UserAgent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:54.0) Gecko/20100101 Firefox/54.0

Steps to reproduce the problem:
1. Attempt to build V8 on a system with Jinja 2.9 (it appears Jinja is not bundled with V8 even if it is bundled with Chromium?)

What is the expected behavior?
Successful build

What went wrong?
Build failure. See https://bugs.gentoo.org/show_bug.cgi?id=605140 for example.

Did this work before? N/A 

Chrome version:   Channel: n/a
OS Version: 
Flash Version: 

inspector_protocol/templates/TypeBuilder_h.template is using Jinja assignments incorrectly. I believe the fix looks like this (line numbers are from the version of inspector_protocol in V8 6.0.186):

@@ -110,12 +110,8 @@ public:
     public:
         enum {
             NoFieldsSet = 0,
-    {% set count = 0 %}
-    {% for property in type.properties %}
-      {% if not(property.optional) %}
-        {% set count = count + 1 %}
-          {{property.name | to_title_case}}Set = 1 << {{count}},
-      {% endif %}
+    {% for property in type.properties|rejectattr("optional") %}
+      {{property.name | to_title_case}}Set = 1 << {{loop.index}},
     {% endfor %}
             AllFieldsSet = (
     {%- for property in type.properties %}

The Jinja documentation (at least for 2.9) suggests that this code was always buggy:

Scoping Behavior

Please keep in mind that it is not possible to set variables inside a block and have them show up outside of it. This also applies to loops. The only exception to that rule are if statements which do not introduce a scope. As a result the following template is not going to do what you might expect:

{% set iterated = false %}
{% for item in seq %}
    {{ item }}
    {% set iterated = true %}
{% endfor %}
{% if not iterated %} did not iterate {% endif %}

It is not possible with Jinja syntax to do this. Instead use alternative constructs like the loop else block or the special loop variable:

{% for item in seq %}
    {{ item }}
{% else %}
    did not iterate
{% endfor %}

Sorry for the cursory bug report. It's late on a Friday...