[Issue 24797] New: isZeroInit is wrongly false for some struct/union nesting

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24797

          Issue ID: 24797
           Summary: isZeroInit is wrongly false for some struct/union
                    nesting
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: minor
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: nick at geany.org

The fix for issue 24776 does not catch cases like this:

union U3
{
    int y;
    struct {
        float z, w; // z ignored, but w has a different offset to y
    }
}
static assert(__traits(isZeroInit, U3)); // fails

Using `StructDeclaration.members` rather than `fields` may be necessary as a
fallback when there are overlapped fields - see:
https://github.com/dlang/dmd/pull/16858#discussion_r1769869617

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