Use function-valued variable for ERC query predicate

* lisp/erc/erc-notify.el (erc-querypoll-mode, erc-querypoll-enable)
(erc-querypoll-disable): Override `erc-query-table-synced-predicate'
when mode is active.
(erc--queries-current-p): Remove method.
(erc--querypoll-active-p): New function.
* lisp/erc/erc-speedbar.el (erc-speedbar-buttons)
(erc-speedbar-insert-target): Call function-valued variable
`erc--query-table-synced-predicate' instead of its now default value
`erc--queries-current-p' directly.
* lisp/erc/erc.el (erc--query-table-synced-predicate): New variable.
(erc--queries-current-p, erc--query-participant-present-p): Convert
former from method to normal function and rename to latter.  Original
was added as part of bug#70928.

lisp/erc/erc-notify.el

@@ -300,7 +300,9 @@ Once ERC implements the `monitor' extension, this module will serve as
 an optional fallback for keeping query-participant rolls up to date on
 servers that lack support or are stingy with their allotments.  Until
 such time, this module should be considered experimental and only really
-useful for bots and other non-interactive Lisp programs.
+useful for bots and other non-interactive Lisp programs.  Please note
+that reporting is unreliable for short periods while a query participant
+is parting, joining, quitting, or logging in.
 
 This is a local ERC module, so selectively polling only a subset of
 query targets is possible but cumbersome.  To do so, ensure
@@ -316,6 +318,8 @@ at least the server buffer."
              (erc-with-server-buffer
                (unless erc-querypoll-mode
                  (erc-querypoll-mode +1)))
+             (add-function :override (local 'erc--query-table-synced-predicate)
+                           #'erc--querypoll-active-p)
              (erc--querypoll-subscribe (current-buffer)))
          (erc-querypoll-mode -1))
      (cl-assert (not erc--decouple-query-and-channel-membership-p))
@@ -331,6 +335,8 @@ at least the server buffer."
                    (index (ring-member ring (current-buffer)))
                    ((not (erc--querypoll-target-in-chan-p (current-buffer)))))
          (ring-remove ring index)
+         (remove-function (local 'erc--query-table-synced-predicate)
+                          #'erc--querypoll-active-p)
          (unless (erc-current-nick-p (erc-target))
            (erc-remove-current-channel-member (erc-target))))
      (erc-with-all-buffers-of-server erc-server-process #'erc-query-buffer-p
@@ -339,7 +345,9 @@ at least the server buffer."
    (kill-local-variable 'erc--querypoll-timer))
   localp)
 
-(cl-defmethod erc--queries-current-p (&context (erc-querypoll-mode (eql t))) t)
+(defun erc--querypoll-active-p ()
+  "Return non-nil if `erc-querypoll-mode' is active in the current buffer."
+  erc-querypoll-mode)
 
 (defvar erc-querypoll-period-params '(10 10 1)
   "Parameters affecting the delay with respect to the number of buffers.

lisp/erc/erc-speedbar.el

@@ -146,7 +146,7 @@ This will add a speedbar major display mode."
       (setq serverp (erc--server-buffer-p))
       (setq chanp (erc-channel-p (erc-default-target)))
       (setq queryp (erc-query-buffer-p)
-            queries-current-p (erc--queries-current-p)))
+            queries-current-p (funcall erc--query-table-synced-predicate)))
     (defvar erc-nickbar-mode)
     (cond ((and erc-nickbar-mode (null (get-buffer-window speedbar-buffer)))
            (run-at-time 0 nil #'erc-nickbar-mode -1))
@@ -207,7 +207,8 @@ This will add a speedbar major display mode."
 
 (defun erc-speedbar-insert-target (buffer depth)
   (if (with-current-buffer buffer
-        (or (erc--target-channel-p erc--target) (erc--queries-current-p)))
+        (or (erc--target-channel-p erc--target)
+            (funcall erc--query-table-synced-predicate)))
       (progn
         (speedbar-make-tag-line
          'bracket ?+ 'erc-speedbar-expand-channel buffer

lisp/erc/erc.el

@@ -559,8 +559,14 @@ user from `erc-server-users'.  Note that enabling this compatibility
 flag degrades the user experience and isn't guaranteed to correctly
 restore the described historical behavior.")
 
-(cl-defmethod erc--queries-current-p ()
-  "Return non-nil if ERC actively updates query manifests."
+(defvar erc--query-table-synced-predicate #'erc--query-participant-present-p
+  "Predicate for whether a query buffer's member table dynamically updates.
+By default, ERC flies half blind by managing membership based on shared
+channels.  This rules out false positives but accepts the chance of
+participants being on the server but absent from local tables.")
+
+(defun erc--query-participant-present-p ()
+  "Return non-nil if the query participant is present in the member table."
   (and (not erc--decouple-query-and-channel-membership-p)
        (erc-query-buffer-p) (erc-get-channel-member (erc-target))))