java - Adding int to short

A colleague of mine asked this question to me and I am kind of confused.

int i = 123456;
short x = 12;

The statement

x += i;

Compiles fine however

x = x + i;

doesn't

What is Java doing here?

Answer

int i = 123456;
short x = 12;
x += i;

is actually

int i = 123456;

short x = 12;
x = (short)(x + i);

Whereas x = x + i is simply x = x + i. It does not automatically cast as a short and hence causes the error (x + i is of type int).

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A compound assignment expression of the form E1 op= E2 is equivalent to E1 = (T)((E1) op (E2)), where T is the type of E1, except that E1 is evaluated only once.

_{- JLS §15.26.2}