Prioritize underline over subscript inside parenthesis

* lisp/org-element.el (org-element--object-lex): Prioritise underline
parser over subscript.  `org-element-underline-parser' is more strict
compared to `org-element-subscript-parser'.
* testing/lisp/test-org-element.el (test-org-element/underline-parser):
Add test.
* etc/ORG-NEWS (Underline syntax now takes priority over subscript
when both are applicable): Announce the breaking change.

Reported-by: Juan Manuel Macías <maciaschain@posteo.net>
Link: https://list.orgmode.org/87v8z52eom.fsf@posteo.net/T/#t

etc/ORG-NEWS

@@ -13,6 +13,17 @@ Please send Org bug reports to mailto:emacs-orgmode@gnu.org.
 
 * Version 9.7 (not released yet)
 ** Important announcements and breaking changes
+*** Underline syntax now takes priority over subscript when both are applicable
+
+Previously, Org mode interpreted =(_text_)= as subscript.
+Now, the interpretation is changed to underline.
+
+=(_text_)= matches both subscript and underline markup.  The
+interpretation is changed to keep consistency with other emphasis like
+=(*bold*)=.
+
+Most of the users should not be affected by this change - it only applies when character immediately preceding =_= is one of =-=, =(=, ='=, and ={=.
+
 *** ~org-latex-to-mathml-convert-command~ and ~org-latex-to-html-convert-command~ may need to be adjusted
 
 Previously, =%i= placeholders in the

lisp/org-element.el

@@ -5258,10 +5258,10 @@ to an appropriate container (e.g., a paragraph)."
 		       (pcase (char-after)
 		         (?^ (and (memq 'superscript restriction)
 			          (org-element-superscript-parser)))
-		         (?_ (or (and (memq 'subscript restriction)
-				      (org-element-subscript-parser))
-			         (and (memq 'underline restriction)
-				      (org-element-underline-parser))))
+		         (?_ (or (and (memq 'underline restriction)
+				      (org-element-underline-parser))
+			         (and (memq 'subscript restriction)
+				      (org-element-subscript-parser))))
 		         (?* (and (memq 'bold restriction)
 			          (org-element-bold-parser)))
 		         (?/ (and (memq 'italic restriction)

testing/lisp/test-org-element.el

@@ -3306,7 +3306,16 @@ Outside list"
    (= 2
       (org-test-with-temp-text "__test__"
 	(length
-	 (org-element-map (org-element-parse-buffer) 'underline 'identity))))))
+	 (org-element-map (org-element-parse-buffer) 'underline 'identity)))))
+  ;; Starting after non-blank
+  (should
+   (eq 'underline
+       (org-test-with-temp-text "(_under<point>line_)"
+         (org-element-type (org-element-context)))))
+  (should-not
+   (eq 'underline
+       (org-test-with-temp-text "x_under<point>line_)"
+         (org-element-type (org-element-context))))))
 
 
 ;;;; Verbatim