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

[Petar]

On Friday, 10 August 2018 at 06:44:20 UTC, Andrea Fontana wrote:
> 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

`is(T == U)` evaluates to true iff `T` is exactly `U`. `is(T : 
U)` tests if `T` is a subtype of (can be implicitly converted to) 
`U` [0]. So we have:

`is(typeof(assert(0)) == T)` is false, unless `T` is 
`typeof(assert(0))`
`is(typeof(assert(0)) : T)` is always true (as bottom is 
implicitly convertible to any other type, including itself).

[0]: https://dlang.org/spec/expression#is_expression