Special Type Challenge

[Jonathan Marler via Digitalmars-d]

On Saturday, 7 February 2015 at 03:48:24 UTC, Adam D. Ruppe wrote:
> wrote:
>>  b = -256;
>
> that won't fit in a byte btw.

Woops, I typed that example too fast :)

>
> The rest of the assignment stuff is easy. I'd prolly even do it 
> with a template:
>
> this(T)(T t) { this.opAssign(t); } // for construction
> Byte opAssign(T)(T t) if(T.sizeof == 1) { // for other 
> assignment
>     data_holder = cast(typeof(data_holder) t);
> }
>
> and that should do it.

This code is almost exactly what I came up with.  But what about 
the function problem?

void echo(Byte b);

echo('c'); // won't work

I can't figure out a way to make this work (without making echo a 
template).  If D supported implicit conversions from built-in 
types to user types then it could work.  Do you know if D might 
support that later or if there's a reason for not supporting it?