cc-mode: extend regexp used by ‘c-or-c++-mode’

* lisp/progmodes/cc-mode (c-or-c++-mode--regexp): Expand the regexp to
match some more C++-only constructs and recognise a few more standard
C++ header files.  Also make sure identifiers start with non-digit.
(c-or-c++-mode): Add ‘(interactive)’ declaration.

* test/lisp/progmodes/cc-mode-tests.el (c-or-c++-mode): Add test case
for the newly recognised constructs.

1 files changed, 15 insertions, 6 deletions

diff --git a/lisp/progmodes/cc-mode.el b/lisp/progmodes/cc-mode.el
index f92d3efdeb7..e3a924efb06 100644
--- a/lisp/progmodes/cc-mode.el
+++ b/lisp/progmodes/cc-mode.el
@@ -2541,13 +2541,21 @@ Key bindings:
 
 (defconst c-or-c++-mode--regexp
   (eval-when-compile
-    (let ((id "[a-zA-Z0-9_]+") (ws "[ \t\r]+") (ws-maybe "[ \t\r]*"))
+    (let ((id "[a-zA-Z_][a-zA-Z0-9_]*") (ws "[ \t\r]+") (ws-maybe "[ \t\r]*")
+          (headers '("string" "string_view" "iostream" "map" "unordered_map"
+                     "set" "unordered_set" "vector" "tuple")))
       (concat "^" ws-maybe "\\(?:"
-                    "using"     ws "\\(?:namespace" ws "std;\\|std::\\)"
-              "\\|" "namespace" "\\(:?" ws id "\\)?" ws-maybe "{"
-              "\\|" "class"     ws id ws-maybe "[:{\n]"
-              "\\|" "template"  ws-maybe "<.*>"
-              "\\|" "#include"  ws-maybe "<\\(?:string\\|iostream\\|map\\)>"
+                    "using"     ws "\\(?:namespace" ws
+                                     "\\|" id "::"
+                                     "\\|" id ws-maybe "=\\)"
+              "\\|" "\\(?:inline" ws "\\)?namespace"
+                    "\\(:?" ws "\\(?:" id "::\\)*" id "\\)?" ws-maybe "{"
+              "\\|" "class"     ws id
+                    "\\(?:" ws "final" "\\)?" ws-maybe "[:{;\n]"
+              "\\|" "struct"     ws id "\\(?:" ws "final" ws-maybe "[:{\n]"
+                                         "\\|" ws-maybe ":\\)"
+              "\\|" "template"  ws-maybe "<.*?>"
+              "\\|" "#include"  ws-maybe "<" (regexp-opt headers) ">"
               "\\)")))
   "A regexp applied to C header files to check if they are really C++.")
 
@@ -2563,6 +2571,7 @@ should be used.
 This function attempts to use file contents to determine whether
 the code is C or C++ and based on that chooses whether to enable
 `c-mode' or `c++-mode'."
+  (interactive)
   (if (save-excursion
         (save-restriction
           (save-match-data