From 018648792b9ad70441bcda164a1930a930d83f4b Mon Sep 17 00:00:00 2001
From: Daniel Vandersluis <daniel.vandersluis@gmail.com>
Date: Thu, 26 Nov 2020 16:06:20 -0500
Subject: [PATCH] Update `Lint/UnmodifiedReduceAccumulator` to handle numblocks
 and more than 2 arguments.

---
 ..._update_lintunmodifiedreduceaccumulator.md |  1 +
 config/default.yml                            |  1 +
 .../cop/lint/unmodified_reduce_accumulator.rb | 13 +++++---
 rubocop.gemspec                               |  2 +-
 .../unmodified_reduce_accumulator_spec.rb     | 31 +++++++++++++++++++
 5 files changed, 42 insertions(+), 6 deletions(-)
 create mode 100644 changelog/change_update_lintunmodifiedreduceaccumulator.md

diff --git a/changelog/change_update_lintunmodifiedreduceaccumulator.md b/changelog/change_update_lintunmodifiedreduceaccumulator.md
new file mode 100644
index 00000000000..2e061d61a94
--- /dev/null
+++ b/changelog/change_update_lintunmodifiedreduceaccumulator.md
@@ -0,0 +1 @@
+* [#9108](https://github.com/rubocop-hq/rubocop/pull/9108): Update `Lint/UnmodifiedReduceAccumulator` to handle numblocks and more than 2 arguments. ([@dvandersluis][])
diff --git a/config/default.yml b/config/default.yml
index 76bfa45e1ce..f66793187fe 100644
--- a/config/default.yml
+++ b/config/default.yml
@@ -2007,6 +2007,7 @@ Lint/UnmodifiedReduceAccumulator:
   Description: Checks for `reduce` or `inject` blocks that do not update the accumulator each iteration.
   Enabled: pending
   VersionAdded: '1.1'
+  VersionChanged: <<next>>
 
 Lint/UnreachableCode:
   Description: 'Unreachable code.'
diff --git a/lib/rubocop/cop/lint/unmodified_reduce_accumulator.rb b/lib/rubocop/cop/lint/unmodified_reduce_accumulator.rb
index 01b3e495592..c0129d74348 100644
--- a/lib/rubocop/cop/lint/unmodified_reduce_accumulator.rb
+++ b/lib/rubocop/cop/lint/unmodified_reduce_accumulator.rb
@@ -12,8 +12,7 @@ module Lint
       # could be rewritten as such without a loop.
       #
       # Also catches instances where an index of the accumulator is returned, as
-      # this may change the type of object being retained. As well, detects when
-      # fewer than 2 block arguments are specified.
+      # this may change the type of object being retained.
       #
       # NOTE: For the purpose of reducing false positives, this cop only flags
       # returns in `reduce` blocks where the element is the only variable in
@@ -68,7 +67,10 @@ class UnmodifiedReduceAccumulator < Base
         MSG_INDEX = 'Do not return an element of the accumulator in `%<method>s`.'
 
         def_node_matcher :reduce_with_block?, <<~PATTERN
-          (block (send _recv {:reduce :inject} ...) args ...)
+          {
+            (block (send _recv {:reduce :inject} ...) args ...)
+            (numblock (send _recv {:reduce :inject} ...) ...)
+          }
         PATTERN
 
         def_node_matcher :accumulator_index?, <<~PATTERN
@@ -107,10 +109,11 @@ class UnmodifiedReduceAccumulator < Base
 
         def on_block(node)
           return unless reduce_with_block?(node)
-          return unless node.arguments.length >= 2
+          return unless node.argument_list.length >= 2
 
           check_return_values(node)
         end
+        alias on_numblock on_block
 
         private
 
@@ -148,7 +151,7 @@ def check_return_values(block_node)
         end
 
         def block_arg_name(node, index)
-          node.arguments[index].node_parts[0]
+          node.argument_list[index].name
         end
 
         # Look for an index of the accumulator being returned, except where the index
diff --git a/rubocop.gemspec b/rubocop.gemspec
index 114bf03e44a..e0c220c6e68 100644
--- a/rubocop.gemspec
+++ b/rubocop.gemspec
@@ -35,7 +35,7 @@ Gem::Specification.new do |s|
   s.add_runtime_dependency('rainbow', '>= 2.2.2', '< 4.0')
   s.add_runtime_dependency('regexp_parser', '>= 2.0')
   s.add_runtime_dependency('rexml')
-  s.add_runtime_dependency('rubocop-ast', '>= 1.1.1')
+  s.add_runtime_dependency('rubocop-ast', '>= 1.2.0')
   s.add_runtime_dependency('ruby-progressbar', '~> 1.7')
   s.add_runtime_dependency('unicode-display_width', '>= 1.4.0', '< 2.0')
 
diff --git a/spec/rubocop/cop/lint/unmodified_reduce_accumulator_spec.rb b/spec/rubocop/cop/lint/unmodified_reduce_accumulator_spec.rb
index 4b77f993036..f593ad6e704 100644
--- a/spec/rubocop/cop/lint/unmodified_reduce_accumulator_spec.rb
+++ b/spec/rubocop/cop/lint/unmodified_reduce_accumulator_spec.rb
@@ -585,6 +585,37 @@
             (1..4).#{method}(0) { |acc| acc.foo }
           RUBY
         end
+
+        it 'ignores when there is a splat argument' do
+          expect_no_offenses(<<~RUBY, method: method)
+            values.#{method}(0) { |*x| x[0] + x[1] }
+          RUBY
+        end
+
+        it 'registers an offense when there are more than two arguments but the element is returned' do
+          expect_offense(<<~RUBY)
+            (1..4).each_with_index.#{method}([]) do |acc, (el, index)|
+              acc[el] = method(index)
+              el
+              ^^ Ensure the accumulator `acc` will be modified by `#{method}`.
+            end
+          RUBY
+        end
+      end
+
+      context 'numblocks', :ruby27 do
+        it 'registers an offense when returning the element' do
+          expect_offense(<<~RUBY, method: method)
+            (1..4).#{method}(0) { _2 }
+                   _{method}      ^^ Ensure the accumulator `_1` will be modified by `#{method}`.
+          RUBY
+        end
+
+        it 'does not register an offense when when returning the accumulator' do
+          expect_no_offenses(<<~RUBY)
+            values.#{method}(0) { _1 + _2 }
+          RUBY
+        end
       end
     end
   end