static assert(version(x)) ?

[Johan Engelen]

On Tuesday, 26 November 2019 at 12:53:02 UTC, Jonathan M Davis 
wrote:
> On Tuesday, November 26, 2019 4:29:18 AM MST S.G via 
> Digitalmars-d-learn wrote:
>> On Tuesday, 26 November 2019 at 10:24:00 UTC, Robert M. Münch
>>
>> wrote:
>> > How can I write something like this to check if any of a set 
>> > of specific versions is used?
>> >
>> > static assert(!(version(a) | version(b) | version(c)):
>> >
>> > The problem is that I can use version(a) like a test, and the
>> > symbol a is not accessbile from assert (different,
>> > non-accessible namespace).
>>
>> BTW D language designers are against boolean eval of version. 
>> It's not a technical restriction, it's just that they don't 
>> want this to work.
>
> ...
> static if can be used instead of version blocks to get boolean 
> conditions, and local version identifiers can be defined which 
> combine some set of version identifiers, but such practices are 
> discouraged for D programmers in general, and they're basically 
> forbidden in official source code. The only case I'm aware of 
> where anything like that is used in druntime or Phobos is for 
> darwin stuff, since darwin isn't a predefined identifier.

`xversion` is a simple and effective and useful tool, used in dmd 
source:
https://github.com/dlang/dmd/blob/53b533dc7fc5da604e7ebf457734766b4e96d900/src/dmd/globals.d#L21-L35

```
template xversion(string s)
{
     enum xversion = mixin(`{ version (` ~ s ~ `) return true; 
else return false; }`)();
}

enum version_a = xversion!`a`;
```

-Johan