Lint/UselessAssignment: bad behavior with pattern matching

[akimd]

Expected behavior

In the following example, res is obviously needed:

def f
  case '0'
  in String
    res = 1
  else
    res = 2
  end
  res
end

puts f

so Lint/UselessAssignment should not claim it is useless.

Actual behavior

Offenses:

patmatch.rb:1:1: C: [Correctable] Style/FrozenStringLiteralComment: Missing frozen string literal comment.
def f
^
patmatch.rb:4:5: W: Lint/UselessAssignment: Useless assignment to variable - res.
    res = 1
    ^^^

1 file inspected, 2 offenses detected, 1 offense auto-correctable

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For /private/tmp/foo: configuration from /private/tmp/foo/.rubocop.yml
Inheriting configuration from /private/tmp/foo/.rubocop_todo.yml
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Please also note that can also opt-in to new cops by default by adding this to your config:
  AllCops:
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Gemspec/DateAssignment: # (new in 1.10)
  Enabled: true
Layout/SpaceBeforeBrackets: # (new in 1.7)
  Enabled: true
Lint/AmbiguousAssignment: # (new in 1.7)
  Enabled: true
Lint/DeprecatedConstants: # (new in 1.8)
  Enabled: true
Lint/DuplicateBranch: # (new in 1.3)
  Enabled: true
Lint/DuplicateRegexpCharacterClassElement: # (new in 1.1)
  Enabled: true
Lint/EmptyBlock: # (new in 1.1)
  Enabled: true
Lint/EmptyClass: # (new in 1.3)
  Enabled: true
Lint/LambdaWithoutLiteralBlock: # (new in 1.8)
  Enabled: true
Lint/NoReturnInBeginEndBlocks: # (new in 1.2)
  Enabled: true
Lint/NumberedParameterAssignment: # (new in 1.9)
  Enabled: true
Lint/OrAssignmentToConstant: # (new in 1.9)
  Enabled: true
Lint/RedundantDirGlobSort: # (new in 1.8)
  Enabled: true
Lint/SymbolConversion: # (new in 1.9)
  Enabled: true
Lint/ToEnumArguments: # (new in 1.1)
  Enabled: true
Lint/TripleQuotes: # (new in 1.9)
  Enabled: true
Lint/UnexpectedBlockArity: # (new in 1.5)
  Enabled: true
Lint/UnmodifiedReduceAccumulator: # (new in 1.1)
  Enabled: true
Style/ArgumentsForwarding: # (new in 1.1)
  Enabled: true
Style/CollectionCompact: # (new in 1.2)
  Enabled: true
Style/DocumentDynamicEvalDefinition: # (new in 1.1)
  Enabled: true
Style/EndlessMethod: # (new in 1.8)
  Enabled: true
Style/HashConversion: # (new in 1.10)
  Enabled: true
Style/HashExcept: # (new in 1.7)
  Enabled: true
Style/IfWithBooleanLiteralBranches: # (new in 1.9)
  Enabled: true
Style/NegatedIfElseCondition: # (new in 1.2)
  Enabled: true
Style/NilLambda: # (new in 1.3)
  Enabled: true
Style/RedundantArgument: # (new in 1.4)
  Enabled: true
Style/StringChars: # (new in 1.12)
  Enabled: true
Style/SwapValues: # (new in 1.1)
  Enabled: true
For more information: https://docs.rubocop.org/rubocop/versioning.html
Inspecting 1 file
Scanning /private/tmp/foo/patmatch.rb
W

Offenses:

patmatch.rb:1:1: C: [Correctable] Style/FrozenStringLiteralComment: Missing frozen string literal comment.
def f
^
patmatch.rb:4:5: W: Lint/UselessAssignment: Useless assignment to variable - res.
    res = 1
    ^^^

1 file inspected, 2 offenses detected, 1 offense auto-correctable
Finished in 0.1654020000132732 seconds

Steps to reproduce the problem

See above.

RuboCop version

$ rubocop -V
1.13.0 (using Parser 3.0.1.0, rubocop-ast 1.4.1, running on ruby 3.0.1 x86_64-darwin18)

Greetings

Thanks for rubocop!