DIP 1017--Add Bottom Type--Community Review Round 1

[Andrea Fontana]

On Thursday, 9 August 2018 at 15:56:31 UTC, w0rp wrote:
> On Thursday, 9 August 2018 at 10:37:36 UTC, Andrea Fontana 
> wrote:
>> On Thursday, 9 August 2018 at 04:10:47 UTC, Nicholas Wilson 
>> wrote:
>>> The DIP makes the claim that:
>>>  * "[@noreturn] has the awkward result of a function 
>>> specifying it has a return type T, but never returns that 
>>> type". When it is deliberate (such as annotating a fatal 
>>> error function) the is almost exclusively `void` (I know of 
>>> no examples to the contrary).
>>
>> Let's say we need to implement an interface with a int func(); 
>> member. We can mark it with @noreturn but we can't use TBottom 
>> return type: we're going to break interface implementation.
>>
>> Andrea
>
> It will work, and why it will work requires some understanding 
> of bottom types. You can define the function as `TBottom 
> func()` and it should work, because `TBottom` is a subtype of 
> `int`. In the same way you can implement `ParentClass func()` 
> as `SubClass func()`, because `SubClass` is a subtype of 
> `ParentClass`. Similarly, you can assign values of `TBottom` to 
> `int`, because `TBottom` is a subtype of all types, but you 
> cannot assign `int` to `TBottom`, because `int` is not a 
> subtype of `TBottom`.

is(T == int) will work with T==tbottom too?
is(T:int) ?

I feel that all this things are going to complicate 
implementation and add corner cases, but maybe I'm wrong...

Andrea