Error(s):

source_file.cpp:13:3: error: call to 'f' is ambiguous
  f(f1);
  ^
source_file.cpp:6:6: note: candidate function
void f(std::function<void(std::shared_ptr<Message>)> arg) {}
     ^
source_file.cpp:7:6: note: candidate function
void f(std::function<void(std::unique_ptr<Message>)> arg) {}
     ^
1 error generated.