Update ISUPPORT handling in ERC

* lisp/erc/erc-backend (erc--isupport-params): Add new variable to
hold a hashmap of parsed `erc-server-parameters' in a more useful
format.  But keep `erc-server-parameters' around for public use.  We
currently lack dedicated local variables for certain discovered IRC
session properties, such as what prefix characters are supported for
channels, etc.  And the truth of this needs querying many times per
second at various points.  As such, caching here seems justified but
can be easily removed if deemed otherwise because all ingredients are
internal.
(erc--parse-isupport-value): Add helper function that parses an
ISUPPORT value and returns the component parts with backslash-x hex
escapes removed.  This can probably use some streamlining.
(erc--with-memoization): Add compat alias for use in internal ISUPPORT
getter.  Should be moved to `erc-compat.el' when that library is fully
reincorporated.
(erc--get-isupport-entry): Add internal getter to look up ISUPPORT
items.
(erc-server-005): Treat `erc-server-response' "command args" field as
read-only.  Previously, this field was set to nil after processing,
which was unhelpful to other parts of the library.  Also call above
mentioned helper to parse values.  And add some bookkeeping to handle
negation.

* lisp/erc/erc-capab.el (erc-capab-identify-send-messages): Use
internal ISUPPORT getter.

* lisp/erc/erc.el (erc-cmd-NICK, erc-parse-prefix,
erc-nickname-in-use): Use internal ISUPPORT getter.

* test/lisp/erc/erc-tests.el: Add tests for the above mentioned
changes in erc-backend.el.

lisp/erc/erc-backend.el

@@ -185,6 +185,11 @@ SILENCE=10 - supports the SILENCE command, maximum allowed number of entries
 TOPICLEN=160 - maximum allowed topic length
 WALLCHOPS - supports sending messages to all operators in a channel")
 
+(defvar-local erc--isupport-params nil
+  "Hash map of \"ISUPPORT\" params.
+Keys are symbols.  Values are lists of zero or more strings with hex
+escapes removed.")
+
 ;;; Server and connection state
 
 (defvar erc-server-ping-timer-alist nil
@@ -1625,6 +1630,67 @@ Then display the welcome message."
      ?U (nth 3 (erc-response.command-args parsed))
      ?C (nth 4 (erc-response.command-args parsed)))))
 
+(defun erc--parse-isupport-value (value)
+  "Return list of unescaped components from an \"ISUPPORT\" VALUE."
+  ;; https://tools.ietf.org/html/draft-brocklesby-irc-isupport-03#section-2
+  ;;
+  ;; > The server SHOULD send "X", not "X="; this is the normalised form.
+  ;;
+  ;; Note: for now, assume the server will only send non-empty values,
+  ;; possibly with printable ASCII escapes.  Though in practice, the
+  ;; only two escapes we're likely to see are backslash and space,
+  ;; meaning the pattern is too liberal.
+  (let (case-fold-search)
+    (mapcar
+     (lambda (v)
+       (let ((start 0)
+             m
+             c)
+         (while (and (< start (length v))
+                     (string-match "[\\]x[0-9A-F][0-9A-F]" v start))
+           (setq m (substring v (+ 2 (match-beginning 0)) (match-end 0))
+                 c (string-to-number m 16))
+           (if (<= ?\  c ?~)
+               (setq v (concat (substring v 0 (match-beginning 0))
+                               (string c)
+                               (substring v (match-end 0)))
+                     start (- (match-end 0) 3))
+             (setq start (match-end 0))))
+         v))
+     (if (if (>= emacs-major-version 28)
+             (string-search "," value)
+           (string-match-p "," value))
+         (split-string value ",")
+       (list value)))))
+
+;; FIXME move to erc-compat (once we decide how to load it)
+(defalias 'erc--with-memoization
+  (cond
+   ((fboundp 'with-memoization) #'with-memoization) ; 29.1
+   ((fboundp 'cl--generic-with-memoization) #'cl--generic-with-memoization)
+   (t (lambda (_ v) v))))
+
+(defun erc--get-isupport-entry (key &optional single)
+  "Return an item for \"ISUPPORT\" token KEY, a symbol.
+When a lookup fails return nil.  Otherwise return a list whose
+CAR is KEY and whose CDR is zero or more strings.  With SINGLE,
+just return the first value, if any.  The latter is potentially
+ambiguous and only useful for tokens supporting a single
+primitive value."
+  (if-let* ((table (or erc--isupport-params
+                       (erc-with-server-buffer erc--isupport-params)))
+            (value (erc--with-memoization (gethash key table)
+                     (when-let ((v (assoc (symbol-name key)
+                                          erc-server-parameters)))
+                       (if (cdr v)
+                           (erc--parse-isupport-value (cdr v))
+                         '--empty--)))))
+      (pcase value
+        ('--empty-- (unless single (list key)))
+        (`(,head . ,_) (if single head (cons key value))))
+    (when table
+      (remhash key table))))
+
 (define-erc-response-handler (005)
   "Set the variable `erc-server-parameters' and display the received message.
 
@@ -1636,21 +1702,25 @@ certain commands are accepted and more.  See documentation for
 
 A server may send more than one 005 message."
   nil
-  (let ((line (mapconcat #'identity
-                         (setf (erc-response.command-args parsed)
-                               (cdr (erc-response.command-args parsed)))
-                         " ")))
-    (while (erc-response.command-args parsed)
-      (let ((section (pop (erc-response.command-args parsed))))
-        ;; fill erc-server-parameters
-        (when (string-match "^\\([A-Z]+\\)=\\(.*\\)$\\|^\\([A-Z]+\\)$"
+  (unless erc--isupport-params
+    (setq erc--isupport-params (make-hash-table)))
+  (let* ((args (cdr (erc-response.command-args parsed)))
+         (line (string-join args " ")))
+    (while args
+      (let ((section (pop args))
+            key
+            value
+            negated)
+        (when (string-match "^\\([A-Z]+\\)=\\(.*\\)$\\|^\\(-\\)?\\([A-Z]+\\)$"
                             section)
-          (add-to-list 'erc-server-parameters
-                       `(,(or (match-string 1 section)
-                              (match-string 3 section))
-                         .
-                         ,(match-string 2 section))))))
-    (erc-display-message parsed 'notice proc line)))
+          (setq key (or (match-string 1 section) (match-string 4 section))
+                value (match-string 2 section)
+                negated (and (match-string 3 section) '-))
+          (setf (alist-get key erc-server-parameters '- 'remove #'equal)
+                (or value negated))
+          (remhash (intern key) erc--isupport-params))))
+    (erc-display-message parsed 'notice proc line)
+    nil))
 
 (define-erc-response-handler (221)
   "Display the current user modes." nil