[clang-tools-extra] Fix the issue #139467 ([clang-tidy] false positive narrowing conversion) (PR #139474)

[via cfe-commits]

https://github.com/AndreyG updated 
https://github.com/llvm/llvm-project/pull/139474

>From 26937479d0eee670946d6c4165c0522e271b5c76 Mon Sep 17 00:00:00 2001
From: Andrey Davydov <andrey.davy...@jetbrains.com>
Date: Sun, 11 May 2025 22:23:38 +0200
Subject: [PATCH] [clang-tidy] false positive narrowing conversion

Let's consider the following code from the issue #139467:

void test(int cond, char c) {
    char ret = cond > 0 ? ':' : c;
}

Initializer of 'ret' looks the following:

-ImplicitCastExpr 'char' <IntegralCast>
 `-ConditionalOperator 'int'
   |-BinaryOperator 'int' '>'
   | |-ImplicitCastExpr 'int' <LValueToRValue>
   | | `-DeclRefExpr 'int' lvalue ParmVar 'cond' 'int'
   | `-IntegerLiteral 'int' 0
   |-CharacterLiteral 'int' 58
   `-ImplicitCastExpr 'int' <IntegralCast>
     `-ImplicitCastExpr 'char' <LValueToRValue>
       `-DeclRefExpr 'char' lvalue ParmVar 'c' 'char'

So it could be seen that 'RHS' of the conditional operator is
DeclRefExpr 'c' which is casted to 'int' and then the whole conditional 
expression is casted to 'char'.
But this last conversion is not narrowing, because 'RHS' was 'char' _initially_.
We should just remove the cast from 'char' to 'int' before the narrowing 
conversion check.
---
 .../bugprone/NarrowingConversionsCheck.cpp         | 14 ++++++++++----
 .../bugprone/NarrowingConversionsCheck.h           |  2 ++
 clang-tools-extra/docs/ReleaseNotes.rst            |  4 ++++
 ...narrowing-conversions-conditional-expressions.c |  6 ++++++
 4 files changed, 22 insertions(+), 4 deletions(-)
 create mode 100644 
clang-tools-extra/test/clang-tidy/checkers/bugprone/narrowing-conversions-conditional-expressions.c

diff --git 
a/clang-tools-extra/clang-tidy/bugprone/NarrowingConversionsCheck.cpp 
b/clang-tools-extra/clang-tidy/bugprone/NarrowingConversionsCheck.cpp
index bafcd402ca851..949d3f0e5b7aa 100644
--- a/clang-tools-extra/clang-tidy/bugprone/NarrowingConversionsCheck.cpp
+++ b/clang-tools-extra/clang-tidy/bugprone/NarrowingConversionsCheck.cpp
@@ -554,15 +554,21 @@ bool NarrowingConversionsCheck::handleConditionalOperator(
     // We have an expression like so: `output = cond ? lhs : rhs`
     // From the point of view of narrowing conversion we treat it as two
     // expressions `output = lhs` and `output = rhs`.
-    handleBinaryOperator(Context, CO->getLHS()->getExprLoc(), Lhs,
-                         *CO->getLHS());
-    handleBinaryOperator(Context, CO->getRHS()->getExprLoc(), Lhs,
-                         *CO->getRHS());
+    handleConditionalOperatorArgument(Context, Lhs, CO->getLHS());
+    handleConditionalOperatorArgument(Context, Lhs, CO->getRHS());
     return true;
   }
   return false;
 }
 
+void NarrowingConversionsCheck::handleConditionalOperatorArgument(
+    const ASTContext &Context, const Expr &Lhs, const Expr *Arg) {
+  if (const auto *ICE = llvm::dyn_cast<ImplicitCastExpr>(Arg))
+    if (!Arg->getIntegerConstantExpr(Context))
+      Arg = ICE->getSubExpr();
+  handleBinaryOperator(Context, Arg->getExprLoc(), Lhs, *Arg);
+}
+
 void NarrowingConversionsCheck::handleImplicitCast(
     const ASTContext &Context, const ImplicitCastExpr &Cast) {
   if (Cast.getExprLoc().isMacroID())
diff --git a/clang-tools-extra/clang-tidy/bugprone/NarrowingConversionsCheck.h 
b/clang-tools-extra/clang-tidy/bugprone/NarrowingConversionsCheck.h
index 20403f920b925..ebddbc2869675 100644
--- a/clang-tools-extra/clang-tidy/bugprone/NarrowingConversionsCheck.h
+++ b/clang-tools-extra/clang-tidy/bugprone/NarrowingConversionsCheck.h
@@ -85,6 +85,8 @@ class NarrowingConversionsCheck : public ClangTidyCheck {
   bool handleConditionalOperator(const ASTContext &Context, const Expr &Lhs,
                                  const Expr &Rhs);
 
+  void handleConditionalOperatorArgument(const ASTContext &Context, const Expr 
&Lhs,
+                                         const Expr *Arg);
   void handleImplicitCast(const ASTContext &Context,
                           const ImplicitCastExpr &Cast);
 
diff --git a/clang-tools-extra/docs/ReleaseNotes.rst 
b/clang-tools-extra/docs/ReleaseNotes.rst
index 579fca54924d5..be4a195708dd2 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -142,6 +142,10 @@ New check aliases
 Changes in existing checks
 ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+- Improved :doc:`bugprone-narrowing-conversions
+  <clang-tidy/checks/bugprone/narrowing-conversions>` check by fixing
+  false positive from analysis of a conditional expression in C.
+
 - Improved :doc:`bugprone-optional-value-conversion
   <clang-tidy/checks/bugprone/optional-value-conversion>` check to detect
   conversion in argument of ``std::make_optional``.
diff --git 
a/clang-tools-extra/test/clang-tidy/checkers/bugprone/narrowing-conversions-conditional-expressions.c
 
b/clang-tools-extra/test/clang-tidy/checkers/bugprone/narrowing-conversions-conditional-expressions.c
new file mode 100644
index 0000000000000..754d6425b07cd
--- /dev/null
+++ 
b/clang-tools-extra/test/clang-tidy/checkers/bugprone/narrowing-conversions-conditional-expressions.c
@@ -0,0 +1,6 @@
+// RUN: %check_clang_tidy %s bugprone-narrowing-conversions %t -- --
+
+char test(int cond, char c) {
+       char ret = cond > 0 ? ':' : c;
+       return ret;
+}

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