That patch seems to do the trick for me. I don't make particularly
extensive use of the flymake diagnostics buffer, but for regular use it
seems to work as expected.

Thanks,
Aidan

On Sun, Apr 12, 2020 at 1:15 PM Eli Zaretskii <eliz@gnu.org> wrote:

> > 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"