[clang] [C] Diagnose use of C++ keywords in C (PR #137234)

[Richard Smith via cfe-commits]

================
@@ -258,8 +283,13 @@ static void AddKeyword(StringRef Keyword,
                        const LangOptions &LangOpts, IdentifierTable &Table) {
   KeywordStatus AddResult = getKeywordStatus(LangOpts, Flags);
 
-  // Don't add this keyword if disabled in this language.
-  if (AddResult == KS_Disabled) return;
+  // Don't add this keyword if disabled in this language and isn't otherwise
+  // special.
+  if (AddResult == KS_Disabled) {
----------------
zygoloid wrote:

Optional:

It'd be more consistent with the existing code to add an extra KeywordStatus value for this case, analogous to KS_Future. To do that, maybe we'd need to also replace the special macro for C++ operator keywords with a normal flag so that it can be taken into account when computing the KeywordStatus too.

I don't think it's necessary to be consistent here, though; we can always refactor later.

https://github.com/llvm/llvm-project/pull/137234