Trait for "can be instantiated"?

[Basile B.]

On Tuesday, 10 May 2022 at 16:10:26 UTC, Ben Jones wrote:
> On Tuesday, 10 May 2022 at 16:05:15 UTC, H. S. Teoh wrote:
>> Using wrapper structs, etc., for this is IMO total overkill. 
>> Just use an enum for your token types.  Something like this 
>> would suffice:
>
> That's basically what sumtype is going to do for me, but 
> (hopefully) more safely. Also, the token types are "user 
> defined,"  my lexer just grabs everything annotated with @Token 
> and passes those types/wrapped enums to sumtype.

How about being more explicit in the UDA ?
The idea would be to associate the enum value to a type or not:

```d
import std.traits;
import std.stdio;

struct Token(T);

struct Token(T...)
if (T.length == 0) { }

@Token!(string) enum str_tok;
@Token!(float)  enum float_tok;
@Token!()       enum lparen_tok;

void main()
{
     alias toks = getSymbolsByUDA!(mixin(__MODULE__), Token);

     static foreach (t; toks)
     {{
         alias U = getUDAs!(t, Token);
         alias A = TemplateArgsOf!(U);
         static if (A.length)
             pragma(msg, "add a `" ~ A[0].stringof ~ "`for `" ~ 
t.stringof ~ "`");
         else
             pragma(msg, "no SumType data needed for `" ~ 
t.stringof ~ "`");
     }}
}
```