Freeze Random::Formatter::ALPHANUMERIC and its elements

Merged: https://github.com/ruby/ruby/pull/12356

lib/random/formatter.rb

@@ -340,7 +340,7 @@ module Random::Formatter
   end
 
   # The default character list for #alphanumeric.
-  ALPHANUMERIC = [*'A'..'Z', *'a'..'z', *'0'..'9']
+  ALPHANUMERIC = [*'A'..'Z', *'a'..'z', *'0'..'9'].map(&:freeze).freeze
 
   # Generate a random alphanumeric string.
   #

test/ruby/test_random_formatter.rb

@@ -165,6 +165,11 @@ module Random::Formatter
     def setup
       @it = Random
     end
+
+    def test_alphanumeric_frozen
+      assert_predicate @it::Formatter::ALPHANUMERIC, :frozen?
+      assert @it::Formatter::ALPHANUMERIC.all?(&:frozen?)
+    end
   end
 
   class TestInstanceMethods < Test::Unit::TestCase