`assign` tag mutates cached template context

The source-code documentation suggests caching Liquid templates and using them to render multiple data:
https://github.com/Shopify/liquid/blob/0847bf560fbb99d20410408c27d750b7b412d9fa/lib/liquid/template.rb#L9-L10

Fair enough. But if a cached template contains an assign tag, rendering it mutates the template's context for subsequent renders.

This may be illustrated with the following Ruby script and its output:

Script

require 'liquid'

CONTENTS = <<~TEXT

       Neo: "I am just {{ title | default: 'a regular guy' }}.."
  {%- if agent == '   Smith' %}{% assign title = 'A DISEASE' %}{% endif %}
  {{ agent }}: "Neo, you're {{ title | default: 'The One' }}!!"
TEXT

TEMPLATE = Liquid::Template.parse(CONTENTS)

puts TEMPLATE.render("agent" => "Morpheus")
puts TEMPLATE.render("agent" => " Trinity")
puts TEMPLATE.render("agent" => "  Oracle")
puts TEMPLATE.render("agent" => " Trinity")
puts TEMPLATE.render("agent" => "   Smith")
puts TEMPLATE.render("agent" => "  Oracle")
puts TEMPLATE.render("agent" => " Trinity")
puts TEMPLATE.render("agent" => "Morpheus")

Output

     Neo: "I am just a regular guy.."
Morpheus: "Neo, you're The One!!"

     Neo: "I am just a regular guy.."
 Trinity: "Neo, you're The One!!"

     Neo: "I am just a regular guy.."
  Oracle: "Neo, you're The One!!"

     Neo: "I am just a regular guy.."
 Trinity: "Neo, you're The One!!"

     Neo: "I am just a regular guy.."
   Smith: "Neo, you're A DISEASE!!"

     Neo: "I am just A DISEASE.."
  Oracle: "Neo, you're A DISEASE!!"

     Neo: "I am just A DISEASE.."
 Trinity: "Neo, you're A DISEASE!!"

     Neo: "I am just A DISEASE.."
Morpheus: "Neo, you're A DISEASE!!"

Workaround

The workaround is to explicitly reset the assigned variable in an else tag

{% if foo %}
  {% assign bar = 'lipsum' %}
{% else %}
  {% assign bar = '' %} (or {% assign bar = false %})
{% endif %}

But I feel that the workaround is just a hack and that there shouldn't be a persisting mutation in the first place.

Start with the caching guidance in lib/liquid/template.rb at the linked lines, then reproduce the behavior using the Ruby script in the issue. Trace how repeated render calls handle assign state; done means rendering the same parsed template no longer carries an assigned value into later renders.