[clang-tidy] fix false positives when using name-independent variables after C++26 for bugprone-unused-local-non-trivial-variable

[HerrCai0907]

Fixed: #121731
According to https://eel.is/c++draft/basic.scope.scope#5, name independent declaration should not be warned as unused

[llvmbot]

@llvm/pr-subscribers-clang-tools-extra

@llvm/pr-subscribers-clang-tidy

Author: Congcong Cai (HerrCai0907)

Changes

Fixed: #121731
According to https://eel.is/c++draft/basic.scope.scope#5, name independent declaration should not be warned as unused

---

Full diff: https://github.com/llvm/llvm-project/pull/121783.diff

4 Files Affected:

• (modified) clang-tools-extra/clang-tidy/bugprone/UnusedLocalNonTrivialVariableCheck.cpp (+7-1)
• (modified) clang-tools-extra/docs/ReleaseNotes.rst (+4)
• (modified) clang-tools-extra/docs/clang-tidy/checks/bugprone/unused-local-non-trivial-variable.rst (+1)
• (added) clang-tools-extra/test/clang-tidy/checkers/bugprone/unused-local-non-trivial-variable-name-independence.cpp (+21)

diff --git a/clang-tools-extra/clang-tidy/bugprone/UnusedLocalNonTrivialVariableCheck.cpp b/clang-tools-extra/clang-tidy/bugprone/UnusedLocalNonTrivialVariableCheck.cpp
index 37baae7a6f0c3a..f15fd4d9ad9fab 100644
--- a/clang-tools-extra/clang-tidy/bugprone/UnusedLocalNonTrivialVariableCheck.cpp
+++ b/clang-tools-extra/clang-tidy/bugprone/UnusedLocalNonTrivialVariableCheck.cpp
@@ -29,6 +29,12 @@ static constexpr StringRef DefaultIncludeTypeRegex =
 
 AST_MATCHER(VarDecl, isLocalVarDecl) { return Node.isLocalVarDecl(); }
 AST_MATCHER(VarDecl, isReferenced) { return Node.isReferenced(); }
+AST_MATCHER_P(VarDecl, explicitMarkUnused, LangOptions, LangOpts) {
+  // Implementations should not emit a warning that a name-independent
+  // declaration is used or unused.
+  return Node.hasAttr<UnusedAttr>() ||
+         (LangOpts.CPlusPlus26 && Node.isPlaceholderVar(LangOpts));
+}
 AST_MATCHER(Type, isReferenceType) { return Node.isReferenceType(); }
 AST_MATCHER(QualType, isTrivial) {
   return Node.isTrivialType(Finder->getASTContext()) ||
@@ -60,7 +66,7 @@ void UnusedLocalNonTrivialVariableCheck::registerMatchers(MatchFinder *Finder) {
       varDecl(isLocalVarDecl(), unless(isReferenced()),
               unless(isExceptionVariable()), hasLocalStorage(), isDefinition(),
               unless(hasType(isReferenceType())), unless(hasType(isTrivial())),
-              unless(hasAttr(attr::Kind::Unused)),
+              unless(explicitMarkUnused(getLangOpts())),
               hasType(hasUnqualifiedDesugaredType(
                   anyOf(recordType(hasDeclaration(namedDecl(
                             matchesAnyListedName(IncludeTypes),
diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index 3cab440155250b..eba72e3d8d014e 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -236,6 +236,10 @@ Changes in existing checks
   <clang-tidy/checks/bugprone/unsafe-functions>` check to allow specifying
   additional functions to match.
 
+- Improved :doc:`bugprone-unused-local-non-trivial-variable
+  <clang-tidy/checks/bugprone/unused-local-non-trivial-variable>` check to avoid
+  false positives when using name-independent variables after C++26.
+
 - Improved :doc:`bugprone-use-after-move
   <clang-tidy/checks/bugprone/use-after-move>` to avoid triggering on
   ``reset()`` calls on moved-from ``std::optional`` and ``std::any`` objects,
diff --git a/clang-tools-extra/docs/clang-tidy/checks/bugprone/unused-local-non-trivial-variable.rst b/clang-tools-extra/docs/clang-tidy/checks/bugprone/unused-local-non-trivial-variable.rst
index 9f283de78fbdec..672eab62b4af63 100644
--- a/clang-tools-extra/docs/clang-tidy/checks/bugprone/unused-local-non-trivial-variable.rst
+++ b/clang-tools-extra/docs/clang-tidy/checks/bugprone/unused-local-non-trivial-variable.rst
@@ -12,6 +12,7 @@ The following types of variables are excluded from this check:
 * static or thread local
 * structured bindings
 * variables with ``[[maybe_unused]]`` attribute
+* name-independent variables
 
 This check can be configured to warn on all non-trivial variables by setting
 `IncludeTypes` to `.*`, and excluding specific types using `ExcludeTypes`.
diff --git a/clang-tools-extra/test/clang-tidy/checkers/bugprone/unused-local-non-trivial-variable-name-independence.cpp b/clang-tools-extra/test/clang-tidy/checkers/bugprone/unused-local-non-trivial-variable-name-independence.cpp
new file mode 100644
index 00000000000000..bcc8b810acabf5
--- /dev/null
+++ b/clang-tools-extra/test/clang-tidy/checkers/bugprone/unused-local-non-trivial-variable-name-independence.cpp
@@ -0,0 +1,21 @@
+// RUN: %check_clang_tidy -std=c++23 -check-suffixes=,CXX23 %s bugprone-unused-local-non-trivial-variable %t -- \
+// RUN:       -config="{CheckOptions: {bugprone-unused-local-non-trivial-variable.IncludeTypes: '::async::Foo'}}" \
+// RUN:       --
+// RUN: %check_clang_tidy -std=c++26 %s bugprone-unused-local-non-trivial-variable %t -- \
+// RUN:       -config="{CheckOptions: {bugprone-unused-local-non-trivial-variable.IncludeTypes: '::async::Foo'}}" \
+// RUN:       --
+
+namespace async {
+class Foo {
+  public:
+    ~Foo();
+  private:
+};
+} // namespace async
+
+void check() {
+  async::Foo C;
+  // CHECK-MESSAGES: :[[@LINE-1]]:14: warning: unused local variable 'C' of type 'async::Foo' [bugprone-unused-local-non-trivial-variable]
+  async::Foo _;
+  // CHECK-MESSAGES-CXX23: :[[@LINE-1]]:14: warning: unused local variable '_' of type 'async::Foo' [bugprone-unused-local-non-trivial-variable]
+}

[5chmidti]

LGTM

[carlosgalvezp]

Out of curiosity, is the CplusPlus26 needed? Won't isPlaceholderVar return false otherwise?

[PiotrZSL]

And, you dont need to pass LangOptions via argument.
From matcher you got access to ASTContext, and that class got getLangOpts

[HerrCai0907]

Out of curiosity, is the CplusPlus26 needed? Won't isPlaceholderVar return false otherwise?

Yes, isPlaceholderVar does not check cpp version.