bug#66912: With `require', the byte compiler reports the wrong file for errors.

[Alan Mackenzie <acm@muc.de>]

Hello, Emacs.

When byte compiling file1, and file1 requires file2, should there be an
error in file2, it is reported as being in file1, at the requires line.
This is entirely unhelpful; the reported position should be that of the
error in file2.  Things get even more unhelpful if there is a nesting of
required files, and the error occurs in a deeply nested file.

For an example, create the files ~/test-byte-compile-errors.el:

#########################################################################
;; -*- lexical-binding:t -*-
(require 'test-byte-compile-errors-2 "~/test-byte-compile-errors-2") 
#########################################################################

, and ~/test-byte-compile-errors-2.el:

#########################################################################
;; -*- lexical-binding:t -*-
(defvar foo nil)
(defun bar ()
  (setq foo)) ; <==================
(provide 'test-byte-compile-errors-2)
#########################################################################

..  From an Emacs session, do

    M-x byte-compile-file RET ~/test-byte-compile-errors.el RET

..  This will report the error as

Compiling file /home/acm/test-byte-compile-errors.el at Fri Nov  3 10:14:40 2023
test-byte-compile-errors.el:2:2: Error: Wrong number of arguments: setq, 1

..  This is not the error location.

#########################################################################

Preliminary analysis:

The pertinent error information is discarded by one of two
condition-cases in the macro displaying-byte-compile-warnings in
emacs-lisp/bytecomp.el.

If these condition-case's are disabled (for example by spiking the
enclosing `if' forms) and the necessary defuns recompiled, there instead
appears an error message in the display area.  On setting debug-on-error
to t and repeating the compilation, one gets a backtrace, which whilst
not ideal, is considerably more helpful than the original error message.

This appears to be a fundamental problem with condition-case.  When an
error occurs, the stack gets unwound before the error handlers have a
chance to analyse it.

-- 
Alan Mackenzie (Nuremberg, Germany).