[clang] [clang][diagnostics] add '-Wundef-true' warning option (PR #128265)

[Richard Smith via cfe-commits]

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@@ -257,7 +257,10 @@ static bool EvaluateValue(PPValue &Result, Token &PeekTok, DefinedTracker &DT,
         // preprocessor keywords and it wasn't macro expanded, it turns
         // into a simple 0
         if (ValueLive) {
-          PP.Diag(PeekTok, diag::warn_pp_undef_identifier) << II;
+          if (II->getName() == "true")
+            PP.Diag(PeekTok, diag::warn_pp_undef_true_identifier);
+          else
+            PP.Diag(PeekTok, diag::warn_pp_undef_identifier) << II;
 
           const DiagnosticsEngine &DiagEngine = PP.getDiagnostics();
           // If 'Wundef' is enabled, do not emit 'undef-prefix' diagnostics.
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zygoloid wrote:

We should check the same DiagID we used above in the `isIgnored` check too; the logic is meant to be "if we just issued a diagnostic then don't produce another".

https://github.com/llvm/llvm-project/pull/128265