bug#32795: Acknowledgement (26.1; provided-mode-derived-p does not support parent modes set with defalias)

[Andrew Schwartzmeyer <andrew@schwartzmeyer.com>]

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Patch with tests attached. Please let me know if I missed anything, or 
if the code could be improved. I read through CONTRIBUTE and may have 
accidentally sent this patch in as a new bug report, sorry!

This was generated from 'emacs-26' where the bug occurs, and will merge 
just fine into 'master'. The bug also exists on 'emacs-25' but the code 
is slightly different ('provided-mode-derived-p' had not been extracted 
yet from 'derived-mode-p'). I can supply a back-ported patch as well.

Thanks,

Andy

On 09/21/2018 8:22 am, help-debbugs@gnu.org wrote:
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> 
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From f39d7f2b5e92e085927918cf4f9da4ba8df8f366 Mon Sep 17 00:00:00 2001
From: Andrew Schwartzmeyer <andrew@schwartzmeyer.com>
Date: Fri, 21 Sep 2018 00:10:47 -0700
Subject: [PATCH] Fix 'provided-mode-derived-p' when the parent is an alias

Authors of Emacs packages often derive from an alias of a mode instead
of the mode directly.  This is especially the case when deriving from
'prog-mode' as it is relatively new.  Unfortunately, using
'derived-mode-p' to check if some mode is derived from an alias of
'prog-mode' does not work as expected.  The bug is that
'provided-mode-derived-p' should not only return non-nil when MODE is
one of MODES, but also when MODE is an alias of one of MODES.
* lisp/subr.el (provided-mode-derived-p):
Return non-nil when MODE is an alias of any of MODES (Bug#32795).
* test/lisp/subr-tests.el: Add tests for the above.
---
 lisp/subr.el            |  9 +++++++--
 test/lisp/subr-tests.el | 12 ++++++++++++
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/lisp/subr.el b/lisp/subr.el
index 7582b6cdb..6c625b40d 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -1919,9 +1919,14 @@ delay-mode-hooks
 (defun provided-mode-derived-p (mode &rest modes)
   "Non-nil if MODE is derived from one of MODES.
 Uses the `derived-mode-parent' property of the symbol to trace backwards.
+Non-nil even if MODE is derived from an alias of any of MODES.
 If you just want to check `major-mode', use `derived-mode-p'."
-  (while (and (not (memq mode modes))
-              (setq mode (get mode 'derived-mode-parent))))
+  (while
+      (and
+       (not (memq mode modes))
+       (let* ((parent (get mode 'derived-mode-parent))
+              (parentfn (symbol-function parent)))
+         (setq mode (if (and parentfn (symbolp parentfn)) parentfn parent)))))
   mode)
 
 (defun derived-mode-p (&rest modes)
diff --git a/test/lisp/subr-tests.el b/test/lisp/subr-tests.el
index 430d71903..a9f72c26d 100644
--- a/test/lisp/subr-tests.el
+++ b/test/lisp/subr-tests.el
@@ -62,6 +62,18 @@
                      (quote
                       (0 font-lock-keyword-face))))))))
 
+(ert-deftest provided-mode-derived-p ()
+  ;; base case: `derived-mode' directly derives `prog-mode'
+  (should (progn
+            (define-derived-mode derived-mode prog-mode "test")
+            (provided-mode-derived-p 'derived-mode 'prog-mode)))
+  ;; edge case: `derived-mode' derives an alias of `prog-mode'
+  (should (progn
+            (defalias 'parent-mode
+              (if (fboundp 'prog-mode) 'prog-mode 'fundamental-mode))
+            (define-derived-mode derived-mode parent-mode "test")
+            (provided-mode-derived-p 'derived-mode 'prog-mode))))
+
 (ert-deftest number-sequence-test ()
   (should (= (length
               (number-sequence (1- most-positive-fixnum) most-positive-fixnum))
-- 
2.19.0