[clang-tidy] fix false positives when using name-independent variables after C++26 for bugprone-unused-local-non-trivial-variable (#121783)

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

Author: HerrCai0907

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),

clang-tools-extra/docs/ReleaseNotes.rst

@@ -241,6 +241,10 @@ 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,

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`.

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]
+}