[Issue 20603] New: 'cannot use non-constant CTFE pointer in an initializer' in recursive structure with overlap

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20603

          Issue ID: 20603
           Summary: 'cannot use non-constant CTFE pointer in an
                    initializer' in recursive structure with overlap
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: dkorpel at live.nl

This does not compile:

```
struct Base {
    union {
        int overlap;
        immutable(Sub)* sub;
    }

    this(Sub) {
        sub = new Sub;
    }
}

struct Sub {
    Base base;
}

immutable c0 = Base(Sub.init);
```

Error: cannot use non-constant CTFE pointer in an initializer Base(,
&Sub0(Base(0, __void)))

If you swap the order of the `overlap` and `sub` field, it compiles.
If you change the field in `Sub` to be of a different type than `Base`, it
compiles too.
The exact limitations of CTFE and pointers to the data segment are not clear to
me, but the above behavior is inconsistent and unclear.

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