Fix get_fallback for DualObjectiveValue with non-affine objective

src/Utilities/results.jl

@@ -74,6 +74,14 @@ function get_fallback(
     if status == MOI.NO_SOLUTION
         throw(MOI.GetAttributeNotAllowed(attr, "No dual solution is available"))
     end
+    F = MOI.get(model, MOI.ObjectiveFunctionType())
+    if !(F <: Union{MOI.VariableIndex,MOI.ScalarAffineFunction})
+        err = MOI.GetAttributeNotAllowed(
+            attr,
+            "Cannot get the dual objective with an objective function of type `$F`",
+        )
+        throw(err)
+    end
     value = zero(T) # sum will not work if there are zero constraints
     for (F, S) in MOI.get(model, MOI.ListOfConstraintTypesPresent())
         value += _dual_objective_value(model, F, S, T, attr.result_index)::T
@@ -83,9 +91,7 @@ function get_fallback(
     end
     if !is_ray(status)
         # The objective constant should not be present in rays
-        F = MOI.get(model, MOI.ObjectiveFunctionType())
-        f = MOI.get(model, MOI.ObjectiveFunction{F}())
-        value += MOI.constant(f, T)
+        value += MOI.constant(MOI.get(model, MOI.ObjectiveFunction{F}()), T)
     end
     return value::T
 end

test/Bridges/Objective/test_QuadratizeBridge.jl

@@ -71,7 +71,8 @@ function test_solve_result_index()
             (MOI.VariableIndex, MOI.GreaterThan{Float64}) => [1.0],
         ),
     )
-    MOI.Test.test_solve_result_index(model, MOI.Test.Config())
+    config = MOI.Test.Config(; exclude = Any[MOI.DualObjectiveValue])
+    MOI.Test.test_solve_result_index(model, config)
     return
 end
 

test/Utilities/test_results.jl

@@ -291,6 +291,30 @@ function test_get_fallback_DualObjectiveValue()
     return
 end
 
+function test_get_fallback_DualObjectiveValue_Quadratic()
+    T = Float64
+    model = MOI.Utilities.MockOptimizer(
+        MOI.Utilities.UniversalFallback(MOI.Utilities.Model{T}()),
+        T,
+    )
+    x = MOI.add_variable(model)
+    MOI.set(model, MOI.ObjectiveSense(), MOI.MIN_SENSE)
+    f = 1.0 * x * x
+    MOI.set(model, MOI.ObjectiveFunction{typeof(f)}(), f)
+    MOI.set(model, MOI.ResultCount(), 1)
+    MOI.set(model, MOI.PrimalStatus(), MOI.FEASIBLE_POINT)
+    MOI.set(model, MOI.DualStatus(), MOI.FEASIBLE_POINT)
+    MOI.set(model, MOI.VariablePrimal(), x, 0.0)
+    @test_throws(
+        MOI.GetAttributeNotAllowed(
+            MOI.DualObjectiveValue(),
+            "Cannot get the dual objective with an objective function of type `$(typeof(f))`",
+        ),
+        MOI.Utilities.get_fallback(model, MOI.DualObjectiveValue(), T),
+    )
+    return
+end
+
 end  # module TestResults
 
 TestResults.runtests()