GNU G++ 5.1.0 doesn't compile next code:

#include <vector>

using namespace std;

vector <pair <int, int>> a(0);

int main()
{
	return 0;
}

But succeыsfully compile this code:

#include <vector>

using namespace std;

#define pii pair <int, int>

vector <pii> a(0);

int main()
{
	return 0;
}

Why the second code compile without space (" ") after "pii"?