Enum that can be 0 or null

[ParticlePeter via Digitalmars-d-learn]

On Tuesday, 7 June 2016 at 14:31:40 UTC, Alex Parrill wrote:
> I don't think opCast gets called for implicit conversions; it 
> only gets called for explicit casts. I'll test it later.

It does for type bool, but I fear that's the only exception.