[Utilities] fix modify_function! for ScalarQuadraticCoefficientChange (#2408)

Author: odow

src/Test/test_modification.jl

@@ -1035,6 +1035,8 @@ function test_modification_objective_scalarquadraticcoefficientchange(
     @test ≈(MOI.get(model, attr), T(1) * x * x + T(2) * x * y, config)
     MOI.modify(model, attr, MOI.ScalarQuadraticCoefficientChange(x, y, T(4)))
     @test ≈(MOI.get(model, attr), T(1) * x * x + T(4) * x * y, config)
+    MOI.modify(model, attr, MOI.ScalarQuadraticCoefficientChange(y, x, T(6)))
+    @test ≈(MOI.get(model, attr), T(1) * x * x + T(6) * x * y, config)
     return
 end
 
@@ -1055,5 +1057,7 @@ function test_modification_constraint_scalarquadraticcoefficientchange(
     MOI.modify(model, c, MOI.ScalarQuadraticCoefficientChange(x, x, T(2)))
     MOI.modify(model, c, MOI.ScalarQuadraticCoefficientChange(x, y, -T(3)))
     @test ≈(MOI.get(model, MOI.ConstraintFunction(), c), g, config)
+    MOI.modify(model, c, MOI.ScalarQuadraticCoefficientChange(y, x, -T(3)))
+    @test ≈(MOI.get(model, MOI.ConstraintFunction(), c), g, config)
     return
 end

src/Utilities/functions.jl

@@ -1402,8 +1402,9 @@ function modify_function!(
     change::MOI.ScalarQuadraticCoefficientChange{T},
 ) where {T}
     indices = findall(f.quadratic_terms) do term
-        return term.variable_1 == change.variable_1 &&
-               term.variable_2 == change.variable_2
+        x, y = term.variable_1, term.variable_2
+        return (x == change.variable_1 && y == change.variable_2) ||
+               (y == change.variable_1 && x == change.variable_2)
     end
     for j in reverse(indices)
         deleteat!(f.quadratic_terms, j)