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Ruby 2.2.2 contains a nasty bug around passing a hash with keys that are symbolized strings to a method. A coworker and I stumbled onto this trap and wrote this proof before finding a bug report on ruby-lang.

The essence of the bug is that when you start with a hash which has string keys, symbolize those keys, and splat that hash into a method parameter, the receiver will not always get all the intended parameters.

# require 'byebug'
puts "ruby -v is #{RUBY_VERSION} but should be '2.2.2'" unless RUBY_VERSION == '2.2.2'

class Hash
  def symbolize_keys
    dup = self.class.new

    each_key do |key|
      dup[ key.to_sym ] = self[key]
    end

    dup
  end
end

class MagicDance
  def initialize
    @rolled_back = false
  end

  def dance(one:, two:, **splat)
    puts "#{two} #{one}"
    puts splat
  end

  def one
    # ============================
    # Buggy, but only when it's by itself in the method:
    splat = { "three" => :anything }.symbolize_keys
    GC.start # fixes it
    dance two: :not, one: :broken, **splat

    # ============================

    # Uncomment to see above statement succeed
    # dance two: :not, one: :broken, **{ three: :anything }

    # ============================

    # Uncomment to see above statement succeed
    # dance two: :not, one: :broken, three: :anything
  end

  # Even in different methods!
  # Uncomment to see above statement succeed
  # def two
  #   dance two: :not, one: :broken, **{ three: :anything }
  # end
end

MagicDance.new.one

Ruby 2.2.2 Splat Bug