overloading Base.promote_type is not permitted

[nsajko]

Several methods are added for promote_type in this package. Xref JuliaLang/julia#54138

[schlichtanders]

Can you help how to replace it? This mirrors the typejoin and other behaviour of Nothing and Missing, how to do without promote_type? (I remember that Nothing and missing used this, but I may be wrong)

[nsajko]

Would simply overloading promote_rule instead of promote_type work?

I'm not otherwise familiar with this package, but I took a look at the implementation now, and it seems to me that it's deeply flawed. For example, this:

const Either{L, R} = Union{Const{L}, Identity{R}}

... should instead look something like:

struct Either{L, R}
  v::R
end

So, it seems to me that you're misusing Union when you should be defining a new, named, type.

[schlichtanders]

Please, I ask you to be more respectful towards code you may not understand.
The types are carefully chosen and have advantages over defining structs.

As already said, the types Const and Identity behave similar to Nothing and Missing in that Union types get explicitly constructed. If you can tell me how to change the implementation without using promote_type, while not changing the type definitions, I am very happy to change it.