Appender cannot add struct with immutable members

[Namespace]

> AFAIK, the problem is actually within emplace, that tries to 
> call opAssign when it should really be just (post)blitting. 
> This is just one of the many emplace-related bugs.
>
> I have an open pull for fixing emplace that should fix this. 
> I'll add your snippet to its unittests to confirm that it also 
> fixes this issue.

You said in my bug report:
> There is a lot of broken in appender that I've tried to fix 
> before, but it is
> very tricky because:
What exactly is broken?