comment #1:

> I'll file a separate bug for the incorrect diagnosis

If this was ever done, I couldn't find it, so I just opened bug #64439 for this.

A bracket-delimited escape used in an argument to a string must be quoted.  Commit 17ee46a8, part of which was motivated by http://lists.gnu.org/r/groff/2022-11/msg00185.html, documented this clearly.  But a user who forgets this requirement is met with a cryptic diagnostic.

$ cat string_test
.ds str hello \\$1
\*[str \[aq]]
$ groff string_test > /dev/null
troff:string_test:2: error: a newline character is not allowed in an escape sequence parameter

In the email thread (which continues briefly at http://lists.gnu.org/r/groff/2022-12/msg00001.html, which is not linked to the rest of the thread due to the archival software's limitations), Branden explains how the parser processes this input, and surmises that at the point it realizes it's in trouble, it may not be a in a good position to explain why.  But he admits the matter requires further investigation, hence this low-priority ticket to put the issue on the radar after more pressing matters have been resolved.