sum doesn't work as expected on variables

[non-Jedi]

Illustrated with an example:

julia> using JuMP, GLPKMathProgInterface;

julia> solver = GLPKSolverLP();

julia> m = Model(solver=solver);

julia> @variable(m, x[1:2,1:2])
2×2 Array{JuMP.Variable,2}:
 x[1,1]  x[1,2]
 x[2,1]  x[2,2]

julia> x[1,1] + x[1,2]
x[1,1] + x[1,2]

julia> sum(x[1,:])
x[1,1] + x[1,2]

julia> sum(x)
x[1,1] + x[2,1] + x[1,2] + x[2,2]

julia> sum(x,2)
ERROR: MethodError: Cannot `convert` an object of type JuMP.GenericAffExpr{Float64,JuMP.Variable} to an object of type JuMP.Variable
This may have arisen from a call to the constructor JuMP.Variable(...),
since type constructors fall back to convert methods.
Stacktrace:
 [1] fill!(::Array{JuMP.Variable,2}, ::JuMP.GenericAffExpr{Float64,JuMP.Variable}) at ./multidimensional.jl:841
 [2] reducedim_initarray(::Array{JuMP.Variable,2}, ::Int64, ::JuMP.GenericAffExpr{Float64,JuMP.Variable}, ::Type{JuMP.Variable}) at ./reducedim.jl:73
 [3] _reducedim_init(::Base.#identity, ::Base.#+, ::Base.#zero, ::Base.#sum, ::Array{JuMP.Variable,2}, ::Int64) at ./reducedim.jl:102
 [4] reducedim_init(::Function, ::Base.#+, ::Array{JuMP.Variable,2}, ::Int64) at ./reducedim.jl:87
 [5] mapreducedim(::Function, ::Function, ::Array{JuMP.Variable,2}, ::Int64) at ./reducedim.jl:242
 [6] sum(::Array{JuMP.Variable,2}, ::Int64) at ./reducedim.jl:585
 [7] macro expansion at ./REPL.jl:97 [inlined]
 [8] (::Base.REPL.##1#2{Base.REPL.REPLBackend})() at ./event.jl:73

The last line should give me the same thing as e.g. [x[1,1] + x[1,2]; x[2,1] + x[2,2]] but throws an error instead. Let me know if there's another issue I missed about this or if this is somehow by design. Thanks!

[mlubin]

This is more of a feature request than a bug report, but yes, it would be nice if this worked.

[blegat]

The issue is due to this line:
https://github.com/JuliaLang/julia/blob/39d8aef7c6288d3fef481d56d49e3b054d3512e8/base/reducedim.jl#L98
In our case, z and x have type JuMP.GenericAffExpr{Float64,JuMP.Variable} and T is JuMP.Variable which is not isbits so Tr is T and not typeof(z).

[blegat]

I have opened an issue on Julia Base JuliaLang/julia#26709

[non-Jedi]

Fixed by JuliaLang/julia#28089 once JuMP works on 1.0.