std::char_traits

#include <cctype>
#include <iostream>
#include <string>
#include <string_view>

struct ci_char_traits : public std::char_traits<char>
{
    static char to_upper(char ch)
    {
        return std::toupper((unsigned char) ch);
    }
    
    static bool eq(char c1, char c2)
    {
        return to_upper(c1) == to_upper(c2);
    }
    
    static bool lt(char c1, char c2)
    {
         return to_upper(c1) < to_upper(c2);
    }
    
    static int compare(const char* s1, const char* s2, std::size_t n)
    {
        while (n-- != 0)
        {
            if (to_upper(*s1) < to_upper(*s2))
                return -1;
            if (to_upper(*s1) > to_upper(*s2))
                return 1;
            ++s1;
            ++s2;
        }
        return 0;
    }
    
    static const char* find(const char* s, std::size_t n, char a)
    {
        const auto ua{to_upper(a)};
        while (n-- != 0) 
        {
            if (to_upper(*s) == ua)
                return s;
            s++;
        }
        return nullptr;
    }
};

template<class DstTraits, class CharT, class SrcTraits>
constexpr std::basic_string_view<CharT, DstTraits>
    traits_cast(const std::basic_string_view<CharT, SrcTraits> src) noexcept
{
    return {src.data(), src.size()};
}

int main()
{
    using namespace std::literals;
    
    constexpr auto s1 = "Hello"sv;
    constexpr auto s2 = "heLLo"sv;
    
    if (traits_cast<ci_char_traits>(s1) == traits_cast<ci_char_traits>(s2))
        std::cout << s1 << "  " << s2 << " \n";
}