Introduce #bisect_over_recursive to handle independent failures

Recursive strategy implicitly assumes that at least one candidate is culpable;
therefore introduce an initial step (#bisect_over) and a recursive helper
method (#bisect_over_recursive) to check that the expected failures do not
occur without any of the passing examples.

Author: urbanautomaton

lib/rspec/core/bisect/example_minimizer.rb

@@ -17,7 +17,7 @@ def find_minimal_repro
           prep
 
           self.remaining_ids, duration = track_duration do
-            bisect_over(non_failing_example_ids, [])
+            bisect_over(non_failing_example_ids)
           end
 
           notify(:bisect_complete, :round => 10, :duration => duration,
@@ -27,7 +27,13 @@ def find_minimal_repro
           remaining_ids + failed_example_ids
         end
 
-        def bisect_over(candidate_ids, fixed)
+        def bisect_over(candidate_ids)
+          return [] if get_expected_failures_for?([])
+
+          bisect_over_recursive(candidate_ids, [])
+        end
+
+        def bisect_over_recursive(candidate_ids, fixed)
           return candidate_ids if candidate_ids.length == 1
           slice_size = (candidate_ids.length / 2.0).ceil
           slices = candidate_ids.each_slice(slice_size).to_a
@@ -50,10 +56,10 @@ def bisect_over(candidate_ids, fixed)
               :ids_to_ignore => ids_to_ignore,
               :remaining_ids => remaining_ids
             )
-            bisect_over(remaining_ids, fixed)
+            bisect_over_recursive(remaining_ids, fixed)
           else
             slices.permutation(2).map do |slice, other_slice|
-              bisect_over(slice, fixed + other_slice)
+              bisect_over_recursive(slice, fixed + other_slice)
             end.flatten(1)
           end
         end

spec/rspec/core/bisect/example_minimizer_spec.rb

@@ -85,7 +85,7 @@ def bisect_individual_run_start
       it 'detects an unminimisable run in the minimum number of rounds' do
         minimizer.find_minimal_repro
 
-        expect(counting_reporter.round_count).to eq(14)
+        expect(counting_reporter.round_count).to eq(15)
       end
     end