@variable declarations returning empty container should have VariableRef element type

[ExpandingMan]

Is your feature request related to a problem? Please describe.
On latest tagged, @variable(m, x[1:0]) returns a Vector{Any}. Because of this, code which uses x as an input cannot infer the containers element type. An example of how this can go wrong is if one calls sum(x) it will throw an error because there is no zero(Any), where as zero(VariableRef) gives a zero AffExpr.

Describe the solution you'd like
The simplest solution would be to check for empty returns and convert them to Array{VariableRef}. However, I realize that this ignores the possibility of other variable types, so, alternatively, it might be nice to add a keyword argument. Preferably, this would still default to something other than Any.

Describe alternatives you've considered
You can convert the result, but it seems undesirable to have to do the check.

Additional context
I think previous versions of JuMP returned a VariableRef container, but I haven't verified this.

[odow]

JuMP@1.0.0 produced:

julia> using JuMP

julia> model = Model();

julia> @variable(model, x[1:0])
VariableRef[]

Latest master produced:

julia> using JuMP

julia> model = Model();

julia> @variable(model, x[1:0])
GenericVariableRef[]

so this is a regression.

Let me take a look.

[odow]

Ah. This is because model is a non-const global, so inference can't figure out the return type of the variable:

julia> using JuMP

julia> model = Model();

julia> @variable(model, x[1:0])
GenericVariableRef[]

julia> add_var(model) = @variable(model, y[1:0])
add_var (generic function with 1 method)

julia> add_var(model)
VariableRef[]

[ExpandingMan]

I'm confused, I was getting Any[] when the type of Model should have been inferable. I'll try to come up with a MWE when I get a chance.

[odow]

I didn't try the latest tag yet. I could see how that might return Any.

I think I have a fix for the underlying issue. Just running the tests.

[odow]

I found your Any:

julia> m = (;m = Model());

julia> @variable(m.m, x[1:0])
Any[]