captures.

c++/src/kj/common.h

@@ -1270,7 +1270,7 @@ namespace _ {  // private
 template <typename Func>
 class Deferred {
 public:
-  inline Deferred(Func func): func(func), canceled(false) {}
+  inline Deferred(Func&& func): func(kj::fwd<Func>(func)), canceled(false) {}
   inline ~Deferred() noexcept(false) { if (!canceled) func(); }
   KJ_DISALLOW_COPY(Deferred);
 
@@ -1286,7 +1286,7 @@ private:
 }  // namespace _ (private)
 
 template <typename Func>
-_::Deferred<Decay<Func>> defer(Func&& func) {
+_::Deferred<Func> defer(Func&& func) {
   // Returns an object which will invoke the given functor in its destructor.  The object is not
   // copyable but is movable with the semantics you'd expect.  Since the return type is private,
   // you need to assign to an `auto` variable.
@@ -1294,7 +1294,7 @@ _::Deferred<Decay<Func>> defer(Func&& func) {
   // The KJ_DEFER macro provides slightly more convenient syntax for the common case where you
   // want some code to run at function exit.
 
-  return _::Deferred<Decay<Func>>(kj::fwd<Func>(func));
+  return _::Deferred<Func>(kj::fwd<Func>(func));
 }
 
 #define KJ_DEFER(code) auto KJ_UNIQUE_NAME(_kjDefer) = ::kj::defer([&](){code;})