I’m trying to do a MIP-start on a multicommodity capacitated fixed-charge network design problem using JuMP 0.19 and CPLEX 12.8 with the `set_start_value`

function, but I keep getting the following error :

`MathOptInterface.UnsupportedAttribute{MathOptInterface.VariablePrimalStart}: Attribute MathOptInterface.VariablePrimalStart() is not supported by the model.`

Here is a made up example that produces the error:

```
using JuMP
using CPLEX
model = Model(with_optimizer(CPLEX.Optimizer))
@variable(model,0<= x[1:10] <= 12)
@variable(model, y[1:5],Bin)
@constraint(model, x[3] == 4y[2])
@constraint(model, con, sum(y->1-y,y)>=3)
@objective(model,Max, sum(x)+15*sum(y))
optimize!(model)#1
for i in y
set_start_value(i,0)
end
optimize!(model)#2
```

If I remove #1, the error becomes a warning saying that `MathOptInterface.VariablePrimalStart() is not supported by MathOptInterface.Bridges.LazyBridgeOptimizer`

and that the information will be discarded.

Since I am rather new to Julia, I would like to understand whether the error is caused by a mistake or misunderstanding of the documentation on my part or by a known problem in MOI or JuMP and how I should go about fixing it.

It would also be nice to know why the error sometimes becomes a warning.