[Issue 21207] New: Mixin get processed in lexical order, resulting in forward reference issues

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21207

          Issue ID: 21207
           Summary: Mixin get processed in lexical order, resulting in
                    forward reference issues
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Keywords: rejects-valid
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: pro.mathias.lang at gmail.com

```
void foo (Callback cb);
mixin(q{ alias Callback = void delegate(); });
```

> test.d(1): Error: undefined identifier Callback

Swap the order and it works.

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