Make it easier to use "inner type parameters" in wrapper types.

[jakobnissen]

This might be a duplicate of #36377, not sure.

Suppose I want to make a type

struct Foo{I <: Integer}
    v::Vector{Complex{I}}
end

Now, at runtime, I have a vector of integer Complex, but I don't know which integer type, so I can just make an abstractly typed array:

julia> v = Complex{<:Integer}[Complex{Int}(1, 1)]
1-element Array{Complex{var"#s16"} where var"#s16"<:Integer,1}:
 1 + 1im

But now, how do I wrap v in a Foo?

julia> Foo{<:Integer}(v)
ERROR: MethodError: no method matching Foo{var"#s12"} where var"#s12"<:Integer(::Array{Complex{var"#s16"} where var"#s16"<:Integer,1})

One workaround is to instead define

struct Foo{C <: Complex{<:Integer}}
    v::Vector{C}
end

But I think having Complex as part of Foo's signature is unnecessary. Would it be possible to enable the "direct" use of <:Integer in this case?

[JeffBezanson]

Basically, no --- it's possible to define a constructor for Foo{<:Integer}(v), but that constructor would still have to return an instance of a concrete type, so it would still have to pick a specific value for I.

[jakobnissen]

Ah, I see. So the reason for having to have the wrapper type Complex in Foo{C <: Complex{<:Integer}} is that Foo{<:Integer} is a UnionAll type whereas Foo{Complex{<:Integer}} is concrete.

Thanks for answering this, closing the issue.