compile-time if can have code not able to be compiled with regular if

constexpr_if.cpp

// Example from: C++17-The Complete Guide, by Nicolai M. Josuttis
// Chapter 10. Compile-Time if
// http://www.cppstd17.com/

#include <string>
#include <iostream>

template <typename T>
std::string toString_constexpr_if(T x)
{
	if constexpr(std::is_same_v<T, std::string>) {
		return x; // statement invalid, if no conversion to string
	}
	else if constexpr(std::is_arithmetic_v<T>) {
		return std::to_string(x); // statement invalid, if x is not numeric
	}
	else {
		return std::string(x); // statement invalid, if no conversion to string
	}
}

template <typename T>
std::string toString_regular_if(T x)
{
	if (std::is_same_v<T, std::string>) {
		return x; // statement invalid, if x is numeric
	}
	else if (std::is_arithmetic_v<T>) {
		return std::to_string(x); // statement invalid, if x is std::string
	}
	else {
		return std::string(x); // statement invalid, if x is numeric
	}
}

int main()
{
	std::cout << toString_constexpr_if(42) << '\n';
	std::cout << toString_constexpr_if(std::string("hello")) << '\n';
	std::cout << toString_constexpr_if("hello") << '\n';
	
	// The following statements will have compilation error 
	//std::cout << toString_regular_if(42) << '\n';
	//std::cout << toString_regular_if(std::string("hello")) << '\n';
	//std::cout << toString_regular_if("hello") << '\n';
	return 0;
}