DIP66 - Multiple alias this

[IGotD-]

On Wednesday, 30 September 2020 at 01:36:59 UTC, Mike wrote:
>
> 1.  Deprecate `alias this` for classes, and implement multiple 
> `alias this` for structs.  That will give users the composition 
> feature they need, and in addition, because structs cannot 
> inherit, it removes the complexity Walter spoke of.
>
> 2.  Add `opImplicit`, implicit copy constructors, or something 
> of that ilk.  Then users only need to forward members using D's 
> metaprogramming facilities.  Both uses of `alias this` are 
> covered, but no `alias this` is required.  `alias this` can 
> then be deprecated entirely.
>
> 3.  Add struct inheritance like C++.

4. Remove "alias this" completely, use mixin templates instead. I 
originally misunderstood template mixins believing it was more or 
less just expansion into the scope. It turns out that there is a 
lot more going on under the hood, which makes the template mixin 
work like inheritance. This currently badly documented which 
makes people believe that template mixins are more limited.