GNU bug report logs - #65622
Inappropriate suppression of backtrace on an error

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From: Alan Mackenzie <acm <at> muc.de>
To: 65622 <at> debbugs.gnu.org
Subject: bug#65622: Inappropriate suppression of backtrace on an error
Date: Wed, 30 Aug 2023 13:08:26 +0000

Hello, Emacs.

On a recent master branch Emacs:
(i) emacs -Q
(ii) Insert the following into *scratch*:

(defmacro hash-if (condition then-form &rest else-forms)
  "A conditional compilation macro analogous to C's #if.
Evaluate CONDITION at macro-expansion time.  If it is non-nil,
expand the macro to THEN-FORM.  Otherwise expand it to ELSE-FORMS
enclosed in a `progn' form.  ELSE-FORMS may be empty."
  (declare (indent 2)
           (debug (form sexp &rest sexp)))
  (if (eval condition lexical-binding)
      then-form
    (cons 'progn else-forms)))

(defun foo (bar)
  (hash-if (< emacs-major-version 19)
      (car bar)
    (cons bar bar)))

(iii) Evaluate hash-if by putting point after it and doing C-x C-e.
(iv) Attempt to instrument foo for edebug by putting point inside foo and
  doing C-u C-M-x.  This throws the error: "Ignoring macroexpansion
  error: (void-function edebug-after)".
(v) Set debug-on-error to t with M-: (setq debug-on-error t).
(vi) Repeat (iv).  This throws the same error, without a backtrace.  This
  lack of a backtrace is a bug.
(vii) This backtrace is almost certainly being suppressed by a frivolous
  condition-case, whose main purpose appears to be making debugging more
  difficult.  ;-)
(viii) There would appear to be no justification for "ignoring" the error
  (void-function edebug-after).  Such error should not occur, and needs
  debugging.

-- 
Alan Mackenzie (Nuremberg, Germany).

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