Why there is too many uneccessary casts?

[Adam D. Ruppe]

On Tuesday, 11 June 2013 at 10:12:27 UTC, Temtaime wrote:
> ubyte k = 10;
> ubyte c = k + 1;
>
> This code fails to compile because of: Error: cannot implicitly 
> convert expression (cast(int)k + 1) of type int to ubyte

The reason is arithmetic operations transform the operands into 
ints, that's why the error says cast(int)k. Then it thinks int is 
too big for ubyte. It really isn't about overflow, it is about 
truncation.

That's why uint + 1 is fine. The result there is still 32 bits so 
assigning it to a 32 bit number is no problem, even if it does 
overflow. But k + 1 is promoted to int first, so it is a 32 bit 
number and now the compiler complains that you are trying to 
shove it into an 8 bit variable. Unless it can prove the result 
still fits in 8 bits, it complains, and it doesn't look outside 
the immediate line of code to try to prove it. So it thinks k can 
be 255, and 255 + 1 = 256, which doesn't fit in 8 bits.

The promotion to int is something D inherited from C and probably 
isn't going anywhere.