I have this piece of code

#include <iostream>

namespace ns{
    int a = 10;
}

using namespace ns;
int a = 20;

int main(){
    std::cout << a;
}

As per my understanding, when std::cout << a is evaluated, a is searched for in main‘s local scope. If not found, the lookup is performed in the global scope, where a is found. So, I was simply expecting it to print 20, but the code doesn’t compile, claiming that

error: reference to 'a' is ambiguous

However, this code works :

#include <iostream>

namespace ns{
    int a = 10;
}

using namespace ns;
int a = 20;

int main(){
    std::cout << ::a;
}

It prints 20. Why does this difference arise? Shouldn’t the two codes be equivalent, considering that a is not defined in main‘s scope, and the lookup is performed in the global scope in both the cases?