SumType! seemingly results in cryptic error, with dub suggesting to make an issue ticket

[Gimbles]

On Tuesday, 30 May 2023 at 08:42:38 UTC, Gimbles wrote:
> My code is this
> ```d
> struct ConstValueIndex
> {
>     ushort const_value_index;
> }
>
> [...]

https://gist.github.com/run-dlang/5dd783c750f04329405af1b1e4a83cde

Here's the full source