[clang-tools-extra] [clang-tidy] fix bugprone-narrowing-conversions false positive for conditional expression #139474 (PR #151874)

[via cfe-commits]

https://github.com/AndreyG updated https://github.com/llvm/llvm-project/pull/151874

>From 5bd55220bc5a7f0d4fa5b43978068e944abfd231 Mon Sep 17 00:00:00 2001
From: Andrey <andrey.a.davydov at gmail.com>
Date: Sun, 3 Aug 2025 12:39:39 +0200
Subject: [PATCH] [clang-tidy] fix `bugprone-narrowing-conversions` false
 positive for conditional expression (#139474)

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

Fixes #139467

The added tests contains the implementation-defined warning about
'int' to 'char' conversion, which is not applicable to all platforms.
And so the target is explictly set to 'x86_64' (the line 'RUN: -- -target x86_64-unknown-linux').
---
 .../bugprone/NarrowingConversionsCheck.cpp    | 15 +++++++++----
 .../bugprone/NarrowingConversionsCheck.h      |  2 ++
 clang-tools-extra/docs/ReleaseNotes.rst       |  4 ++++
 ...wing-conversions-conditional-expressions.c | 22 +++++++++++++++++++
 ...ng-conversions-conditional-expressions.cpp | 22 +++++++++++++++++++
 5 files changed, 61 insertions(+), 4 deletions(-)
 create mode 100644 clang-tools-extra/test/clang-tidy/checkers/bugprone/narrowing-conversions-conditional-expressions.c
 create mode 100644 clang-tools-extra/test/clang-tidy/checkers/bugprone/narrowing-conversions-conditional-expressions.cpp

diff --git a/clang-tools-extra/clang-tidy/bugprone/NarrowingConversionsCheck.cpp b/clang-tools-extra/clang-tidy/bugprone/NarrowingConversionsCheck.cpp
index 53782231b6421..249c77ca0c432 100644
--- a/clang-tools-extra/clang-tidy/bugprone/NarrowingConversionsCheck.cpp
+++ b/clang-tools-extra/clang-tidy/bugprone/NarrowingConversionsCheck.cpp
@@ -555,15 +555,22 @@ 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..116a8cba8d321 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 85b31bc0b42a6..004130c9472b0 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -134,6 +134,10 @@ Changes in existing checks
   <clang-tidy/checks/bugprone/infinite-loop>` check by adding detection for
   variables introduced by structured bindings.
 
+- 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-reserved-identifier
   <clang-tidy/checks/bugprone/reserved-identifier>` check by ignoring
   declarations in system headers.
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..08594dd104b52
--- /dev/null
+++ b/clang-tools-extra/test/clang-tidy/checkers/bugprone/narrowing-conversions-conditional-expressions.c
@@ -0,0 +1,22 @@
+// RUN: %check_clang_tidy %s bugprone-narrowing-conversions %t \
+// RUN: -- -target x86_64-unknown-linux
+
+char test_char(int cond, char c) {
+	char ret = cond > 0 ? ':' : c;
+	return ret;
+}
+
+short test_short(int cond, short s) {
+	short ret = cond > 0 ? ':' : s;
+	return ret;
+}
+
+int test_int(int cond, int i) {
+	int ret = cond > 0 ? ':' : i;
+	return ret;
+}
+
+void test(int cond, int i) {
+  char x = cond > 0 ? ':' : i;
+  // CHECK-MESSAGES: :[[@LINE-1]]:29: warning: narrowing conversion from 'int' to signed type 'char' is implementation-defined [bugprone-narrowing-conversions]
+}
diff --git a/clang-tools-extra/test/clang-tidy/checkers/bugprone/narrowing-conversions-conditional-expressions.cpp b/clang-tools-extra/test/clang-tidy/checkers/bugprone/narrowing-conversions-conditional-expressions.cpp
new file mode 100644
index 0000000000000..08594dd104b52
--- /dev/null
+++ b/clang-tools-extra/test/clang-tidy/checkers/bugprone/narrowing-conversions-conditional-expressions.cpp
@@ -0,0 +1,22 @@
+// RUN: %check_clang_tidy %s bugprone-narrowing-conversions %t \
+// RUN: -- -target x86_64-unknown-linux
+
+char test_char(int cond, char c) {
+	char ret = cond > 0 ? ':' : c;
+	return ret;
+}
+
+short test_short(int cond, short s) {
+	short ret = cond > 0 ? ':' : s;
+	return ret;
+}
+
+int test_int(int cond, int i) {
+	int ret = cond > 0 ? ':' : i;
+	return ret;
+}
+
+void test(int cond, int i) {
+  char x = cond > 0 ? ':' : i;
+  // CHECK-MESSAGES: :[[@LINE-1]]:29: warning: narrowing conversion from 'int' to signed type 'char' is implementation-defined [bugprone-narrowing-conversions]
+}