Parametric Subtyping?

[ParadaCarleton]

I'm not sure if parametric subtyping would be the right term, but the pattern I'm interested in is this:

# minimum example
struct Thing{T} <: supertype(T)
	field::T
end

# actual use case
struct UnitfulNumber{T <: Number} <: supertype(T)
	field::T
end

The motivation being that many Unitful objects can't deal with functions that have Real restrictions, because units have to subtype Number instead of Real. How difficult would it be to support this?

[vchuravy]

I think type predicates might be the right term, but I suspect for your use-case a limited set of predicates might be acceptable.

We have wanted something similar in the GPU ecosystem for wrapper arrays as well.

SubArray{T<:AbstractArray} <: storagetype(T)

[Seelengrab]

This is basically a duplicate of #5 and/or traits. See also #23429, #18161, #6975 and numerous others, both here and on discourse. I don't think having a new issue for this is going to lead to a more centralized discussion.

[vchuravy]

No it is not equivalent to #5 or interfaces/traits.

This is not about UnitfulNumber implementing the interface of the class number, but that the interface it implements is dependent on a parameter of the class is self.

Multiple Inheritance/Interface are an orthogonal concern.

[mbauman]

Yeah, this is #8322.