Whats the difference between r += x and r = r + x

Question

Why is that the following code will compile and execute but will yield a wrong result due to integer overflow?

int r = 0;
long x = 1_000_000_000_000l;

r += x;

While this one will generate a compiler error incompatible types: possible lossy conversion from long to int.

int r = 0;
long x = 1_000_000_000_000l;

r = r + x;