fix: Replace deprecated std::result_of with std::invoke_result

Remove trailing return type from enqueue; rely on C++14+ deduction.
Reference: progschj/ThreadPool#130

Author: soonhokong

third_party/com_github_progschj_threadpool/ThreadPool/ThreadPool.h

@@ -16,8 +16,7 @@ class ThreadPool {
  public:
   ThreadPool(size_t);
   template <class F, class... Args>
-  auto enqueue(F&& f, Args&&... args)
-      -> std::future<typename std::result_of<F(Args...)>::type>;
+  auto enqueue(F&& f, Args&&... args);
   ~ThreadPool();
 
   // Returns the thread ID.
@@ -69,9 +68,8 @@ inline ThreadPool::ThreadPool(size_t threads) : stop(false) {
 
 // add new work item to the pool
 template <class F, class... Args>
-auto ThreadPool::enqueue(F&& f, Args&&... args)
-    -> std::future<typename std::result_of<F(Args...)>::type> {
-  using return_type = typename std::result_of<F(Args...)>::type;
+auto ThreadPool::enqueue(F&& f, Args&&... args) {
+  using return_type = typename std::invoke_result<F, Args...>::type;
 
   auto task = std::make_shared<std::packaged_task<return_type()> >(
       std::bind(std::forward<F>(f), std::forward<Args>(args)...));