future_parser doesn't appear to reject puppet 3 syntax

[WillSewell]

I have a Rakefile

require 'puppet-syntax/tasks/puppet-syntax'

PuppetSyntax.future_parser = true

I expect it to throw an error when I use a Puppet 3 feature not supported in Puppet 4, e.g.

node 'parent' {}

node 'child' inherits 'parent' {}

However when I run bundle exec rake, no errors are logged.

I know it is checking this file because when I make an obvious syntax change, it reports the error, whether or not PuppetSyntax.future_parser is set.