alias not valid with ~

[rikki cattermole via Digitalmars-d-learn]

On 19/01/2017 3:35 PM, Ignacious wrote:
> On Thursday, 19 January 2017 at 02:25:44 UTC, Adam D. Ruppe wrote:
>> On Thursday, 19 January 2017 at 02:15:04 UTC, rikki cattermole wrote:
>>> On 19/01/2017 3:08 PM, Ignacious wrote:
>>>>
>>>> class Y
>>>> {
>>>>    int y;
>>>>    alias y this;
>>>> }
>>>>
>>>> class X
>>>> {
>>>>    Y[] x;
>>>>    alias x this;
>>>> }
>>
>>> This should not fail:
>>>
>>> X x = new X;
>>> x ~= 3;
>>
>>
>> Yes, it should fail. 3 is not implicitly convertible to Y under any
>> circumstance. D does not support implicit constructors.
>>
>> alias this only works if you ALREADY HAVE a Y, then it will implicitly
>> convert Y to int. It will never go the other way around.
>
> Huh?
>
> But this is alias this, the whole point of alias this is to treat the
> type as as the alias?
>
> You are saying it basically only works one way, seems to make alias this
> quite useless(50% at least). Is there any real reason why this doesn't
> work?
>
> X x;
> Y y;
> y = 3;
>
> x ~= y; works fine
> x ~= 3; fails.
>
> Yet, logically, 3 is convertible to Y(3rd line above) and Y is
> appendable to X.
>
> Seems to me that D simply hasn't added the logic to handle the case for
> implicit construction for alias this, why not add it?

It is not implicitly convertible in any form.
An integer is just a value, probably stored in a register or directly 
encoded into an instruction.

A class instance is always allocated into memory, in pretty much all 
cases the heap (stack is explicit in D). So what you're suggesting would 
require an allocation + calling of a constructor to make it equal.

Now, lets say Y was a struct, then yeah it can work. Because a struct is 
nothing more than a set of values that go together. Which are commonly 
allocated on the stack and for smaller ones, be passed around by only 
registers.