[Issue 18545] Casting away const with cast() triggers alias this, but returns the supertype anyway

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18545

--- Comment #1 from FeepingCreature <default_357-line at yahoo.de> ---
It turns out what's happening here is that cast() constNullable is equivalent
to Nullable(constNullable)... which implicitly casts to constNullable.get.

alias get this strikes again!

Nullable should maybe have a copy constructor from const?

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