inline_if.hpp

#include<iostream>

template<auto A, auto B, auto first_, auto second_, typename to_compare>
struct inline_if
{
  constexpr auto operator()()
  {
    if constexpr(to_compare{}.template operation<A, B>())
    {
      return first_;
    }
    else
    {
      return second_;
    }
  }
};

struct is_same_value
{
    template<auto Az, auto Bz>
    constexpr auto operation()
    {
        return Az == Bz;
    }
};

template<auto X>
struct foo
{
  static constexpr auto value = inline_if<X, 42, 'c', true, is_same_value>{}.operator()();
};

int main()
{
    std::cout << foo<42>::value; // outputs 'c'
    std::cout << foo<'x'>::value; // outputs 1
}