Examine the following code:

1| struct Point { int x, y; };
2|
3| void foo(int& a, int&& b) {
4| int x; Point p, q;
5| x = a + b;
6| a = x + 1;
7| p = Point{ x, a };
8| p.x = p.y + 1;
9| q = std move(p);
10| return x;
11| }

Tell whether the following expressions is an l-value or an r-value:

line 5: a + b