Enum conversion

[tsbockman]

On Tuesday, 21 April 2020 at 16:03:20 UTC, Russel Winder wrote:
> then which of these is the right way of accessing the value?
>
> cast(ubyte)ZoneNumber.One
> to!ubyte(ZoneNumber.One)

Either is acceptable because there is no way that this operation 
can fail. Using a naked `cast` makes less work for the compiler 
and performs optimally with or without inlining, though.

> conversely what is the right way of going the other way:
>
> cast(ZoneNumber)1
> to!ZoneNumber(1)

Use `to` except where you can gaurantee that the input value maps 
to a valid enum member, because `cast` does not check your work:

     writeln(cast(ZoneNumber)17); // breaks the type system
     writeln(to!ZoneNumber(17)); // throws a ConvException: Value 
(17) does not match any member value of enum 'ZoneNumber'

So, `cast` is faster, but unsafe. `to` is slower, but protects 
the enum's invariant.