DIP 1017--Add Bottom Type--Final Review

[Johannes Loher]

On Tuesday, 15 January 2019 at 18:51:13 UTC, Johannes Loher wrote:
> [...]
18.
> A point was raised that Tbottom* == typeof(null) and Tbottom[] 
> == typeof([]) would be preferable to both cases equating to 
> Tbottom.

I totally agree with this criticism. What does `Tbottom*` mean 
conceptually? It is a type which can hold either an address where 
a `Tbottom` is located, or `null`. However, there cannot be any 
address where a `Tbottom` is located, because `Tbottom` does not 
have any values. This means that `Tbottom*` is a type which can 
hold exactly one value: `null`.

By the way, Kotlin does basically the same thing. Kotlin does not 
have pointers, but it has optional types and the type `Nothing?` 
is a type which can hold exactly one value: `null`. This also 
means that it is basically a unit type. While it is not actually 
Kotlin's `Unit` type, it could have been. Similarly, should we 
decide to go down that road and add proper top, bottom and unit 
types, we could define

```
alias Tbottom = typeof(assert(0));
alias void = Tbottom*;
```