Template error raised in an {% extends %} child template shows incorrect source location on debug page

[Matt Westcott]

If an error occurs in the processing of a template tag on a template that extends another via {% extends %}, the "Error during template rendering" section of the debug page incorrectly shows an excerpt from the parent template rather than the child.

To reproduce:

base.html:

{% block body %}{% endblock %}

home.html:

{% extends "base.html" %}

{% block body %}
    {% include "does_not_exist" %}
{% endblock %}

views.py:

from django.shortcuts import render

def home(request):
    return render(request, 'home.html')

The resulting error page displays:

Error during template rendering
In template /Users/matthew/Development/tbx/wagtail/debug/brokendebug/core/templates/base.html, error at line 0

does_not_exist
1	{% block body %}{% endblock %}
2	

instead of the expected error output:

Error during template rendering
In template /Users/matthew/Development/tbx/wagtail/debug/brokendebug/core/templates/home.html, error at line 4

does_not_exist
1	{% extends "base.html" %}
2	
3	{% block body %}
4	    {% include "does_not_exist" %}
5	{% endblock %}
6	

Bisecting shows that this bug was introduced in #27956 (e643ba8bcf0b1da517cbab689ac157ee031202a3), which was a fix for the opposite case: an error occurring on the parent template. (In the above test case, moving the line {% include "does_not_exist" %} to base.html turns it into a test case for #27956, and fails on pre-e643ba8bcf0b1da517cbab689ac157ee031202a3 revisions as expected.)

[Ankur Jain]

When the parser runs then it appends all the child template nodes to the parent template nodes and hence when template not found error comes then it's reported from the parent html instead of the child html.

[Simon Charette]

#29740 was a duplicate.

[Michael F. Lamb]

Replying to Ankur Jain:
Hi, I'm interested in working on this bug. Are you still working on it, Ankur?

[pascal chambon]

Error confirmed on Django2.2.1, it makes the debug page much less helpful in case of TemplateError, since most websites inherit their templates from some kind of "base.html" skeleton.