Honor arbitrary CHANTYPES in ERC

* lisp/erc/erc.el (erc-channel-p): Favor "CHANTYPES" ISUPPORT item
before falling back to well known prefixes.
* test/lisp/erc/erc-tests.el (erc-channel-p): Add test.  Arbitrarily
bundled with bug#60954.

2 files changed, 29 insertions, 4 deletions

diff --git a/lisp/erc/erc.el b/lisp/erc/erc.el
index 6d35a62518d..ef51f100f8b 100644
--- a/lisp/erc/erc.el
+++ b/lisp/erc/erc.el
@@ -1549,10 +1549,14 @@ effect when `erc-join-buffer' is set to `frame'."
 (defun erc-channel-p (channel)
   "Return non-nil if CHANNEL seems to be an IRC channel name."
   (cond ((stringp channel)
-         (memq (aref channel 0) '(?# ?& ?+ ?!)))
-        ((and (bufferp channel) (buffer-live-p channel))
-         (with-current-buffer channel
-           (erc-channel-p (erc-default-target))))
+         (memq (aref channel 0)
+               (if-let ((types (erc--get-isupport-entry 'CHANTYPES 'single)))
+                   (append types nil)
+                 '(?# ?& ?+ ?!))))
+        ((and-let* (((bufferp channel))
+                    ((buffer-live-p channel))
+                    (target (buffer-local-value 'erc--target channel)))
+           (erc-channel-p (erc--target-string target))))
         (t nil)))
 
 ;; For the sake of compatibility, a historical quirk concerning this
diff --git a/test/lisp/erc/erc-tests.el b/test/lisp/erc/erc-tests.el
index d6c63934163..bbf3269161d 100644
--- a/test/lisp/erc/erc-tests.el
+++ b/test/lisp/erc/erc-tests.el
@@ -447,6 +447,27 @@
       (should (equal (erc-downcase "Tilde~") "tilde~" ))
       (should (equal (erc-downcase "\\O/") "|o/" )))))
 
+(ert-deftest erc-channel-p ()
+  (let ((erc--isupport-params (make-hash-table))
+        erc-server-parameters)
+
+    (should (erc-channel-p "#chan"))
+    (should (erc-channel-p "##chan"))
+    (should (erc-channel-p "&chan"))
+    (should (erc-channel-p "+chan"))
+    (should (erc-channel-p "!chan"))
+    (should-not (erc-channel-p "@chan"))
+
+    (push '("CHANTYPES" . "#&@+!") erc-server-parameters)
+
+    (should (erc-channel-p "!chan"))
+    (should (erc-channel-p "#chan"))
+
+    (with-current-buffer (get-buffer-create "#chan")
+      (setq erc--target (erc--target-from-string "#chan")))
+    (should (erc-channel-p (get-buffer "#chan"))))
+  (kill-buffer "#chan"))
+
 (ert-deftest erc--valid-local-channel-p ()
   (ert-info ("Local channels not supported")
     (let ((erc--isupport-params (make-hash-table)))