Request: Allow type aliases to be used as outer constructors

[kmsquire]

I'd like to be able to use type aliases as outer constructors. My use case is in my latest incarnation of OrderedDicts, although the same applies (I think) to, e.g., Vector{T}.

In my case, I define a base type OrderedDictBase

abstract DictItem{K,V}

type OrderedDictBase{K,V,Item<:DictItem,Itr} <: Associative{K,V}
    ht::Dict{K,Item} # really: Item{K,V}
    order::Itr       # really: Itr{Item{K,V}}

    OrderedDictBase() = OrderedDictBase(K,V,Item,Itr)  # callback to outer constructor
    OrderedDictBase(ht::Dict{K,Item}, order::Itr) = new(ht, order)
end

and specific versions as

typealias OrderedDict{K,V} OrderedDictBase{K,V,LinkedDictItem,LinkedDictItem}
typealias ArrayOrderedDict{K,V} OrderedDictBase{K,V,ArrayDictItem,Array}

I'd like to be able to define OrderedDict() as an outer constructor,

OrderedDict() = OrderedDict(Any,Any)
OrderedDict(K::Type, V::Type) = OrderedDictBase{K,V,LinkedDictItem,LinkedDictItem}()

But when I do, I get

julia> using OrderedCollections
ERROR: invalid method definition: not a generic function
 in include_from_node1 at loading.jl:91 (repeats 2 times)
 in reload_path at loading.jl:114
 in require at loading.jl:48
at /home/kmsquire/.julia/v0.2/OrderedCollections/src/OrderedDict.jl:201
at /home/kmsquire/.julia/v0.2/OrderedCollections/src/OrderedCollections.jl:3

My current solution is to define the typealias as _OrderedDict{K,V} (https://github.com/kmsquire/OrderedCollections.jl/blob/simplified/src/OrderedDict.jl#L199).

This works, but is a little inelegant.

Can this be done?

Somewhat related to @amitmurthy's comment in #3214 (comment)

[JeffBezanson]

Apologies, but this will probably not happen.

[StefanKarpinski]

Can you comment on why it won't happen? I've often wanted this as well. Seems related to #1470.

[kmsquire]

I was just going to ask the same thing.

[cbecker]

I would also like to find out why, has been discussed here as well https://groups.google.com/forum/#!topic/julia-users/FO-xpNDrfoM

[JeffBezanson]

#1470 might make this possible.