[Issue 24425] New: @standalone attribute should be usable on thread-local static constructors

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24425

          Issue ID: 24425
           Summary: @standalone attribute should be usable on thread-local
                    static constructors
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: schveiguy at gmail.com

This is a valid use of standalone constructors and should be allowed:

```d
import core.attribute;

int *foo;

@standlone @system static this() {
   foo = new int(10);
}
```

Yet, the compiler doesn't allow this usage.

I'm unsure whether the compiler needs to do anything fancy here. There is an
`MIstandalone` flag, which means all shared *and* thread-local static ctors of
a module are standalone. And there is also an `ictor` function pointer which
appears to be independent construction for a module. I'm not sure how each is
used, but I would imagine constructing a "dummy" moduleinfo that contains only
standalone ctors/dtors (by using the MIstandalone flag) is possible whenever
needed.

I know that druntime already would handle this case properly.

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