bug#74294: Master: debug-on-error is ineffective whilst requiring a file from the byte compiler.

[Alan Mackenzie <acm@muc.de>]

Hello, Emacs.

In master (any recent version):

Create the following files:

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

2: ~/test-byte-compile-errors-2.el:
#########################################################################
;; -*- lexical-binding:t -*-
(car 'baz)

(provide 'test-byte-compile-errors-2)
#########################################################################

(Note the erroneous (car 'baz) in this file.)

(i) emacs -Q
(ii) M-: (setq debug-on-error t) RET
(iii) M-x byte-compile-file RET ~/test-byte-compile-errors.el RET

(iv) The error message:

    test-byte-compile-errors.el:2:11: Error: Wrong type argument: listp, baz

is displayed.  The debugger is not run.  This is surely a bug.

(Note also, the file name displayed is not that where the error is: See
bug#66912.)

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

Diagnosis
---------

In the function bytecomp--displaying-warnings (bytecomp.el), a blocking
condition-case is set up, except when byte-compile-debug is non-nil.
This prevents the debugger running.  The problem is, the effect of this
condition-case prevails even outside the byte compiler whilst loading a
file with `require'.

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

Proposed Resolution
-------------------

Set up one or more handler-binds around the parts of Fload that evaluate
the contents of the file being loaded.  In the event of an error, the
handler-bind handler would trigger, test debug-on-error, and if it's set
appropriately, call the debugger, then quit.  Otherwise, the handler
would return, and the error handling would eventually reach the
condition-case set up by bytecomp--displaying-warnings.

-- 
Alan Mackenzie (Nuremberg, Germany).