Allow querying conflicts from within JuMP

Project.toml

@@ -19,7 +19,7 @@ Statistics = "10745b16-79ce-11e8-11f9-7d13ad32a3b2"
 Calculus = "0.5"
 DataStructures = "0.17"
 ForwardDiff = "~0.5.0, ~0.6, ~0.7, ~0.8, ~0.9, ~0.10"
-MathOptInterface = "~0.9.11"
+MathOptInterface = "~0.9.14"
 MutableArithmetics = "0.2"
 NaNMath = "0.3"
 julia = "1"

docs/src/solutions.md

@@ -52,7 +52,7 @@ This function will return a `MOI.TerminationStatusCode` `enum`.
 MOI.TerminationStatusCode
 ```
 
-Additionally, we can receive a solver specific string explaning why the
+Additionally, we can receive a solver specific string explaining why the
 optimization stopped with [`raw_status`](@ref).
 
 ## Solution statuses
@@ -249,6 +249,28 @@ lp_objective_perturbation_range
 lp_rhs_perturbation_range
 ```
 
+## Conflicts
+
+Sometimes, the model you input is infeasible, and some solvers can help you
+find the cause of this infeasibility by offering a conflict, i.e. a subset of
+the constraints that create this infeasibility.
+
+The function [`compute_conflict!`](@ref) is used to start the computation of
+a conflict. Once this process is finished, the attribute `MOI.ConflictStatus`
+returns a `MOI.ConflictStatusCode` `enum`.
+
+```@docs
+MOI.ConflictStatusCode
+```
+
+If there is a conflict, you can query each constraint whether it participates
+in the conflict or not using the attribute `MOI.ConstraintConflictStatus`.
+It returns a `MOI.ConstraintConflictStatus` `enum`.
+
+```@docs
+MOI.ConflictParticipationStatusCode
+```
+
 ## Multiple solutions
 
 Some solvers support returning multiple solutions. You can check how many
@@ -289,6 +311,7 @@ JuMP.dual
 JuMP.solve_time
 OptimizeNotCalled
 MOI.optimize!
+JuMP.compute_conflict!
 JuMP.result_count
 JuMP.relative_gap
 JuMP.simplex_iterations

src/optimizer_interface.jl

@@ -144,6 +144,27 @@ function optimize!(model::Model,
     return
 end
 
+"""
+    compute_conflict!(model::Model)
+
+Compute a conflict if the model is infeasible. If an optimizer has not
+been set yet (see [`set_optimizer`](@ref)), a [`NoOptimizer`](@ref)
+error is thrown.
+
+The status of the conflict can be checked with the `MOI.ConflictStatus`
+model attribute. Then, the status for each constraint can be queried with
+the `MOI.ConstraintConflictStatus` attribute.
+"""
+function compute_conflict!(model::Model)
+    if mode(model) != DIRECT && MOIU.state(backend(model)) == MOIU.NO_OPTIMIZER
+        throw(NoOptimizer())
+    end
+
+    MOI.compute_conflict!(backend(model))
+
+    return
+end
+
 """
     result_count(model::Model)
 

test/model.jl

@@ -507,3 +507,11 @@ function dummy_optimizer_hook(::JuMP.AbstractModel) end
         @test_throws ErrorException JuMP.copy(model)
     end
 end
+
+@testset "Conflict computation" begin
+    @testset "NoOptimizer()" begin
+        err = NoOptimizer()
+        model = Model()
+        @test_throws err compute_conflict!(model)
+    end
+end