[Fix rubocop#3430] Prevent exception in Performance/RedundantMerge …

…when receiver is implicit

This cop would blow up when inspecting code that involved sending
`#merge!` to `self` implicitly, e.g.:

```
class Foo
  def bar(baz)
    merge!(baz)
  end
end
```

This change fixes that.

CHANGELOG.md

@@ -17,6 +17,7 @@
 * [#3540](https://github.com/bbatsov/rubocop/issues/3540): Fix `Style/GuardClause` to register offense for instance and singleton methods. ([@tejasbubane][])
 * [#3311](https://github.com/bbatsov/rubocop/issues/3311): Detect incompatibilities with the external encoding to prevent bad autocorrections in `Style/StringLiterals`. ([@deivid-rodriguez][])
 * [#3499](https://github.com/bbatsov/rubocop/issues/3499): Ensure `Lint/UnusedBlockArgument` doesn't make recommendations that would change arity for methods defined using `#define_method`. ([@drenmi][])
+* [#3430](https://github.com/bbatsov/rubocop/issues/3430): Fix exception in `Performance/RedundantMerge` when inspecting a `#merge!` with implicit receiver. ([@drenmi][])
 
 ### Changes
 

lib/rubocop/cop/performance/redundant_merge.rb

@@ -44,6 +44,7 @@ def autocorrect(node)
 
         def each_redundant_merge(node)
           redundant_merge(node) do |receiver, pairs|
+            next unless receiver
             next if node.value_used? &&
                     !EachWithObjectInspector.new(node, receiver).value_used?
             next if pairs.size > 1 && !receiver.pure?

spec/rubocop/cop/performance/redundant_merge_spec.rb

@@ -36,6 +36,13 @@
     end
   end
 
+  context 'when receiver is implicit' do
+    it "doesn't autocorrect" do
+      new_source = autocorrect_source(cop, 'merge!(foo: 1, bar: 2)')
+      expect(new_source).to eq('merge!(foo: 1, bar: 2)')
+    end
+  end
+
   context 'when internal to each_with_object' do
     it 'autocorrects when the receiver is the object being built' do
       source = ['foo.each_with_object({}) do |f, hash|',