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

[Richard Smith via cfe-commits]

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@@ -393,6 +393,9 @@ def pp_macro_not_used : Warning<"macro is not used">, DefaultIgnore,
 def warn_pp_undef_identifier : Warning<
   "%0 is not defined, evaluates to 0">,
   InGroup<DiagGroup<"undef">>, DefaultIgnore;
+def warn_pp_undef_true_identifier : Warning<
+  "'true' is not defined, evaluates to 0">,
+  InGroup<DiagGroup<"undef-true">>;
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zygoloid wrote:

`-Wundef-true` should be a child warning group under `-Wundef`, so that `-Wundef` and `-Wno-undef` continue to cover both cases. You'll need to define the groups in DiagnosticGroups.td

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