No need for version expression is a lie

[Adam D Ruppe]

On Thursday, 24 August 2023 at 16:47:55 UTC, Walter Bright wrote:
> That was something different.

No, it is a case of the same general problem. It even literally 
uses the `version` keyword in the language!

The best advice I have is to never use any version specifier 
except the built in platform ones. `version(Windows)` is not 
great but not demonstrably harmful the way user-defined versions 
or `version(unittest)` and others influenced by build flags are 
just because those are *so* easy to get out of sync. And 
`version(unittest)` is actually extraordinarily dangerous because 
`unittest` is so generic.

At least `version(no_fork)`, while it could be defined by 
multiple libraries in differing ways, is somewhat unlikely to be 
seen. But virtually all D code agrees on `unittest` being a 
condition they use since it is actually standardized.

This would be mitigated if the -unittest switch took a module 
pattern. -version taking a module pattern would help limit its 
damage. You'd still want to match when building, but at least 
then it would be intentionally namespaced and avoid accidental 
conflicts.

> I wouldn't set it up that way. I'd version on IA64 within 
> CRuntime_DigitalMars. I.e. overlapping version declarations are 
> not a good plan, for precisely the reason you mention. Better 
> to version along orthogonal axes.

https://dlang.org/spec/version.html#predefined-versions

Plenty of overlapping things in that existing list.

(and i expect you meant X86_64 rather than IA64, but that falls 
apart if it ran on AArch64 too, so that'd be all the more reason 
to have a separately defined thing like Phobos did)

> It failed because none of them had ever ported code from 16 to 
> 32 before, and so the future-proof solutions were all the wrong 
> solutions.

Indeed. Just like with D's `version` feature, it seemed like a 
good idea at the time, but didn't stand up to real world use in 
the long run.

> At some point, you're just going to have to run:
>
>     grep -r version *

The problem with this is that `-version` is global, including 
across dependencies of dependencies that might not even be in 
tree.