Abbreviated Function Templates

#include <concepts>
#include <iostream>
template <typename T> 
requires std::integral<T> 
T gcd(T a,T b) {
    if( b == 0 ) return a;
    else return gcd(b, a % b); 
}
template<typename T> 
T gcd1(T a,T b) requires std::integral<T> {
    if( b == 0 ) return a;
    else return gcd1(b, a % b); 
}
template<std::integral T> 
T gcd2(T a,T b) {
    if( b == 0 ) return a;
    else return gcd2(b, a % b); 
}
std::integral auto gcd3(std::integral auto a,std::integral auto b) { 
    if( b == 0 ) return a;
    else return gcd3(b, a % b);
}
auto gcd4(auto a,auto b) {
    if( b == 0 ) return a;
    return gcd4(b, a % b);
}
int main() {
    std::cout << '\n';
    std::cout << "gcd(100, 10)= " << gcd(100, 10) << '\n';
    std::cout << "gcd1(100, 10)= " << gcd1(100, 10) << '\n';
    std::cout << "gcd2(100, 10)= " << gcd2(100, 10) << '\n';
    std::cout << "gcd3(100, 10)= " << gcd3(100, 10) << '\n';
    std::cout << "gcd4(100, 10)= " << gcd4(100, 10) << '\n';
    std::cout << '\n';
}