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

[Congcong Cai via cfe-commits]

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@@ -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));
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HerrCai0907 wrote:

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

Yes, `isPlaceholderVar` does not check cpp version.

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