Multiple alias this, what's the hold up?

[Mike Franklin]

On Saturday, 15 June 2019 at 10:12:46 UTC, Amex wrote:
> How many years has it been in limbo? 1? 2? 3? 5? 10?
>
> What is the hold up?
>

DIP66 is in need of champion.  Do you wish to adopt 
https://github.com/dlang/dmd/pull/8378 and see it across the 
finish line?

But I'm curious.  Is there anything that `alias this` can do that 
can't be replicated with a mataprogramming library?

Contrived, naive, very incomplete illustration:

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

template AliasThis(alias f)
{
     static foreach(m; __traits(allMembers, typeof(f)))
     {
         mixin(
             ReturnType!(__traits(getMember, typeof(f), 
m)).stringof
             ~ " " ~ m ~ "()"
             ~ "{" ~ __traits(identifier, f) ~ "." ~ m ~ "(); }"
         );
     }
}

struct Base
{
     void print()
     {
         writeln("Base.print");
     }

     void print2()
     {
         writeln("Base.print2");
     }
}

struct Base2
{
     void print3()
     {
         writeln("Base2.print3");
     }

     void print4()
     {
         writeln("Base2.print4");
     }
}



struct Derived
{
     Base base;
     Base2 base2;
     mixin AliasThis!(base);
     mixin AliasThis!(base2);
}

void main()
{
     Derived d;
     d.print();
     d.print2();
     d.print3();
     d.print4();
}
```

Mike