Cannot assign valid string contents

I was playing around with the idea of this post and I found that my take on it just breaks Liquid.
/cc @SLaks

This is what I would expect to work:

{% assign endraw = '{% endraw %}' %} contents: _{{endraw}}_

’ %} contents: __

This is how I managed to work around it:

{% assign endraw = '{% endraw %' | append: '}' %} contents: _{{endraw}}_

contents: _{% endraw %}_

... and as a bonus this one even throws an error:

{% assign endraw = '{% endraw ' | append: '%}' %}

Liquid error (line 123): wrong number of arguments (1 for 2)

Start by reproducing the issue with the Liquid assignment examples in the report, including the case that raises an argument-count error. Trace how assign values containing endraw tags are parsed; done means the literal string is preserved in output and the error case no longer fails unexpectedly.