The {% comment %} tag parses its body

Problem

It would be reasonable to expect any text between {% comment %} and {% endcomment %} to be completely ignored.

However, that is not the case:

❯ irb
irb(main):001:0> require 'liquid'
=> true
irb(main):002:0> Liquid::VERSION
=> "4.0.0"
irb(main):003:0> Liquid::Template.parse("{% comment %}{% if 'foo' %}{% endcomment %}")
Liquid::SyntaxError: Liquid syntax error: Unknown tag 'endcomment'
	from /Users/david/.gem/ruby/2.3.3/bundler/gems/liquid-c582b86f16b6/lib/liquid/block.rb:38:in `unknown_tag'
	from /Users/david/.gem/ruby/2.3.3/bundler/gems/liquid-c582b86f16b6/lib/liquid/tags/if.rb:36:in `unknown_tag'
	from /Users/david/.gem/ruby/2.3.3/bundler/gems/liquid-c582b86f16b6/lib/liquid/block.rb:68:in `block in parse_body'
	from /Users/david/.gem/ruby/2.3.3/bundler/gems/liquid-c582b86f16b6/lib/liquid/block_body.rb:33:in `parse'
	from /Users/david/.gem/ruby/2.3.3/bundler/gems/liquid-c582b86f16b6/lib/liquid/block.rb:58:in `parse_body'
	from /Users/david/.gem/ruby/2.3.3/bundler/gems/liquid-c582b86f16b6/lib/liquid/tags/if.rb:24:in `parse'
	from /Users/david/.gem/ruby/2.3.3/bundler/gems/liquid-c582b86f16b6/lib/liquid/tag.rb:10:in `parse'
	from /Users/david/.gem/ruby/2.3.3/bundler/gems/liquid-c582b86f16b6/lib/liquid/block_body.rb:27:in `parse'
	from /Users/david/.gem/ruby/2.3.3/bundler/gems/liquid-c582b86f16b6/lib/liquid/block.rb:58:in `parse_body'
	from /Users/david/.gem/ruby/2.3.3/bundler/gems/liquid-c582b86f16b6/lib/liquid/block.rb:12:in `parse'
	from /Users/david/.gem/ruby/2.3.3/bundler/gems/liquid-c582b86f16b6/lib/liquid/tag.rb:10:in `parse'
	from /Users/david/.gem/ruby/2.3.3/bundler/gems/liquid-c582b86f16b6/lib/liquid/block_body.rb:27:in `parse'
	from /Users/david/.gem/ruby/2.3.3/bundler/gems/liquid-c582b86f16b6/lib/liquid/document.rb:10:in `parse'
	from /Users/david/.gem/ruby/2.3.3/bundler/gems/liquid-c582b86f16b6/lib/liquid/document.rb:5:in `parse'
	from /Users/david/.gem/ruby/2.3.3/bundler/gems/liquid-c582b86f16b6/lib/liquid/template.rb:132:in `parse'
	from /Users/david/.gem/ruby/2.3.3/bundler/gems/liquid-c582b86f16b6/lib/liquid/template.rb:116:in `parse'

Cause

I believe this is because Liquid::Comment inherits from Liquid::Block

https://github.com/Shopify/liquid/blob/27c91203ab4adfde2cc5e1142bce2a267634ef8e/lib/liquid/tags/comment.rb#L2

which parses its body

https://github.com/Shopify/liquid/blob/27c91203ab4adfde2cc5e1142bce2a267634ef8e/lib/liquid/block.rb#L12

My initial thought was that this should have been caught by Liquid::Comment#unknown_tag

https://github.com/Shopify/liquid/blob/27c91203ab4adfde2cc5e1142bce2a267634ef8e/lib/liquid/tags/comment.rb#L7-L8

but the exception is being raised from within Liquid::If#unknown_tag

https://github.com/Shopify/liquid/blob/27c91203ab4adfde2cc5e1142bce2a267634ef8e/lib/liquid/tags/if.rb#L32-L36

which makes sense seen as {% if %} owns its own body.

Fix?

A potential solution might be to override Liquid::Block#parse

https://github.com/Shopify/liquid/blob/27c91203ab4adfde2cc5e1142bce2a267634ef8e/lib/liquid/block.rb#L10-L14

in Liquid::Comment seen as #render always returns an empty string.

Start with lib/liquid/tags/comment.rb and lib/liquid/block.rb, then reproduce the reported Liquid::Template.parse example. Check how the comment tag parses its body and ensure content between comment markers is ignored, including nested tags such as if. Done means the example parses without the unknown endcomment error and comment rendering remains empty.