CTFE Assignment to anonymous union shows unexpected behavior

[Rekel]

On Friday, 23 April 2021 at 00:55:50 UTC, H. S. Teoh wrote:
> [...]
> If you read the field during CTFE.  I've never tested 
> initializing a union in CTFE then reading it at runtime, 
> though. Not sure exactly what would happen in that case.
>
>
> T

I'm not referring to reading non-initial variables, though Im 
surprised that's not possible?
In any case, it seems assigning values doesn't work during CTFE 
either ... And that is very disappointing to me to be quite 
honest...

This only seems to happen when I use CTFE to create a static 
singleton of the union by assigning to non-initial variables, I 
couldn't find anything in the docs that say I shouldn't, I might 
have missed something...
I really hope this is a bug... if its not, & assuming its not 
inevitable, this really shouldn't be a thing. Be it by design or 
limitation.