#include <type_traits>

#ifdef B_WITH_HELPER

template<typename T, typename = void>
struct helper : std::false_type {};

template<typename T>
std::enable_if_t<helper<T>::value, int> do_foo(T bb)
{
  static_assert(std::is_same<T, void>::value && false, "Never do this");
}

#endif

#ifdef B_IN_IMPL

template<typename T>
int do_foo(T bb)
{
  static_assert(std::is_same<T, void>::value && false, "Never do this");
}

#endif


template<typename T>
std::enable_if_t<std::is_fundamental<T>::value, int> do_foo(T bb)
{
  // Some generic business logic for all fundamental types here
  // OK to live in a header, not much we can do here
    return 0;
}