Some improvements to json wrapper

lib/gitlab/json.rb

@@ -2,12 +2,14 @@
 
 module Gitlab
   module Json
+    INVALID_LEGACY_TYPES = [String, TrueClass, FalseClass].freeze
+
     class << self
       def parse(string, *args, **named_args)
         legacy_mode = legacy_mode_enabled?(named_args.delete(:legacy_mode))
         data = adapter.parse(string, *args, **named_args)
 
-        raise parser_error if legacy_mode && [String, TrueClass, FalseClass].any? { |type| data.is_a?(type) }
+        raise parser_error if legacy_mode && INVALID_LEGACY_TYPES.any? { |type| data.is_a?(type) }
 
         data
       end
@@ -39,10 +41,11 @@ module Gitlab
       end
 
       def legacy_mode_enabled?(arg_value)
-        # This will change to the following once the `json` gem is upgraded:
-        # arg_value.nil? ? true : arg_value
-
-        true
+        if ::JSON::VERSION_MAJOR >= 2
+          arg_value.nil? ? true : arg_value
+        else
+          true
+        end
       end
     end
   end

spec/lib/gitlab/json_spec.rb

@@ -40,15 +40,15 @@ RSpec.describe Gitlab::Json do
       # will be updated accordingly.
 
       it "raises an error on a string" do
-        expect { subject.parse('"foo"') }.to raise_error(JSON::ParserError)
+        expect { subject.parse('"foo"', legacy_mode: false) }.to raise_error(JSON::ParserError)
       end
 
       it "raises an error on a true bool" do
-        expect { subject.parse("true") }.to raise_error(JSON::ParserError)
+        expect { subject.parse("true", legacy_mode: false) }.to raise_error(JSON::ParserError)
       end
 
       it "raises an error on a false bool" do
-        expect { subject.parse("false") }.to raise_error(JSON::ParserError)
+        expect { subject.parse("false", legacy_mode: false) }.to raise_error(JSON::ParserError)
       end
     end
   end