[Fix rubocop#3249] Do not error when parsing rescue nil in ShadowedEx…

…ception

CHANGELOG.md

@@ -7,6 +7,7 @@
 * [#3248](https://github.com/bbatsov/rubocop/issues/3248): Support 'ruby-' prefix in `.ruby-version`. ([@tjwp][])
 * [#3250](https://github.com/bbatsov/rubocop/pull/3250): Make regexp for cop names less restrictive in CommentConfig lines. ([@tjwp][])
 * [#3261](https://github.com/bbatsov/rubocop/pull/3261): Prefer `TargetRubyVersion` to `.ruby-version`. ([@tjwp][])
+* [#3249](https://github.com/bbatsov/rubocop/issues/3249): Account for `rescue nil` in `Style/ShadowedException`. ([@rrosenblum][])
 
 ## 0.41.1 (2016-06-26)
 

lib/rubocop/cop/lint/shadowed_exception.rb

@@ -73,7 +73,13 @@ def evaluate_exceptions(rescue_group)
 
             rescued_exceptions.each_with_object([]) do |exception, converted|
               begin
-                converted << instance_eval(exception, __FILE__, __LINE__)
+                evaled_exception = instance_eval(exception, __FILE__, __LINE__)
+                # `rescue nil` is valid syntax in all versions of Ruby. In Ruby
+                # 1.9.3, it effectively disables the `rescue`. In versions
+                # after 1.9.3, a `TypeError` is thrown when the statement is
+                # rescued. In order to account for this, we convert `nil` to
+                # `NilClass`.
+                converted << (evaled_exception || NilClass)
               rescue StandardError, ScriptError
                 next
               end

spec/rubocop/cop/lint/shadowed_exception_spec.rb

@@ -122,6 +122,37 @@
 
       expect(cop.offenses).to be_empty
     end
+
+    it 'accepts rescuing nil' do
+      inspect_source(cop, ['begin',
+                           '  a',
+                           'rescue nil',
+                           '  b',
+                           'end'])
+
+      expect(cop.offenses).to be_empty
+    end
+
+    it 'accepts rescuing nil and another exception' do
+      inspect_source(cop, ['begin',
+                           '  a',
+                           'rescue nil, Exception',
+                           '  b',
+                           'end'])
+
+      expect(cop.offenses).to be_empty
+    end
+
+    it 'registers an offense when rescuing nil multiple exceptions of ' \
+       'different levels' do
+      inspect_source(cop, ['begin',
+                           '  a',
+                           'rescue nil, StandardError, Exception',
+                           '  b',
+                           'end'])
+
+      expect(cop.messages).to eq(['Do not shadow rescued Exceptions'])
+    end
   end
 
   context 'multiple rescues' do
@@ -257,5 +288,29 @@
         expect(cop.offenses).to be_empty
       end
     end
+
+    it 'accepts rescuing nil before another exception' do
+      inspect_source(cop, ['begin',
+                           '  a',
+                           'rescue nil',
+                           '  b',
+                           'rescue',
+                           '  c',
+                           'end'])
+
+      expect(cop.offenses).to be_empty
+    end
+
+    it 'accepts rescuing nil after another exception' do
+      inspect_source(cop, ['begin',
+                           '  a',
+                           'rescue',
+                           '  b',
+                           'rescue nil',
+                           '  c',
+                           'end'])
+
+      expect(cop.offenses).to be_empty
+    end
   end
 end