Handle C++17's constexpr keyword in if statements

* lisp/progmodes/cc-engine.el (c-after-conditional): Test for matches to
c-block-stmt-hangon-key.

* lisp/progmodes/cc-langs.el (c-block-stmt-hangon-kwds): New lang const.
(c-block-stmt-hangon-key): New lang const/var matching any element of the
above.

2 files changed, 17 insertions, 0 deletions

diff --git a/lisp/progmodes/cc-engine.el b/lisp/progmodes/cc-engine.el
index 05926c48b04..26ce2c919e8 100644
--- a/lisp/progmodes/cc-engine.el
+++ b/lisp/progmodes/cc-engine.el
@@ -9847,9 +9847,15 @@ comment at the start of cc-engine.el for more info."
   ;; This function might do hidden buffer changes.
   (save-excursion
     (and (zerop (c-backward-token-2 1 t lim))
+         (if (looking-at c-block-stmt-hangon-key)
+             (zerop (c-backward-token-2 1 t lim))
+           t)
          (or (looking-at c-block-stmt-1-key)
              (and (eq (char-after) ?\()
                   (zerop (c-backward-token-2 1 t lim))
+                  (if (looking-at c-block-stmt-hangon-key)
+                      (zerop (c-backward-token-2 1 t lim))
+                    t)
                   (or (looking-at c-block-stmt-2-key)
                       (looking-at c-block-stmt-1-2-key))))
          (point))))
diff --git a/lisp/progmodes/cc-langs.el b/lisp/progmodes/cc-langs.el
index 98cfd2073d1..8fa253f3045 100644
--- a/lisp/progmodes/cc-langs.el
+++ b/lisp/progmodes/cc-langs.el
@@ -2587,6 +2587,17 @@ Keywords here should also be in `c-block-stmt-1-kwds'."
                                   (c-lang-const c-block-stmt-2-kwds))
                           :test 'string-equal))
 
+(c-lang-defconst c-block-stmt-hangon-kwds
+  "Keywords which may directly follow a member of `c-block-stmt-1/2-kwds'."
+  t nil
+  c++ '("constexpr"))
+
+(c-lang-defconst c-block-stmt-hangon-key
+  ;; Regexp matching a "hangon" keyword in a `c-block-stmt-1/2-kwds'
+  ;; construct.
+  t (c-make-keywords-re t (c-lang-const c-block-stmt-hangon-kwds)))
+(c-lang-defvar c-block-stmt-hangon-key (c-lang-const c-block-stmt-hangon-key))
+
 (c-lang-defconst c-opt-block-stmt-key
   ;; Regexp matching the start of any statement that has a
   ;; substatement (except a bare block).  Nil in languages that