Make enum auto castable

[Jonathan M Davis via Digitalmars-d-learn]

On Monday, June 05, 2017 02:14:14 Mike B Johnson via Digitalmars-d-learn 
wrote:
> On Monday, 5 June 2017 at 01:42:55 UTC, Jonathan M Davis wrote:
> > On Monday, June 05, 2017 01:30:47 Mike B Johnson via
> >
> > Digitalmars-d-learn wrote:
> >> [...]
> >
> > It's not a bug. The alias this conversion only goes one way. It
> > provides a way to convert _from_ the type that it's declared on
> > to another type, not from the other type to the type that it's
> > declared on. There is no way in D to declare an implicit
> > conversion in the direction you're trying. So, if you have a
> > struct that wraps an int like this, and you want to assign it
> > an int, you're going to need to explicitly construct the struct
> > - e.g. EnumX(1).
> >
> > - Jonathan M Davis
>
> That's pretty crappy! Defeats the whole point!

Well, implicit conversions tend to cause a lot of bugs in C++ code, so
Walter decided to restrict them quite a bit more in D than they are in C++.
This therefore prevents a number of bugs in D, but it also prevents implicit
conversions from being used in a number of situations where they're useful.
Whether the end result is better or worse, I don't know and certainly YMMV.
As it is, some of the implicit conversions that we still allow have a
tendency to cause bugs (e.g. conversions between integral values and
character types tend to be error-prone - especially when arrays are involved
- and using implicit conversions with templated code is almost always a bad
idea). So, at times, I tend to think that all implicit conversions should be
banned. But at other times, it would be really nice to be able to have more
than we do (e.g. it would be a lot more user-friendly if you could pass a T
to a function that takes a Nullable!T).

In any case, I think that the reality of the matter with D is that you tend
to have to use explicit conversions rather than implicit ones, and if you're
looking to do much with implicit conversions, you're usually going to be
frustrated, and obviously, that sucks, but at least you're less likely to
have bugs related to implicit conversions.

- Jonathan M Davis