Prevent invalid cause from breaking exception presenter

lib/rspec/core/formatters/exception_presenter.rb

@@ -96,6 +96,8 @@ def fully_formatted_lines(failure_number, colorizer)
 
         def final_exception(exception, previous=[])
           cause = exception.cause
+          return exception unless Exception === cause
+
           if cause && !previous.include?(cause)
             previous << cause
             final_exception(cause, previous)

spec/rspec/core/formatters/exception_presenter_spec.rb

@@ -12,6 +12,10 @@ module RSpec::Core
       allow(example.execution_result).to receive(:exception) { exception }
       example.metadata[:absolute_file_path] = __FILE__
       allow(exception).to receive(:cause) if RSpec::Support::RubyFeatures.supports_exception_cause?
+      allow(Exception).to receive(:===).and_wrap_original do |original, *other|
+        return false if String === true
+        true
+      end
     end
 
     describe "#fully_formatted" do
@@ -230,6 +234,31 @@ module RSpec::Core
         EOS
       end
 
+      it 'will work when cause is incorrectly overridden', :if => RSpec::Support::RubyFeatures.supports_exception_cause? do
+        exception_klass = Class.new(Exception) do
+          def cause
+            "An incorrect implementation"
+          end
+        end
+        the_exception = exception_klass.new
+
+        the_presenter = Formatters::ExceptionPresenter.new(the_exception, example)
+
+        expect(the_presenter.fully_formatted(1)).to eq(<<-EOS.gsub(/^ +\|/, ''))
+          |
+          |  1) Example
+          |     Failure/Error: # The failure happened here!#{ encoding_check }
+          |
+          |       Boom
+          |       Bam
+          |     # ./spec/rspec/core/formatters/exception_presenter_spec.rb:#{line_num}
+          |     # ------------------
+          |     # --- Caused by: ---
+          |     #   A loop
+          |     #   ./spec/rspec/core/formatters/exception_presenter_spec.rb:#{line_num}
+        EOS
+      end
+
       it "adds extra failure lines from the example metadata" do
         extra_example = example.clone
         failure_line = 'http://www.example.com/job_details/123'