oc: Allow inserting citations in captions

* lisp/oc.el (org-cite--allowed-p): Allow inserting citations in captions.
* testing/lisp/test-oc.el (test-org-cite/insert-capability): Add test.

lisp/oc.el

@@ -58,6 +58,7 @@
 (require 'org-compat)
 (require 'org-macs)
 
+(declare-function org-at-heading-p "org" (&optional _))
 (declare-function org-collect-keywords "org" (keywords &optional unique directory))
 
 (declare-function org-element-adopt-elements "org-element" (parent &rest children))
@@ -73,6 +74,7 @@
 (declare-function org-element-normalize-string "org-element" (s))
 (declare-function org-element-parse-buffer "org-element" (&optional granularity visible-only))
 (declare-function org-element-parse-secondary-string "org-element" (string restriction &optional parent))
+(declare-function org-element-context "org-element" (&optional element))
 (declare-function org-element-property "org-element" (property element))
 (declare-function org-element-put-property "org-element" (element property value))
 (declare-function org-element-restriction "org-element" (element))
@@ -85,9 +87,11 @@
 (declare-function org-export-get-previous-element "org-export" (blob info &optional n))
 (declare-function org-export-raw-string "org-export" (s))
 
+(defvar org-complex-heading-regexp)
 (defvar org-element-all-objects)
 (defvar org-element-citation-key-re)
 (defvar org-element-citation-prefix-re)
+(defvar org-element-parsed-keywords)
 
 
 ;;; Constants
@@ -1363,12 +1367,24 @@ ARG is the prefix argument received when calling `org-open-at-point', or nil."
 ;;; Meta-command for citation insertion (insert capability)
 (defun org-cite--allowed-p (context)
   "Non-nil when a citation can be inserted at point."
-  (let* ((type (org-element-type context)))
+  (let ((type (org-element-type context)))
     (cond
-     ;; No citation in attributes.
+     ;; No citation in attributes, except in parsed ones.
+     ;;
+     ;; XXX: Inserting citation in a secondary value is not allowed
+     ;; yet.  Is it useful?
      ((let ((post (org-element-property :post-affiliated context)))
 	(and post (< (point) post)))
-      nil)
+      (let ((case-fold-search t))
+        (looking-back
+         (rx-to-string
+          `(seq line-start (0+ (any " \t"))
+                "#+"
+                (or ,@org-element-parsed-keywords)
+                ":"
+                (0+ nonl))
+          t)
+         (line-beginning-position))))
      ;; Paragraphs and blank lines at top of document are fine.
      ((memq type '(nil paragraph)))
      ;; So are contents of verse blocks.

testing/lisp/test-oc.el

@@ -1768,7 +1768,17 @@ arguments.  Replace citation with \"@\" character in the output."
            (org-cite-insert-processor 'foo))
        (org-cite-register-processor 'foo
          :insert (lambda (_ _) (throw :exit 'success)))
-       (call-interactively #'org-cite-insert)))))
+       (call-interactively #'org-cite-insert))))
+  ;; Allow inserting citations in captions.
+  (should
+   (eq 'success
+       (catch :exit
+         (org-test-with-temp-text "#+caption: <point>\n| table |"
+           (let ((org-cite--processors nil)
+                 (org-cite-insert-processor 'foo))
+             (org-cite-register-processor 'foo
+               :insert (lambda (_ _) (throw :exit 'success)))
+             (call-interactively #'org-cite-insert)))))))
 
 (provide 'test-oc)
 ;;; test-oc.el ends here