cc-mode: add ‘c-lineup-ternary-bodies’ (bug#41061)

Introduce ‘c-lineup-ternary-bodies’ function which, when used as
a c lineup function, aligns question mark and colon of a ternary
operator.  For example:

    return arg % 2 == 0 ? arg / 2
                        : (3 * arg + 1);

* lisp/progmodes/cc-align.el (c-lineup-ternary-bodies): New function.
* doc/misc/cc-mode.texi (Operator Line-Up Functions): Document the
new function.
* test/lisp/progmodes/cc-mode-tests.el (c-lineup-ternary-bodies): New
test case.

1 files changed, 32 insertions, 0 deletions

diff --git a/lisp/progmodes/cc-align.el b/lisp/progmodes/cc-align.el
index f30477dc787..c49bdc5c518 100644
--- a/lisp/progmodes/cc-align.el
+++ b/lisp/progmodes/cc-align.el
@@ -790,6 +790,38 @@ arglist-cont-nonempty."
   (or (c-lineup-assignments langelem)
       c-basic-offset))
 
+(defun c-lineup-ternary-bodies (langelem)
+  "Line up true and false branches of a ternary operator (i.e. ‘?:’).
+More precisely, if the line starts with a colon which is a part of
+a said operator it with corresponding question mark; otherwise return
+nil.  For example:
+
+    return arg % 2 == 0 ? arg / 2
+                        : (3 * arg + 1);    <- c-lineup-ternary-bodies
+
+Works with: arglist-cont, arglist-cont-nonempty and statement-cont.
+"
+  (save-excursion
+    (back-to-indentation)
+    (when (and (eq ?: (char-after))
+               (not (eq ?: (char-after (1+ (point))))))
+      (let ((limit (c-langelem-pos langelem)) (depth 1))
+        (catch 'done
+          (while (c-syntactic-skip-backward "^?:" limit t)
+            (goto-char (1- (point)))
+            (cond ((eq (char-after) ??)
+                   ;; If we’ve found a question mark, decrease depth.  If we’re
+                   ;; reached zero, we’ve found the one we were looking for.
+                   (when (zerop (setq depth (1- depth)))
+                     (throw 'done (vector (current-column)))))
+                  ((or (eq ?: (char-before)) (eq ?? (char-before)))
+                   ;; Step over ‘::’ and ‘?:’ operators.  We don’t have to
+                   ;; handle ‘?:’ here but doing so saves an iteration.
+                   (if (eq (point) limit)
+                       (throw 'done nil)
+                     (goto-char (1- (point)))))
+                  ((setq depth (1+ depth)))))))))) ; Otherwise increase depth.
+
 (defun c-lineup-cascaded-calls (langelem)
   "Line up \"cascaded calls\" under each other.
 If the line begins with \"->\" or \".\" and the preceding line ends