Is `alias this` a mistake?

[Kagamin]

On Tuesday, 3 August 2021 at 16:24:26 UTC, Paul Backus wrote:
> On Tuesday, 3 August 2021 at 15:52:07 UTC, NonNull wrote:
>> Here, further down, I just saw Walter indicate his opinion 
>> that `alias this` is a mistake. Any thoughts?
>>
>> https://news.ycombinator.com/item?id=28029184
>
> `alias this` is typically used to
>
> - make a user-defined type behave like a subtype of some other 
> type
> - make a user-defined type implicitly convert to some other type
>
> ...but it does kind of a crappy job at both. It's very easy to 
> use `alias this` in ways that violate the [Liskov substitution 
> principle][1] (see for example [this thread][2]), so it fails 
> at being a good subtyping mechanism. And you can only have one 
> `alias this` per type, so it also fails at being a good 
> mechanism for user-defined implicit conversions (compare to 
> `opCast`, the mechanism for user-defined *explicit* conversion).
>
> [1]: https://en.wikipedia.org/wiki/Liskov_substitution_principle
> [2]: https://forum.dlang.org/thread/q6plhj$1l9$1@digitalmars.com

I use alias this for implicit conversion between reference and 
value type. It's not really subtyping and not really type 
conversion and works fine without humongous amounts of 
metaprogramming.