Robert, thanks for your June 6 patch in Bug#31718#14. A small problem: gensym 
does not guarantee that the resulting symbol is unique, so the generated symbol 
could in theory appear in the input which would trigger the bug. Instead, let's 
use a guaranteed-unique object (cons nil nil). Also, I wrote a test case for 
this bug.

I installed the attached patch into the master branch. Given the practical 
consequences of this bug I expect the bug fix should be backported into the 
emacs-26 branch too.