Switching solver malfunctioning

[haoxiangyang89]

Previous to v.0.19 to switch from a solver to another, or modify the solver parameter, all we have to do is: (the following is an example)

mp = Model(solver = GurobiSolver(OutputFlag = 0));
mp.solver = IpoptSolver(print_level = 0);

Once JuMP was upgraded, MOI employs the solver in this way:

mp = Model();
optimize!(with_optimizer(Gurobi.Optimizer, OutputFlag = 0));

Now if I change the solver by running optimize! again, it throws me an error. Here is the code I run to generate this error:

using JuMP, Ipopt, Gurobi;
mp = Model();
@variable(mp, x1 >= 0);
@variable(mp, x2 >= 0);
@constraint(mp, x1 + 2x2 <= 1);
@constraint(mp, x2 + 2x1 <= 1);
@objective(mp, Max, x1 + x2);
optimize!(mp, with_optimizer(Gurobi.Optimizer));
optimize!(mp, with_optimizer(Ipopt.Optimizer));

Here is the printout of the error. The first solve is successful and then the second fails. If I want to solve the model again, I have to use Gurobi.Optimizer without changing any parameters/to another solver. It appears the same if I set up an optimizer when setting up the JuMP model instead of leaving it blank initially.

ERROR: An optimizer factory cannot both be provided in the Model constructor and at the optimize call.
Stacktrace:
[1] error(::String) at ./error.jl:33
[2] #optimize!#78(::Bool, ::Bool, ::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}, ::typeof(optimize!), ::Model, ::OptimizerFactory) at /home/haoxiang/.julia/packages/JuMP/iGamg/src/optimizer_interface.jl:121
[3] optimize!(::Model, ::OptimizerFactory) at /home/haoxiang/.julia/packages/JuMP/iGamg/src/optimizer_interface.jl:111
[4] top-level scope at REPL[162]:1

[blegat]

You can do

optimize!(mp, with_optimizer(Gurobi.Optimizer))
set_optimizer(mp, with_optimizer(Ipopt.Optimizer))
optimize!(mp)

The error message could be improved. Or we could even make it work.

[mlubin]

The second argument to optimize! makes it look like the optimizer is tossed out after the solve. Given that this is not how it actually works, maybe we should remove this method and force a call to set_optimizer.

[odow]

👍 on removing this in favor of set_optimizer