std::optional

[TurkeyMan]

The DMD union problem is fixed, this might get closer now...

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[TurkeyMan]

Still blocked on https://issues.dlang.org/show_bug.cgi?id=19902
... which is a pretty bad regression!

[Laeeth]

@TurkeyMan with @adamdruppe splitting the traits into two PRs, is this now no longer blocked?

[12345swordy]

ping @TurkeyMan

[TurkeyMan]

This may be able to move forward now. I'll poke at it when I get a few minutes.

[RazvanN7]

@TurkeyMan is this ready for review?