Can't use a 'version' that was set inside 'static if'

[dlangBugzillaToGithub]

Nick Sabalausky reported this on 2012-01-28T03:56:26Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=7386

CC List

• Dennis
• Iain Buclaw (@ibuclaw)
• KytoDragon

Description

static if(true)
	version = Foo;
version(Foo) {}

test.d(2): Error: version Foo defined after use

The compiler's evaluation seems to occur in the wrong order ("versions before static ifs" instead of "top to bottom"), because this works:

static if(true)
	version = Foo;
static if(true)
	version(Foo) {}

[dlangBugzillaToGithub]

ibuclaw (@ibuclaw) commented on 2020-09-09T10:25:42Z

Bumping priority of this one.

[dlangBugzillaToGithub]

dkorpel commented on 2021-07-09T22:56:49Z

> The compiler's evaluation seems to occur in the wrong order ("versions before static ifs" instead of "top to bottom")

This 'evaluation order' is totally underspecified and super tricky. Simply going top to bottom doesn't work since a static if can have forward references:

```
static if (x == 0) {
    version = X1;
}

version(X1) {

}

enum x = 0;
```

Even trickier:
```
static if (x == 0) {
    version = X1;
}

version(X1) {
    version = X2;
}

version(X2) {
    enum x = 0;
}
```

[dlangBugzillaToGithub]

dfj1esp02 commented on 2021-07-12T14:22:10Z

It feels like version is a lightweight feature and is processed early as it doesn't depend much on semantics of the surrounding code, while static if tends to be processed on later stages as it needs module that passed most semantic checks.
As a workaround the original example can simply use static if everywhere; static if can inspect versions too: see issue 7417

[dlangBugzillaToGithub]

kytodragon commented on 2022-04-11T07:41:28Z

(In reply to anonymous4 from comment #3)
> It feels like version is a lightweight feature and is processed early as it
> doesn't depend much on semantics of the surrounding code, while static if
> tends to be processed on later stages as it needs module that passed most
> semantic checks.
> As a workaround the original example can simply use static if everywhere;
> static if can inspect versions too: see issue 7417

There are few bugs related to static if (see https://issues.dlang.org/show_bug.cgi?id=20905), so replacing all versions with static if is not currently possible.

[dlangBugzillaToGithub]

ibuclaw (@ibuclaw) commented on 2022-04-11T11:44:22Z

Potentially very breaking, but declaring versions could be even more restrictive than it currently is.

i.e: When setting a version in a function or user defined type, you expect a compile-time error.

    Error: version `Foo` declaration must be at module level

This could be extended to `static if` as well so that we avoid all these nasty cases of will it, won't it work.  Also noting that it's currently an error to declare a version inside a `static foreach`.

---
static foreach (_; 0 .. 5) {
    version = Foo;  // Error: version `Foo` declaration must be at module level
}
---

So why is `static if` different here?