[Fix #7635] Fix a false positive for Style/MultilineWhenThen

Fixes #7635

This PR fixes a false positive for `Style/MultilineWhenThen`
when `then` required for a body of `when` is used.

CHANGELOG.md

@@ -7,6 +7,7 @@
 * [#7882](https://github.com/rubocop-hq/rubocop/pull/7882): Fix `Style/CaseEquality` when `AllowOnConstant` is `true` and the method receiver is implicit. ([@rafaelfranca][])
 * [#7790](https://github.com/rubocop-hq/rubocop/issues/7790): Fix `--parallel` and `--ignore-parent-exclusion` combination. ([@jonas054][])
 * [#7881](https://github.com/rubocop-hq/rubocop/issues/7881): Fix `--parallel` and `--force-default-config` combination. ([@jonas054][])
+* [#7635](https://github.com/rubocop-hq/rubocop/issues/7635): Fix a false positive for `Style/MultilineWhenThen` when `then` required for a body of `when` is used. ([@koic][])
 
 ### Changes
 

lib/rubocop/cop/style/multiline_when_then.rb

@@ -22,6 +22,12 @@ module Style
       #   when bar then do_something
       #   end
       #
+      #   # good
+      #   case foo
+      #   when bar then do_something(arg1,
+      #                              arg2)
+      #   end
+      #
       class MultilineWhenThen < Cop
         include RangeHelp
 
@@ -32,7 +38,10 @@ def on_when(node)
           return unless node.then?
 
           # Single line usage of `then` is not an offense
-          return if !node.children.last.nil? && !node.multiline? && node.then?
+          return if !node.children.last.nil? && !node.multiline?
+
+          # Requires `then` for write `when` and its body on the same line.
+          return if require_then?(node)
 
           # With more than one statements after then, there's not offense
           return if accept_node_type?(node.body)
@@ -50,6 +59,12 @@ def autocorrect(node)
           end
         end
 
+        def require_then?(when_node)
+          return false unless when_node.body
+
+          when_node.loc.line == when_node.body.loc.line
+        end
+
         def accept_node_type?(node)
           node&.begin_type? || node&.array_type? || node&.hash_type?
         end

manual/cops_style.md

@@ -4206,6 +4206,12 @@ end
 case foo
 when bar then do_something
 end
+
+# good
+case foo
+when bar then do_something(arg1,
+                           arg2)
+end
 ```
 
 ### References

spec/rubocop/cop/style/multiline_when_then_spec.rb

@@ -30,6 +30,16 @@
     RUBY
   end
 
+  it "doesn't register an offense when `then` required for a body of `when` " \
+     'is used' do
+    expect_no_offenses(<<~RUBY)
+      case cond
+      when foo then do_something(arg1,
+                                 arg2)
+      end
+    RUBY
+  end
+
   it "doesn't register an offense for multiline when statement
   with then followed by other lines" do
     expect_no_offenses(<<~RUBY)