Querying dual information of MIP that has previously been solved

[odow]

In the following scenario, it is possible for the dual information to be out of date.

m=Model()
@variable(m, x, Bin)
@objective(m, Max, x)

solve(m)
getdual(x) # errors

solve(m, relaxation=true)
getdual(x) # 1.0

solve(m)
getdual(x) # 1.0

Is this user-beware or should the dual information be set to NaN in the else branch of here.

[mlubin]

The dual information should be cleared. More evidence for reworking MPB to better communicate which solution information is available and what its status is (#938).

[mlubin]

@joehuchette, closed by #963?

[joehuchette]

Yes