std::variant<A,B> v = ...;

std::visit([&v](auto& e){
  if constexpr (std::is_same_v<std::remove_reference_t<decltype(e)>,A>)
    e.some_modifying_operation_on_A();
  else {
    int i = e.some_accessor_of_B();
    v = some_function_returning_A(i);
  }
}, v);