[Fix rubocop#3411] Avoid crash in Performance::CaseWhenSplat#autocorrect

A special case is when there is only one `when` branch, and it has
multiple conditions, with a splat followed by non-splat. This change
only fixes the crash. I think it's possible that reordering the
conditions on the line leads to faster code, but that should be checked.
It's also a more complicated change that could lead to new bugs.

CHANGELOG.md

@@ -18,6 +18,7 @@
 * [#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][])
+* [#3411](https://github.com/bbatsov/rubocop/issues/3411): Avoid auto-correction crash for single `when` in `Performance/CaseWhenSplat`. ([@jonas054][])
 
 ### Changes
 

lib/rubocop/cop/performance/case_when_splat.rb

@@ -106,6 +106,8 @@ def inline_fix_branch(corrector, _node, conditions, new_condition)
         def reorder_condition(corrector, node, new_condition)
           *_conditions, body = *node
           _case_branch, *when_branches, _else_branch = *node.parent
+          return if when_branches.size == 1 # Can't reorder one branch
+
           corrector.remove(when_branch_range(node, when_branches))
 
           correction = if same_line?(node, body)

spec/rubocop/cop/performance/case_when_splat_spec.rb

@@ -80,6 +80,18 @@
     expect(cop.highlights).to eq(['when *cond'])
   end
 
+  # TODO: Examine if changing `when *Foo, Bar` to `when Bar, *Foo` gives faster
+  # code.
+  it 'registers an offense for a single when with splat expansion followed ' \
+     'by another value' do
+    inspect_source(cop, ['case foo',
+                         'when *Foo, Bar',
+                         '  nil',
+                         'end'])
+    expect(cop.messages).to eq([described_class::MSG])
+    expect(cop.highlights).to eq(['when *Foo'])
+  end
+
   it 'registers an offense for multiple splat conditions at the beginning' do
     inspect_source(cop, ['case foo',
                          'when *cond1',
@@ -182,6 +194,18 @@
   end
 
   context 'autocorrect' do
+    # TODO: Either report and auto-correct, or don't do anything. Currently we
+    # report but don't auto-correct.
+    it 'does not correct a single when with splat expansion followed by ' \
+       'another value' do
+      old_source = ['case foo',
+                    'when *Foo, Bar',
+                    '  nil',
+                    'end']
+      new_source = autocorrect_source(cop, old_source)
+      expect(new_source).to eq(old_source.join("\n"))
+    end
+
     it 'moves a single splat condition to the end of the when conditions' do
       new_source = autocorrect_source(cop, ['case foo',
                                             'when *cond',