[clang] [C11] Diagnose C11 keywords as being incompatible w/earlier standards (PR #82015)

[Aaron Ballman via cfe-commits]

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@@ -2742,6 +2742,19 @@ bool Parser::parseMisplacedModuleImport() {
   return false;
 }
 
+void Parser::diagnoseUseOfC11Keyword(const Token& Tok) {
+  // Warn that this is a C11 extension if in an older mode or if in C++.
+  // Otherwise, warn that it is incompatible with standards before C11 if in
+  // C11 or later.
+  unsigned DiagId;
+  if (!getLangOpts().C11) {
+    DiagId = diag::ext_c11_feature;
+  } else {
+    DiagId = diag::warn_c11_compat_keyword;
+  }
+  Diag(Tok, DiagId) << Tok.getName();
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AaronBallman wrote:

Yeah, I had originally structured it this way because I thought we might want to give a different diagnostic in C++ mode (where we call these C11 extensions), but then I realized that diagnostic is reasonable as-is. I'll change.

https://github.com/llvm/llvm-project/pull/82015