Make ERC's format catalogs more extensible

* lisp/erc/erc-common.el (erc--define-catalog): Accept a `:parent'
keyword to allow for extending an existing catalog by overriding some
subset of defined entries.
(erc-define-message-format-catalog): Add edebug spec.
* lisp/erc/erc.el (erc-retrieve-catalog-entry): Check parent for
definition before looking to `default-toplevel-value'.
* test/lisp/erc/erc-tests.el (erc-retrieve-catalog-entry): Add test
case for inheritance.
* test/lisp/erc/resources/erc-tests-common.el
(erc-tests-common-pp-propertized-parts): Fix bug in convenience
command.  (Bug#67677)

2 files changed, 21 insertions, 2 deletions

diff --git a/lisp/erc/erc-common.el b/lisp/erc/erc-common.el
index b8ba0673355..2581e40f850 100644
--- a/lisp/erc/erc-common.el
+++ b/lisp/erc/erc-common.el
@@ -554,9 +554,21 @@ See `erc-define-message-format-catalog' for the meaning of
 ENTRIES, an alist, and `erc-tests-common-pp-propertized-parts' in
 tests/lisp/erc/erc-tests.el for a convenience command to convert
 a literal string into a sequence of `propertize' forms, which are
-much easier to review and edit."
+much easier to review and edit.  When ENTRIES begins with a
+sequence of keyword-value pairs remove them and consider their
+evaluated values before processing the alist proper.
+
+Currently, the only recognized keyword is `:parent', which tells
+ERC to search recursively for a given template key using the
+keyword's associated value, another catalog symbol, if not found
+in catalog NAME."
   (declare (indent 1))
   (let (out)
+    (while (keywordp (car entries))
+      (push (pcase-exhaustive (pop entries)
+              (:parent `(put ',name 'erc--base-format-catalog
+                             ,(pop entries))))
+            out))
     (dolist (e entries (cons 'progn (nreverse out)))
       (push `(defvar ,(intern (format "erc-message-%s-%s" name (car e)))
                ,(cdr e)
@@ -575,7 +587,8 @@ symbol, and FORMAT evaluates to a format string compatible with
 `format-spec'.  Expect modules that only define a handful of
 entries to do so manually, instead of using this macro, so that
 the resulting variables will end up with more useful doc strings."
-  (declare (indent 1))
+  (declare (indent 1)
+           (debug (symbolp [&rest [keywordp form]] &rest (symbolp . form))))
   `(erc--define-catalog ,language ,entries))
 
 (defmacro erc--doarray (spec &rest body)
diff --git a/lisp/erc/erc.el b/lisp/erc/erc.el
index d0c43134f9d..478683a77f5 100644
--- a/lisp/erc/erc.el
+++ b/lisp/erc/erc.el
@@ -9320,6 +9320,12 @@ if yet untried."
   (unless catalog (setq catalog erc-current-message-catalog))
   (symbol-value
    (or (erc--make-message-variable-name catalog key 'softp)
+       (let ((parent catalog)
+             last)
+         (while (and (setq parent (get parent 'erc--base-format-catalog))
+                     (not (setq last (erc--make-message-variable-name
+                                      parent key 'softp)))))
+         last)
        (let ((default (default-toplevel-value 'erc-current-message-catalog)))
          (or (and (not (eq default catalog))
                   (erc--make-message-variable-name default key 'softp))