Warn about duplicated :tag strings in defcustom choices

It is bad user experience when two menu items have identical labels.

* lisp/emacs-lisp/bytecomp.el (bytecomp--check-cus-type): Add check.
* test/lisp/emacs-lisp/bytecomp-tests.el
(bytecomp-test-defcustom-type): Add test case.

2 files changed, 12 insertions, 2 deletions

diff --git a/lisp/emacs-lisp/bytecomp.el b/lisp/emacs-lisp/bytecomp.el
index 1474acc1638..387d7ef4de1 100644
--- a/lisp/emacs-lisp/bytecomp.el
+++ b/lisp/emacs-lisp/bytecomp.el
@@ -5272,7 +5272,8 @@ FORM is used to provide location, `bytecomp--cus-function' and
           (unless tail
             (bytecomp--cus-warn type "`%s' without any types inside" head))
           (let ((clauses tail)
-                (constants nil))
+                (constants nil)
+                (tags nil))
             (while clauses
               (let* ((ty (car clauses))
                      (ty-head (car-safe ty)))
@@ -5291,6 +5292,12 @@ FORM is used to provide location, `bytecomp--cus-function' and
                       (bytecomp--cus-warn
                        ty "duplicated value in `%s': `%S'" head val))
                     (push val constants)))
+                (let ((tag (and (consp ty) (plist-get (cdr ty) :tag))))
+                  (when (stringp tag)
+                    (when (member tag tags)
+                      (bytecomp--cus-warn
+                       ty "duplicated :tag string in `%s': %S" head tag))
+                    (push tag tags)))
                 (bytecomp--check-cus-type ty))
               (setq clauses (cdr clauses)))))
          ((eq head 'cons)
diff --git a/test/lisp/emacs-lisp/bytecomp-tests.el b/test/lisp/emacs-lisp/bytecomp-tests.el
index a335a7fa1f8..e644417c3d4 100644
--- a/test/lisp/emacs-lisp/bytecomp-tests.el
+++ b/test/lisp/emacs-lisp/bytecomp-tests.el
@@ -1875,7 +1875,7 @@ EXPECTED-POINT BINDINGS (MODES \\='\\='(ruby-mode js-mode python-mode)) \
 (FIXTURE-FN \\='#\\='electric-pair-mode))" fill-column)))
 
 (ert-deftest bytecomp-test-defcustom-type ()
-  (cl-flet ((dc (type) `(defcustom mytest nil "doc" :type ',type)))
+  (cl-flet ((dc (type) `(defcustom mytest nil "doc" :type ',type :group 'test)))
     (bytecomp--with-warning-test
      (rx "type should not be quoted") (dc ''integer))
     (bytecomp--with-warning-test
@@ -1891,6 +1891,9 @@ EXPECTED-POINT BINDINGS (MODES \\='\\='(ruby-mode js-mode python-mode)) \
      (rx "duplicated value in `choice': `a'")
      (dc '(choice (const a) (const b) (const a))))
     (bytecomp--with-warning-test
+     (rx "duplicated :tag string in `choice': \"X\"")
+     (dc '(choice (const :tag "X" a) (const :tag "Y" b) (other :tag "X" c))))
+    (bytecomp--with-warning-test
      (rx "`cons' requires 2 type specs, found 1")
      (dc '(cons :tag "a" integer)))
     (bytecomp--with-warning-test