Add inexplicit indexing warning when adding constraints

[jac0320]

So I was using a 3-dimensional variable, when adding constraints, I use the sum() generator and gave an explicit indexing with two indices, it automatically assumed the third index to be 1. It could be challenging in debugging the model especially when a complex variable is given.

Do you think it is necessary to have a warning message to notify users for implied index?

julia> @variable(m, x[1:5,1:5,1:10], Bin);

julia> @constraint(m, sum(x[i,j] for i=1:3, j=1:2)>=4)
x[1,1,1] + x[1,2,1] + x[2,1,1] + x[2,2,1] + x[3,1,1] + x[3,2,1] ≥ 4

Thanks.

[mlubin]

Thanks for the report. This is a duplicate of #653 which goes back to an upstream issue in Julia.