bug#40529: 26.3; global-display-line-numbers-mode and flymake-show-diagnostics-buffer error

[Eli Zaretskii <eliz@gnu.org>]

> From: João Távora <joaotavora@gmail.com>
> Date: Sun, 12 Apr 2020 17:58:16 +0100
> Cc: Aidan Beggs <nadiasggeb001@gmail.com>, 40529@debbugs.gnu.org
> 
> > I'm sure a simple solution for Flymake can be found.  E.g., what about
> > skipping the entire body of flymake--diagnostics-buffer-entries if
> > flymake--diagnostics-buffer-source is nil
> 
> Maybe that works, yes. Feel free to try it and commit it 
> to Emacs 27, I have little time and I'm booted into a machine 
> with no Emacs.

OK.

Aidan, can you try the patch below and see if it solves the problem?
The initial error in the recipe you posted is definitely gone after
applying the patch, but please also try this after that and see that
flymake-show-diagnostics-buffer also works after that, in real-life
use.

Thanks.

diff --git a/lisp/progmodes/flymake.el b/lisp/progmodes/flymake.el
index 25a2152..b37b72e 100644
--- a/lisp/progmodes/flymake.el
+++ b/lisp/progmodes/flymake.el
@@ -1321,35 +1321,36 @@ flymake-goto-diagnostic
    (flymake-show-diagnostic (if (button-type pos) (button-start pos) pos))))
 
 (defun flymake--diagnostics-buffer-entries ()
-  (with-current-buffer flymake--diagnostics-buffer-source
-    (cl-loop for diag in
-             (cl-sort (flymake-diagnostics) #'< :key #'flymake-diagnostic-beg)
-             for (line . col) =
-             (save-excursion
-               (goto-char (flymake--diag-beg diag))
-               (cons (line-number-at-pos)
-                     (- (point)
-                        (line-beginning-position))))
-             for type = (flymake--diag-type diag)
-             collect
-             (list (list :diagnostic diag
-                         :line line
-                         :severity (flymake--lookup-type-property
-                                    type
-                                    'severity (warning-numeric-level :error)))
-                   `[,(format "%s" line)
-                     ,(format "%s" col)
-                     ,(propertize (format "%s"
-                                          (flymake--lookup-type-property
-                                           type 'flymake-type-name type))
-                                  'face (flymake--lookup-type-property
-                                         type 'mode-line-face 'flymake-error))
-                     (,(format "%s" (flymake--diag-text diag))
-                      mouse-face highlight
-                      help-echo "mouse-2: visit this diagnostic"
-                      face nil
-                      action flymake-goto-diagnostic
-                      mouse-action flymake-goto-diagnostic)]))))
+  (when (bufferp flymake--diagnostics-buffer-source)
+    (with-current-buffer flymake--diagnostics-buffer-source
+      (cl-loop for diag in
+               (cl-sort (flymake-diagnostics) #'< :key #'flymake-diagnostic-beg)
+               for (line . col) =
+               (save-excursion
+                 (goto-char (flymake--diag-beg diag))
+                 (cons (line-number-at-pos)
+                       (- (point)
+                          (line-beginning-position))))
+               for type = (flymake--diag-type diag)
+               collect
+               (list (list :diagnostic diag
+                           :line line
+                           :severity (flymake--lookup-type-property
+                                      type
+                                      'severity (warning-numeric-level :error)))
+                     `[,(format "%s" line)
+                       ,(format "%s" col)
+                       ,(propertize (format "%s"
+                                            (flymake--lookup-type-property
+                                             type 'flymake-type-name type))
+                                    'face (flymake--lookup-type-property
+                                           type 'mode-line-face 'flymake-error))
+                       (,(format "%s" (flymake--diag-text diag))
+                        mouse-face highlight
+                        help-echo "mouse-2: visit this diagnostic"
+                        face nil
+                        action flymake-goto-diagnostic
+                        mouse-action flymake-goto-diagnostic)])))))
 
 (define-derived-mode flymake-diagnostics-buffer-mode tabulated-list-mode
   "Flymake diagnostics"