opApply and attributes

[solidstate1991]

On Tuesday, 7 July 2020 at 13:33:41 UTC, Paul Backus wrote:
>
> You can make opApply a template:
>
>     int opApply(Dg)(Dg dg)
>         if (is(Dg : scope int delegate(ref E)))
>     {
>         // etc.
>     }
>
> Because `scope int delegate(ref E) @safe` implicitly converts 
> to `scope int delegate(ref E)`, this version will accept both 
> @safe and non- at safe delegates. (And likewise for the other 
> function attributes.)

Unfortunately this doesn't really work, even with explicitly 
defined foreach arguments.