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

[via cfe-commits]

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

>From 6f08928c9d031ad4850f6f535003627441869712 Mon Sep 17 00:00:00 2001
From: isuckatcs <65320245+isuckatcs at users.noreply.github.com>
Date: Sat, 1 Mar 2025 17:06:34 +0100
Subject: [PATCH] [clang][diagnostics] add '-Wundef-true' warning option

---
 clang/include/clang/Basic/DiagnosticGroups.td |  2 +
 .../include/clang/Basic/DiagnosticLexKinds.td |  5 +-
 clang/lib/Lex/PPExpressions.cpp               |  5 +-
 .../test/Preprocessor/warn-macro-undef-true.c | 78 +++++++++++++++++++
 4 files changed, 88 insertions(+), 2 deletions(-)
 create mode 100644 clang/test/Preprocessor/warn-macro-undef-true.c

diff --git a/clang/include/clang/Basic/DiagnosticGroups.td b/clang/include/clang/Basic/DiagnosticGroups.td
index 05e39899e6f25..66a117a3c45d9 100644
--- a/clang/include/clang/Basic/DiagnosticGroups.td
+++ b/clang/include/clang/Basic/DiagnosticGroups.td
@@ -791,6 +791,8 @@ def ReservedIdAsMacroAlias : DiagGroup<"reserved-id-macro", [ReservedIdAsMacro]>
 def ReservedAttributeIdentifier : DiagGroup<"reserved-attribute-identifier">;
 def RestrictExpansionMacro : DiagGroup<"restrict-expansion">;
 def FinalMacro : DiagGroup<"final-macro">;
+def UndefinedIdentifier : DiagGroup<"undef">;
+def UndefinedTrueIdentifier : DiagGroup<"undef-true", [UndefinedIdentifier]>;
 
 // Just silence warnings about -Wstrict-aliasing for now.
 def : DiagGroup<"strict-aliasing=0">;
diff --git a/clang/include/clang/Basic/DiagnosticLexKinds.td b/clang/include/clang/Basic/DiagnosticLexKinds.td
index 4bcef23ccce16..c2552565fb96c 100644
--- a/clang/include/clang/Basic/DiagnosticLexKinds.td
+++ b/clang/include/clang/Basic/DiagnosticLexKinds.td
@@ -392,7 +392,10 @@ def pp_macro_not_used : Warning<"macro is not used">, DefaultIgnore,
   InGroup<DiagGroup<"unused-macros">>;
 def warn_pp_undef_identifier : Warning<
   "%0 is not defined, evaluates to 0">,
-  InGroup<DiagGroup<"undef">>, DefaultIgnore;
+  InGroup<UndefinedIdentifier>, DefaultIgnore;
+def warn_pp_undef_true_identifier : Warning<
+  "'true' is not defined, evaluates to 0">,
+  InGroup<UndefinedTrueIdentifier>;
 def warn_pp_undef_prefix : Warning<
   "%0 is not defined, evaluates to 0">,
   InGroup<DiagGroup<"undef-prefix">>, DefaultIgnore;
diff --git a/clang/lib/Lex/PPExpressions.cpp b/clang/lib/Lex/PPExpressions.cpp
index a3b1384f0fa1d..adb062e1348ba 100644
--- a/clang/lib/Lex/PPExpressions.cpp
+++ b/clang/lib/Lex/PPExpressions.cpp
@@ -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.
diff --git a/clang/test/Preprocessor/warn-macro-undef-true.c b/clang/test/Preprocessor/warn-macro-undef-true.c
new file mode 100644
index 0000000000000..9a64d577a96ed
--- /dev/null
+++ b/clang/test/Preprocessor/warn-macro-undef-true.c
@@ -0,0 +1,78 @@
+// RUN: %clang_cc1 %s -Eonly -std=c89 -verify=undef-true
+// RUN: %clang_cc1 %s -Eonly -std=c99 -verify=undef-true
+// RUN: %clang_cc1 %s -Eonly -std=c11 -verify=undef-true
+// RUN: %clang_cc1 %s -Eonly -std=c17 -verify=undef-true
+// RUN: %clang_cc1 %s -Eonly -std=c23 -verify=undef-true
+
+#if __STDC_VERSION__ >= 202311L
+/* undef-true-no-diagnostics */
+#endif
+
+#define FOO true
+#if FOO /* #1 */
+#endif
+#if __STDC_VERSION__ < 202311L
+/* undef-true-warning@#1 {{'true' is not defined, evaluates to 0}} */
+#endif
+
+#if true /* #2 */
+#endif
+#if __STDC_VERSION__ < 202311L
+/* undef-true-warning@#2 {{'true' is not defined, evaluates to 0}} */
+#endif
+
+#if false || true /* #3 */
+#endif
+#if __STDC_VERSION__ < 202311L
+/* undef-true-warning@#3 {{'true' is not defined, evaluates to 0}} */
+#endif
+
+#define true 1
+
+#define FOO true
+#if FOO
+#endif
+
+#if true
+#endif
+
+#if false || true
+#endif
+
+#undef true
+
+#define FOO true
+#if FOO /* #4 */
+#endif
+#if __STDC_VERSION__ < 202311L
+/* undef-true-warning@#4 {{'true' is not defined, evaluates to 0}} */
+#endif
+
+#if true /* #5 */
+#endif
+#if __STDC_VERSION__ < 202311L
+/* undef-true-warning@#5 {{'true' is not defined, evaluates to 0}} */
+#endif
+
+#if false || true /* #6 */
+#endif
+#if __STDC_VERSION__ < 202311L
+/* undef-true-warning@#6 {{'true' is not defined, evaluates to 0}} */
+#endif
+
+#define true true
+#if true /* #7 */
+#endif
+#if __STDC_VERSION__ < 202311L
+/* undef-true-warning@#7 {{'true' is not defined, evaluates to 0}} */
+#endif
+#undef true
+
+/* Test that #pragma-enabled 'Wundef' can override 'Wundef-true' */
+#pragma clang diagnostic warning "-Wundef"
+#if true /* #8 */
+#endif
+#pragma clang diagnostic ignored "-Wundef"
+#if __STDC_VERSION__ < 202311L
+/* undef-true-warning@#8 {{'true' is not defined, evaluates to 0}} */
+#endif