transform.toMath, invalid syntax: incorrect position in source file printed out

[deining]

As discussed in this thread on hugo's discourse board:

I'm using this passthrough render hook to render equations using function transform.toMath:

{{- with try (transform.ToMath .Inner ) }}
  {{- with .Err }}
    {{ errorf "KaTeX: %s: see %s." . $.Position }}
  {{- else }}
    {{- .Value }}
  {{- end }}
{{- end -}}

With an invalid syntax given for the TeX content to rendered, I'm getting:

ERROR KaTeX: template: _default/_markup/render-passthrough.html:1:23:
executing "_default/_markup/render-passthrough.html" at <transform.ToMath>:
error calling ToMath: KaTeX parse error: Undefined control sequence:
\foo at position 1: \̲f̲o̲o̲:
see "/path/to/site/content/lang/section/subsection/index.md:0:1".

Bug

The position given in the last line is always 0:1, regardless of the actual position of the offending code in the source file.

Test of render hooks

As requested in the discourse thread, I tested all available render hooks, this is the result:

Render hook	Context has Position	Position correct
Block quote	yes	yes
Code blocks	yes	yes
Headings	no	-
Images	no	-
Links	no	-
Pass through	yes	no
Table	yes	no

What version of Hugo are you using (hugo version)?

$ hugo version
hugo v0.144.0-DEV-9c2f8ec61beb9521fb606848de2d14aa180facd7+extended linux/amd64

Does this issue reproduce with the latest release?

Yes.