How to extract the AA type?

[jmh530]

On Wednesday, 13 December 2017 at 16:00:32 UTC, Steven 
Schveighoffer wrote:
> A nice puzzle for those template gurus out there.
>
> I have a function like this:
>
> auto foo(T: V[K], V, K)(T t)
> {
>    RealAAType!(T, V, K) aa = t;
>    return aa;
> }
>
> So I need to know what to write RealAAType. What I'm looking 
> for is a mechanism to write the exact AA type that is passed 
> in. RealAAType should be an AA, and nothing else.
>
[snip]

How about:

template RealAAType!(T, V, K)
{
     import std.traits : CopyConstness;

     alias RealAAType = CopyConstness!(T, V[K]);
}