[Fix #6755] Prevent Style/TrailingCommaInArgument from breaking when …

…a safe method call is chained on the offending method

This cop would error out on code like:

```
foo.bar(
  baz: 1,
)&.fetch(:qux)
```

This was happening because the cop wasn't taking `csend` (safe navigation)
node types into consideration.

CHANGELOG.md

@@ -4,6 +4,10 @@
 
 ## 0.64.0 (2019-02-10)
 
+### Bug fixes
+
+* [#6755](https://github.com/rubocop-hq/rubocop/issues/6755): Prevent `Style/TrailingCommaInArgument` from breaking when a safe method call is chained on the offending method. ([@drenmi][])
+
 ### New features
 
 * [#6704](https://github.com/rubocop-hq/rubocop/pull/6704): Add new `Rails/ReflectionClassName` cop. ([@Bhacaz][])

lib/rubocop/cop/mixin/trailing_comma.rb

@@ -104,7 +104,7 @@ def allowed_multiline_argument?(node)
       end
 
       def elements(node)
-        return node.children unless node.send_type?
+        return node.children unless %i[csend send].include?(node.type)
 
         node.arguments.flat_map do |argument|
           # For each argument, if it is a multi-line hash without braces,

spec/rubocop/cop/style/trailing_comma_in_arguments_spec.rb

@@ -416,6 +416,23 @@
           )
         RUBY
       end
+
+      it 'does not break when a method call is chaned on the offending one' do
+        expect_no_offenses(<<-RUBY.strip_indent)
+          foo.bar(
+            baz: 1,
+          ).fetch(:qux)
+        RUBY
+      end
+
+      it 'does not break when a safe method call is chained on the ' \
+         'offending one', :ruby23 do
+        expect_no_offenses(<<-RUBY.strip_indent)
+          foo.bar(
+            baz: 1,
+          )&.fetch(:qux)
+        RUBY
+      end
     end
 
     context 'when EnforcedStyleForMultiline is consistent_comma' do