error in including type_traits template functions to other functions

I am writing a bunch of custom type_trait like functions for my code, they basically have two types: either return a boolean, or a type. that is my implementation for one of them.

#ifndef TYPE_TRAITS_IS_DIGIT_H_
#define TYPE_TRAITS_IS_DIGIT_H_

#include <type_traits>

template <typename T>
constexpr bool is_digit_b = std::is_scalar<T>::value;

#endif

I have noticed everytime I try to include those functions that return bool in other tyetriats, I will get

Symbol 'is_digit_b' could not be resolved

#ifndef TYPE_TRAITS_IS_DATA_VECTOR_H_
#define TYPE_TRAITS_IS_DATA_VECTOR_H_

#include "forwards.h"
#include "get_arg_type.h"
#include "removed_cv_type.h"
#include "is_digit.h"

template<typename T, typename VTYPE = removed_cv_t<T>, typename ARG = get_arg_t<
T> >
constexpr bool is_data_vector_b = is_digit_b<ARG>;

#endif

am am I doing something wrong ?!!