Please tell me this is a bug?

[Peter Alexander via Digitalmars-d]

On Sunday, 22 February 2015 at 07:11:24 UTC, deadalnix wrote:
> On Sunday, 22 February 2015 at 02:27:30 UTC, Peter Alexander 
> wrote:
>> On Sunday, 22 February 2015 at 01:24:09 UTC, Almighty Bob 
>> wrote:
>>> a += b; // Compiles with no ERROR!
>>>
>>> Please tell me that's a bug?
>>
>> Not a bug. From spec:
>>
>> http://dlang.org/expression.html#AssignExpression
>>> Assignment operator expressions, such as:
>>> 
>>> a op= b
>>> 
>>> are semantically equivalent to:
>>> 
>>> a = cast(typeof(a))(a op b)
>>
>> Seems questionable to me. Anyone know the rationale? If a = b; 
>> is disallowed, I don't see why a += b; should be more 
>> acceptable.
>
> The rationale make sens for things like :
> byte a; a += 1;
>
> Here, because of type promotion, a + 1 is an int, and if VRP of 
> a is unknown, you can't cast implicitly back to byte.

If VRP is unknown then it should be disallowed for precisely that 
reason! It can't implicitly cast back to a byte, so it shouldn't. 
If you know that the int fits in a byte then do the cast 
yourself, that's what it's for.


> It is true that this create questionable side effect for float, 
> but in the end, that is consistent, and it would be very 
> annoying to not be able to increment/decrement integral smaller 
> than int.

Even if you forget about float, it's still inconsistent.

byte a;
int b;
a = a + b;
a += b;

These do the same thing, and have the same pitfalls if the int is 
too big. Both should be disallowed.