src: simplify is_callable by making it a concept

Using a C++20 `concept` here makes `is_callable` much simpler than relying on SFINAE. It is equivalent for function types, `std::function`, lambdas, and classes with `operator()`, regardless of argument or return types. PR-URL: #58169 Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>

Author: tniessen

src/req_wrap-inl.h

@@ -111,7 +111,7 @@ struct CallLibuvFunction<ReqT, void(*)(ReqT*, Args...)> {
 template <typename ReqT, typename T>
 struct MakeLibuvRequestCallback {
  static T For(ReqWrap<ReqT>* req_wrap, T v) {
- static_assert(!is_callable<T>::value,
+ static_assert(!is_callable<T>,
  "MakeLibuvRequestCallback missed a callback");
  return v;
  }

src/util.h

@@ -661,13 +661,9 @@ struct MallocedBuffer {
 };
 
 // Test whether some value can be called with ().
-template <typename T, typename = void>
-struct is_callable : std::is_function<T> { };
-
 template <typename T>
-struct is_callable<T, typename std::enable_if<
- std::is_same<decltype(void(&T::operator())), void>::value
- >::type> : std::true_type { };
+concept is_callable =
+ std::is_function<T>::value || requires { &T::operator(); };
 
 template <typename T, void (*function)(T*)>
 struct FunctionDeleter {