Improve the code coverage

Author: amontoison

test/small.jl

@@ -97,6 +97,18 @@ end;
                        "Arrowhead"     => Dict(:direct       => Dict(:all_colors => (0,0), :row_colors => (0,0), :column_colors => (0,0)),
                                                :substitution => Dict(:all_colors => (0,0), :row_colors => (0,0), :column_colors => (0,0))))
 
+    @testset "postprocessing_minimizes = :sym_colors" begin
+        problem = ColoringProblem(; structure=:nonsymmetric, partition=:bidirectional)
+        order = NaturalOrder()
+        A = sparse([1 0 0 0; 0 1 0 0; 0 0 1 0; 0 0 0 1])
+
+        star_bicoloring_algorithm = GreedyColoringAlgorithm{:direct}(; postprocessing=true, postprocessing_minimizes=:sym_colors)
+        @test_throws "The value postprocessing_minimizes = :sym_colors is not supported." coloring(A, problem, star_bicoloring_algorithm)
+
+        acyclic_bicoloring_algorithm = GreedyColoringAlgorithm{:substitution}(; postprocessing=true, postprocessing_minimizes=:sym_colors)
+        @test_throws "The value postprocessing_minimizes = :sym_colors is not supported." coloring(A, problem, acyclic_bicoloring_algorithm)
+    end
+
     @testset "postprocessing_minimizes = $target" for target in (:all_colors, :row_colors, :column_colors)
         problem = ColoringProblem(; structure=:nonsymmetric, partition=:bidirectional)
         order = RandomOrder(StableRNG(0), 0)