byte + byte = int: why?

[Mek101]

On Friday, 18 January 2019 at 17:15:09 UTC, Steven Schveighoffer 
wrote:
> What is 127 + 127? Answer: 254. Which if converted to a byte is 
> -127. Not what you might expect if you are doing addition.

Quite similar to int.max + int.max

> In fact, D promotes all integral types smaller than int to int 
> to do arithmetic. And if the result might not fit into what you 
> are assigning it to, it requires a cast or mask.

Then why isn't int + int = long? If i did the example above, they 
wouldn't fit in a int for sure, yet the result is of the same 
type. Why should byte be any different?