Fixing the imaginary/complex mess

[Don]

Georg Wrede wrote:
> Don wrote:
>> D currently allows some conversions between complex/imaginary types 
>> and real types, which are highly dubious.
>>
>> Given creal z, ireal y, these casts are legal:
>> real x = cast(real)z;
>> x = cast(real)y; // always sets x==0, regardless of the value of y.
>> But I believe that should not be legal.
>>
>> For the first case, it should be written as:   real x = z.re;
>> (which is shorter and clearer), and the second case is probably a bug, 
>> and should be x = y.im; (unless the intention really was to set x=0!).
>>
>> By the same logic, we could have sqrt(-1)==0, since the real part is 0.
>>
>> The most important effect of disallowing these casts would be to fix a 
>> host of bugs and wierd behaviour. All the A op= B operations involve a 
>> cast to A. If those nonsensical casts become illegal, the nonsensical 
>> op= operations become illegal automatically.
>>
>> Eg, ireal y;
>> y *= y; // mathematically nonsense, y*y is real, so can't be stored in 
>> a pure imaginary type!
>>
>> There are a few segfault/ICE bugs (eg 718, 2839) which involve 
>> int/=complex, an operation which never makes any sense anyway.
>>
>> I think we're just making problems for ourselves by allowing these 
>> useless operations. I think they should be killed.
>> Does anyone object? (If not, I'll create a patch to do it; it's not 
>> very difficult).
> 
> It might be a good idea to write in the docs why these operations are 
> missing. Even better would of course be if the user got a message 
> stating that this operation isn't implemented because it doesn't make 
> mathematical sense.

I'll make sure the error messages are sensible.

> 
> If the latter isn't done, then many people will just think there's 
> something wrong with the compiler/language. Bad press we don't need.

I don't think anyone expects to be able to divide an integer by an 
imaginary, and then assign it to an integer. I was astonished that the 
compiler accepted it.