Assertion failure while solving MINLPs with fixed variables using AmplNLWriter

[ashwincarvalho]

Originally posted here

This piece of code gives me an assertion failure:

using JuMP, AmplNLWriter

x0 = 1.0   # continuous fixed variable value

m = Model(solver=BonminNLSolver())
@defVar(m, x == x0)
@defVar(m, y, Bin)
@setNLObjective(m, Min, x*y)
solve(m)

Error message:

ERROR: assertion failed: all(x->begin  # /Users/Ashwin/.julia/v0.3/AmplNLWriter/src/AmplNLWriter.jl, line 195:
            $(Expr(:in, :x, :([:Cont,:Bin,:Int])))
        end,cat)
 in setvartype! at /Users/Ashwin/.julia/v0.3/AmplNLWriter/src/AmplNLWriter.jl:195
 in _buildInternalModel_nlp at /Users/Ashwin/.julia/v0.3/JuMP/src/nlp.jl:543
 in buildInternalModel at /Users/Ashwin/.julia/v0.3/JuMP/src/solvers.jl:623
 in solve at /Users/Ashwin/.julia/v0.3/JuMP/src/solvers.jl:39
 in include at /Applications/Juno.app/Contents/Resources/app/julia/lib/julia/sys.dylib
 in include_from_node1 at /Applications/Juno.app/Contents/Resources/app/julia/lib/julia/sys.dylib
 in reload_path at loading.jl:152
 in reload at loading.jl:85
while loading /Users/Ashwin/Documents/GitHub/Ashwin/Research/Research/MIPLaneChange/testBonminAmplSolve2.jl, in expression starting on line 10

I was able to resolve that by using the Variable() method to specify the variable category:

x = Variable(m, x0, x0, :Cont)

Another workaround suggested by @mlubin :

@defVar(m, x0 <= x <= x0)

If no binary variables are involved, fixed continuous variables defined by the @defvar macro work fine. Eg:

@defVar(m, x == x0)
y = 1.0
@setNLObjective(m, Min, x*y)

This is not a problem when I call Bonmin via CoinOptServices. Using BonminNLSolver() or CouenneNLSolver() gives the assertion failure.

[tkelman]

I do explicitly translate :Fixed variables to continuous inside CoinOptServices, here https://github.com/JuliaOpt/CoinOptServices.jl/blob/cc73be365583c6acc969869c5238776494960cfa/src/translations.jl#L64

I think there was a fix at some point to prevent fixed variables from leaking out of JuMP, but it may have gotten broken (or never fixed in the first place?) for MINLP's. I might be wrong but I don't think any of the MINLP tests currently have any fixed variables.

[mlubin]

@tkelman, :Fixed isn't defined in the MPB interface. You should be able to remove that special case now.