Fix MethodError when trying to modify a variable objective (#2278)

Author: odow

src/Utilities/objective_container.jl

@@ -274,8 +274,13 @@ function MOI.modify(
     change::MOI.AbstractFunctionModification,
 ) where {T}
     if o.single_variable !== nothing
-        o.single_variable =
-            modify_function!(something(o.single_variable), change)
+        throw(
+            MOI.ModifyObjectiveNotAllowed(
+                change,
+                "Cannot modify objective when there is a " *
+                "`VariableIndex` objective.",
+            ),
+        )
     elseif o.scalar_affine !== nothing
         o.scalar_affine = modify_function!(something(o.scalar_affine), change)
     elseif o.scalar_quadratic !== nothing
@@ -286,12 +291,17 @@ function MOI.modify(
             MOI.ModifyObjectiveNotAllowed(
                 change,
                 "Cannot modify objective when there is a " *
-                "`ScalarNonlinearFunction` objective",
+                "`ScalarNonlinearFunction` objective.",
             ),
         )
     elseif o.vector_variables !== nothing
-        o.vector_variables =
-            modify_function!(something(o.vector_variables), change)
+        throw(
+            MOI.ModifyObjectiveNotAllowed(
+                change,
+                "Cannot modify objective when there is a " *
+                "`VectorOfVariables` objective.",
+            ),
+        )
     elseif o.vector_quadratic !== nothing
         o.vector_quadratic =
             modify_function!(something(o.vector_quadratic), change)
@@ -302,7 +312,7 @@ function MOI.modify(
             MOI.ModifyObjectiveNotAllowed(
                 change,
                 "Cannot modify objective when there is a " *
-                "`VectorNonlinearFunction` objective",
+                "`VectorNonlinearFunction` objective.",
             ),
         )
     else

test/Utilities/objective_container.jl

@@ -164,6 +164,33 @@ function test_delete_variables_ScalarNonlinearFunction()
     return
 end
 
+function test_modify_VariableIndex()
+    model = MOI.Utilities.Model{Float64}()
+    x = MOI.add_variable(model)
+    MOI.set(model, MOI.ObjectiveSense(), MOI.MAX_SENSE)
+    attr = MOI.ObjectiveFunction{typeof(x)}()
+    MOI.set(model, attr, x)
+    @test_throws(
+        MOI.ModifyObjectiveNotAllowed,
+        MOI.modify(model, attr, MOI.ScalarConstantChange(3.0)),
+    )
+    return
+end
+
+function test_modify_VectorOfVariables()
+    model = MOI.Utilities.Model{Float64}()
+    x = MOI.add_variables(model, 2)
+    MOI.set(model, MOI.ObjectiveSense(), MOI.MAX_SENSE)
+    f = MOI.VectorOfVariables(x)
+    attr = MOI.ObjectiveFunction{typeof(f)}()
+    MOI.set(model, attr, f)
+    @test_throws(
+        MOI.ModifyObjectiveNotAllowed,
+        MOI.modify(model, attr, MOI.VectorConstantChange([3.0, 4.0])),
+    )
+    return
+end
+
 end  # module
 
 TestObjectiveContainer.runtests()