What's the deal with the massive duplication in std.meta?

[Q. Schroll]

On Tuesday, 4 May 2021 at 16:54:13 UTC, Andrei Alexandrescu wrote:
> I put together a bunch other related pull requests:
>
> https://github.com/dlang/phobos/pulls/andralex
>
> Using alias assignment with good measure leads to much simpler 
> and clearer code. I also measured memory consumption 
> improvements, but for the Phobos unittest they are not dramatic.

Is there some resource where the rules for alias assignments are 
spelled out? Googling '"alias assignment" site:dlang.org' 
produces no meaningful results. Having procedural aspects in a 
declarative space looks a bit odd to me. The change logs revealed 
nothing.