Question about iteger literals

[Uranuz via Digitalmars-d-learn]

On Monday, 23 June 2014 at 18:32:38 UTC, Steven Schveighoffer 
wrote:
> On Sun, 22 Jun 2014 08:23:45 -0400, Uranuz <neuranuz at gmail.com> 
> wrote:
>
>> If these rules are not so clear and have some exceptions (but 
>> I don't understand why they are needed) then some 
>> documentation needed about this.
>
> See integer promotion rules:
>
> http://dlang.org/type.html#Integer%20Promotions
>
> And the section below it.
>
> -Steve

I see these rules but when I compile following code and it fails 
with error it looks VERY stupid.

import std.stdio;

void main()
{
	ubyte a = 15;
	ubyte b = 10;
	
	ubyte c = a + b; //What is happening there?! AAAAARGH! Are you 
joking?!
	
}

Compilation output:
/d837/f382.d(9): Error: cannot implicitly convert expression 
(cast(int)a + cast(int)b) of type int to ubyte

I'm just crazy about it! How could it happen?!