[Fix rubocop#8195] Fix an error for Style/RedundantFetchBlock

Fixes rubocop#8195

This PR fixes an error for `Style/RedundantFetchBlock` when using
`#fetch` with empty block.

CHANGELOG.md

@@ -6,6 +6,10 @@
 
 * [#7868](https://github.com/rubocop-hq/rubocop/pull/7868): `Cop::Base` is the new recommended base class for cops. Extensive refactoring of `Team`, `Commissioner`. ([@marcandre][])
 
+### Bug fixes
+
+* [#8195](https://github.com/rubocop-hq/rubocop/issues/8195): Fix an error for `Style/RedundantFetchBlock` when using `#fetch` with empty block. ([@koic][])
+
 ## 0.86.0 (2020-06-22)
 
 ### New features

lib/rubocop/cop/style/redundant_fetch_block.rb

@@ -41,13 +41,13 @@ class RedundantFetchBlock < Cop
           (block
             $(send _ :fetch _)
             (args)
-            ${#basic_literal? const_type?})
+            ${nil? #basic_literal? #const_type?})
         PATTERN
 
         def on_block(node)
           redundant_fetch_block_candidate?(node) do |send, body|
-            return if body.const_type? && !check_for_constant?
-            return if body.str_type? && !check_for_string?
+            return if body&.const_type? && !check_for_constant?
+            return if body&.str_type? && !check_for_string?
 
             range = fetch_range(send, node)
             good = build_good_method(send, body)
@@ -65,15 +65,21 @@ def autocorrect(node)
           redundant_fetch_block_candidate?(node) do |send, body|
             lambda do |corrector|
               receiver, _, key = send.children
-              corrector.replace(node, "#{receiver.source}.fetch(#{key.source}, #{body.source})")
+              default_value = body ? body.source : 'nil'
+
+              corrector.replace(node, "#{receiver.source}.fetch(#{key.source}, #{default_value})")
             end
           end
         end
 
         private
 
         def basic_literal?(node)
-          node.basic_literal?
+          node&.basic_literal?
+        end
+
+        def const_type?(node)
+          node&.const_type?
         end
 
         def fetch_range(send, node)
@@ -82,12 +88,16 @@ def fetch_range(send, node)
 
         def build_good_method(send, body)
           key = send.children[2].source
-          "fetch(#{key}, #{body.source})"
+          default_value = body ? body.source : 'nil'
+
+          "fetch(#{key}, #{default_value})"
         end
 
         def build_bad_method(send, body)
           key = send.children[2].source
-          "fetch(#{key}) { #{body.source} }"
+          block = body ? "{ #{body.source} }" : '{}'
+
+          "fetch(#{key}) #{block}"
         end
 
         def check_for_constant?

spec/rubocop/cop/style/redundant_fetch_block_spec.rb

@@ -67,6 +67,17 @@
       RUBY
     end
 
+    it 'registers an offense and corrects when using `#fetch` with empty block' do
+      expect_offense(<<~RUBY)
+        hash.fetch(:key) {}
+             ^^^^^^^^^^^^^^ Use `fetch(:key, nil)` instead of `fetch(:key) {}`.
+      RUBY
+
+      expect_correction(<<~RUBY)
+        hash.fetch(:key, nil)
+      RUBY
+    end
+
     it 'registers an offense and corrects when using `#fetch` with constant in the block' do
       expect_offense(<<~RUBY)
         hash.fetch(:key) { CONSTANT }