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

[w0rp]

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`.