[Issue 18945] immutable variable is used as if it's an enum

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18945

--- Comment #8 from David Bennett <davidbennett at bravevision.com> ---
(In reply to David Bennett from comment #7)
> Then there are immutable struct members used at compile time, they might not
> be easy to convert to an enum.
> https://run.dlang.io/is/cfGfxw
> I've never seen it used like that... but it does work currently.

Actually, I reduced that last case a bit to much that it would work fine even
with this change...

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