std::shared_ptr(static_cast<DerivedClient*>(this), {})

likely isnt what we want to be doing, you are creating a shared point from this then have a custom deleter [](DerivedClient*) {} that does nothing when delete is called. you are essentially trying to fit this into a shared pointer without using the actual properties of shared.

consider

#include <iostream>
#include <thread>

template<typename crtp_t>
class thing {
public:
  thing(crtp_t& ref) : ptr_(ref) {};
  void do_something() {
    ptr_.work();
  }
private:
  crtp_t& ptr_;
};

template <typename crtp_t>
class mixin {
 public:
  void operation() {
    thing<crtp_t> operation_thing{*static_cast<crtp_t*>(this)};
    operation_thing.do_something();
  }
};

class widget : public mixin<widget> {
public:
  void work() {
    std::cout << "hello from class\n";
  }
};

auto main() -> int {
  widget w{};
  w.operation();
  return 0;
}

which is long way of saying on PagePaginator make client a reference not a pointer

instead of forward declaring DynamoDB why not have ScanPaginationTraits take a template parameter template <typename Client = ClientType> like you have on Invoke then the entire class is tempalted. You already pass DynamoDBClient as a template parameter in DynamoDBClientPagination.h