bug#56201: Backtrace fails to appear in an error situation.

[Alan Mackenzie]

Hello, Emacs.

(i) With the master branch, start Emacs (-Q not needed). 
(ii) Visit a non-empty file in a buffer.
(iii) M-s o <regexp that will generate matches>.
(iv) C-x o to move to the *Occur* buffer.
(v) Move point to somewhere in the displayed matches.
(vi) C-x C-q to swith off read-onliness.
(vii) C-j, an attempt to insert a newline.

This produces the error message "Text is read-only".  This is probably a
bug, but is minor compared to what follows:

(viii) M-: (setq debug-on-error t) RET.
(ix) C-j.

This ought to produce a backtrace, but doesn't.  It merely repeats the
error message "Text is read-only".  This is a major bug; how can one
solve minor bugs if attempts to get diagnostic information are blocked?

It would appear a frivolous condition-case is discarding the backtrace
here.  This appears to happen with lots of error messages.  This is not
a Good Thing.

-- 
Alan Mackenzie (Nuremberg, Germany).