Implicit enum conversions are a stupid PITA

[Lutger]

bearophile wrote:

> To invent a software you can first find the best syntax. This seems a nice
> syntax, very similar to the enum one (that ubyte is optional):
> 
> flags ubyte Todo {
>     do_nothing,
>     walk_dog,
>     cook_breakfast,
>     deliver_newspaper,
>     visit_miss_kerbopple,
>     wash_covers
> }
> 
> Todo todo = Todo.walk_dog | Todo.deliver_newspaper | Todo.wash_covers;
> if (todo == (Todo.walk_dog | Todo.deliver_newspaper)) { ...
> if ((Todo.walk_dog | Todo.deliver_newspaper) in todo) { ...
> if ((Todo.walk_dog | Todo.deliver_newspaper) & todo) { ...
> assert((Todo.walk_dog | Todo.walk_dog) == Todo.walk_dog); // OK
> 
> 
> A way to implement it with current D2 syntax:
> 
> 
> alias Flags!(ubyte, "do_nothing",
>                     "walk_dog"
>                     "cook_breakfast"
>                     "deliver_newspaper"
>                     "visit_miss_kerbopple"
>                     "wash_covers") Todo;
> 
> 
> Where Flags defines a struct, "do_nothing" are compile-time constants. It
> can overload 8 operators:
> =   ==   |    |=    in   &   &=  opBool
> 
> The operator ! too can be defined, but I think it looks too much like the |
> so it can be omitted (other operators like ^ and ~ are possible).
> 

I like this idea of implementing a flag type and tried to work something out.  
Instead of implementing the overloads, it is also possible to generate an 
enum via CTFE inside a struct and forward with alias this, what do you think? 
I have tried this syntax, seems to work ok:

alias Flags!q{ do_nothing, 
               walk_dog, 
               cook_breakfast,
               deliver_newspaper,
               visit_miss_kerbopple,
               wash_covers } Todo;

It does allow this though, but perhaps that can fixed:

Todo todo = Todo.walk_dog;
todo |= 4;
 
With such a type, it is easy to add some small convenience features, such as 
an  enumToString, define property .max and implement a range that iterates 
over all flags set or possible values.