Use modern fallback for channel name detection in ERC

* lisp/erc/erc-backend.el (erc-query-buffer-p): Remove forward declaration.
* lisp/erc/erc.el (erc-query-buffer-p): Defer to `erc-channel-p'.
(erc-channel-p): Refactor and use `erc--fallback-channel-prefixes' for
the default CHANTYPES value.  Honor an empty CHANTYPES value as valid,
e.g., for servers that only support direct messages.
(erc--fallback-channel-prefixes): New variable to hold fallback
CHANTYPES prefixes recommended by RFC1459 and modern authorities on
the matter.
* test/lisp/erc/erc-tests.el (erc-channel-p): Revise test.  (Bug#67220)

lisp/erc/erc-backend.el

@@ -158,7 +158,6 @@
 (declare-function erc-parse-user "erc" (string))
 (declare-function erc-process-away "erc" (proc away-p))
 (declare-function erc-process-ctcp-query "erc" (proc parsed nick login host))
-(declare-function erc-query-buffer-p "erc" (&optional buffer))
 (declare-function erc-remove-channel-member "erc" (channel nick))
 (declare-function erc-remove-channel-users "erc" nil)
 (declare-function erc-remove-user "erc" (nick))

lisp/erc/erc.el

@@ -1663,11 +1663,7 @@ If BUFFER is nil, the current buffer is used."
 (defun erc-query-buffer-p (&optional buffer)
   "Return non-nil if BUFFER is an ERC query buffer.
 If BUFFER is nil, the current buffer is used."
-  (with-current-buffer (or buffer (current-buffer))
+  (not (erc-channel-p (or buffer (current-buffer)))))
-    (let ((target (erc-target)))
-      (and (eq major-mode 'erc-mode)
-           target
-           (not (memq (aref target 0) '(?# ?& ?+ ?!)))))))
 
 (defun erc-ison-p (nick)
   "Return non-nil if NICK is online."
@@ -1882,18 +1878,20 @@ buries those."
   :group 'erc-buffers
   :type 'boolean)
 
-(defun erc-channel-p (channel)
+(defvar erc--fallback-channel-prefixes "#&"
-  "Return non-nil if CHANNEL seems to be an IRC channel name."
+  "Prefix chars for distinguishing channel targets when CHANTYPES is unknown.")
-  (cond ((stringp channel)
+
-         (memq (aref channel 0)
+(defun erc-channel-p (target)
-               (if-let ((types (erc--get-isupport-entry 'CHANTYPES 'single)))
+  "Return non-nil if TARGET is a valid channel name or a channel buffer."
-                   (append types nil)
+  (cond ((stringp target)
-                 '(?# ?& ?+ ?!))))
+         (and-let*
-        ((and-let* (((bufferp channel))
+             (((not (string-empty-p target)))
-                    ((buffer-live-p channel))
+              (value (let ((entry (erc--get-isupport-entry 'CHANTYPES)))
-                    (target (buffer-local-value 'erc--target channel)))
+                       (if entry (cadr entry) erc--fallback-channel-prefixes)))
-           (erc-channel-p (erc--target-string target))))
+              ((erc--strpos (aref target 0) value)))))
-        (t nil)))
+        ((and-let* (((buffer-live-p target))
+                    (target (buffer-local-value 'erc--target target))
+                    ((erc--target-channel-p target)))))))
 
 ;; For the sake of compatibility, a historical quirk concerning this
 ;; option, when nil, has been preserved: all buffers are suffixed with

test/lisp/erc/erc-tests.el

@@ -1167,25 +1167,37 @@
       (should (equal (erc-downcase "\\O/") "|o/" )))))
 
 (ert-deftest erc-channel-p ()
-  (let ((erc--isupport-params (make-hash-table))
+  (erc-tests-common-make-server-buf)
-        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 (erc-channel-p "&chan"))
-    (should (erc-channel-p "+chan"))
+  (should-not (erc-channel-p "+chan"))
-    (should (erc-channel-p "!chan"))
+  (should-not (erc-channel-p "!chan"))
-    (should-not (erc-channel-p "@chan"))
+  (should-not (erc-channel-p "@chan"))
 
-    (push '("CHANTYPES" . "#&@+!") erc-server-parameters)
+  ;; Server sends "CHANTYPES=#&+!"
+  (should-not erc-server-parameters)
+  (setq erc-server-parameters '(("CHANTYPES" . "#&+!")))
+  (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"))
+  (with-current-buffer (erc--open-target "#chan")
-    (should (erc-channel-p "#chan"))
+    (should (erc-channel-p (current-buffer))))
+  (with-current-buffer (erc--open-target "+chan")
+    (should (erc-channel-p (current-buffer))))
+  (should (erc-channel-p (get-buffer "#chan")))
+  (should (erc-channel-p (get-buffer "+chan")))
 
-    (with-current-buffer (get-buffer-create "#chan")
+  ;; Server sends "CHANTYPES=" because it's query only.
-      (setq erc--target (erc--target-from-string "#chan")))
+  (puthash 'CHANTYPES '("CHANTYPES") erc--isupport-params)
-    (should (erc-channel-p (get-buffer "#chan"))))
+  (should-not (erc-channel-p "#spam"))
-  (kill-buffer "#chan"))
+  (should-not (erc-channel-p "&spam"))
+  (should-not (erc-channel-p (save-excursion (erc--open-target "#spam"))))
+
+  (erc-tests-common-kill-buffers))
 
 (ert-deftest erc--valid-local-channel-p ()
   (ert-info ("Local channels not supported")