JuMP v0.19:

```
using JuMP, Clp
m = Model(with_optimizer(Clp.Optimizer))
@variable(m, y>=0)
@objective(m, Max, y)
@constraint(m, con, y <= 3)
JuMP.optimize!(m)
pi_val = JuMP.result_dual(con)
@show pi_val
```

I obtained `pi_val = -1.0`

JuMP v0.18:

```
using JuMP, Clp
m = Model(solver=ClpSolver())
@variable(m, y>=0)
@objective(m, Max, y)
@constraint(m, con, y <= 3)
solve(m)
pi_val = getdual(con)
@show pi_val
```

I obtained `pi_val = 1.0`

Why does this happen? This is independent from the solvers, so I guess something changed in JuMP or MOI.