pitrou commented on code in PR #51328:
URL: https://github.com/apache/arrow/pull/51328#discussion_r4027861697


##########
cpp/src/arrow/util/cache_internal.h:
##########
@@ -167,42 +166,42 @@ struct ThreadUnsafeMemoizer {
 };
 
 template <template <typename...> class Cache, template <typename...> class 
MemoizerType,
-          typename Func, typename Key = 
std::decay_t<call_traits::argument_type<0, Func>>,
-          typename Value = std::decay_t<std::invoke_result_t<Func, const 
Key&>>,
-          typename Memoizer = MemoizerType<Key, Value, Cache<Key, Value>, 
Func>,
-          typename RetType = typename Memoizer::RetType>
-static std::function<RetType(const Key&)> Memoize(Func&& func, int32_t 
cache_capacity) {
+          typename Key, typename Func>
+static auto Memoize(Func&& func, int32_t cache_capacity) {
+  using Value = std::decay_t<std::invoke_result_t<Func, const Key&>>;
+  using Memoizer = MemoizerType<Key, Value, Cache<Key, Value>, Func>;
+  using RetType = typename Memoizer::RetType;
+
   // std::function<> requires copy constructibility
   struct {
     RetType operator()(const Key& key) const { return (*memoized_)(key); }
     std::shared_ptr<Memoizer> memoized_;
   } shared_memoized = {
       std::make_shared<Memoizer>(std::forward<Func>(func), cache_capacity)};
 
-  return shared_memoized;
+  return std::function<RetType(const Key&)>(std::move(shared_memoized));

Review Comment:
   > However, I discovered that no code within Arrow currently uses this 
`arrow::internal::LruCache`; perhaps we can simply delete the file.
   
   Let's open a separate issue for that?



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