Thanks to the implicit casting in compound assignments and increment/decrement operators, the following compiles:

byte b = 0;
++b; b++; --b; b--;
b += b -= b *= b /= b %= b;
b <<= b >>= b >>>= b;
b |= b &= b ^= b;

And thanks to auto-boxing and auto-unboxing, the following also compiles:

Integer ii = 0;
++ii; ii++; --ii; ii--;
ii += ii -= ii *= ii /= ii %= ii;
ii <<= ii >>= ii >>>= ii;
ii |= ii &= ii ^= ii;

And yet, the last line in the following snippet gives compile-time error:

Byte bb = 0;
++bb; bb++; --bb; bb--; // ... okay so far!
bb += bb; // DOESN'T COMPILE!!!
// "The operator += is undefined for the argument type(s) Byte, byte"

Can anyone help me figure out what's going on here? The byte b version compiles just fine, so shouldn't Byte bb just follow suit and do the appropriate boxing and unboxing as necessary to accommodate?