Predefined Versions for Apple Operating Systems

[Quirin Schroll]

On Sunday, 22 June 2025 at 00:44:42 UTC, Jonathan M Davis wrote:
> [I]f we were redoing them, maybe it would be better to just 
> make them all of the predefined ones start with `__` so that 
> any that start with `__` but aren't known are an error, 
> significantly reducing the bugs related to mistyping version 
> identifers - not that that would entirely fix the problem, 
> since someone could still forget a `_` and make it look like a 
> user-defined one (and of course, we can't fix the problem with 
> user-defined ones unless we do something like require that a 
> list of all valid version identifiers be provided somehow).

A different approach could be to special-case version identifiers 
and require them to be ASCII-only and partially case-insensitive. 
The latter means that `X` and `x` are considered essentially the 
same, but only one of them is correct and defining it makes using 
the other an error. That means the built-in ones `linux`, 
`Windows`, `iOS`, etc., make it an error to use `Linux`, 
`windows`, or `IOS`. The error message can tell you what the 
correct spelling is. There’s no use case for the same identifier 
with different casing. I’d also ban version user-defined version 
identifiers with upper-case letters and reserve them for the 
language.

Maybe also change all the built-in ones to include an upper case 
letter.

It’s a breaking change, but one that can be mitigated by search 
and replace.

```d
version(Linux) {} // error: Use `linux`
version(OSx) {} // error: Use `OSX`
version(Win) {} // error: `Win` is not a built-in version 
identifier
version(win) {} // okay
```