[clang-tools-extra] [clang-tidy] Fix false positive in misc-redundant-expression with type aliases (PR #198085)

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llvmorg-github-actions[bot] wrote:


<!--LLVM PR SUMMARY COMMENT-->

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

Author: Caevan Lin (voyager-jhk)

<details>
<summary>Changes</summary>

The `misc-redundant-expression` check previously flagged expressions as redundant if their underlying `DeclRefExpr` pointed to the same declaration. This caused false positives when comparing identical values accessed through distinct type aliases (sugared types).

This patch uses `printPretty` to compare the source-level spellings of the expressions, ensuring nested name specifiers and explicit template arguments are correctly differentiated.

Fixes #<!-- -->145415

---
Full diff: https://github.com/llvm/llvm-project/pull/198085.diff


2 Files Affected:

- (modified) clang-tools-extra/clang-tidy/misc/RedundantExpressionCheck.cpp (+17-3) 
- (added) clang-tools-extra/test/clang-tidy/checkers/misc/redundant-expression-type-aliases.cpp (+24) 


``````````diff
diff --git a/clang-tools-extra/clang-tidy/misc/RedundantExpressionCheck.cpp b/clang-tools-extra/clang-tidy/misc/RedundantExpressionCheck.cpp
index 9db297990b274..b1b6bcee4b118 100644
--- a/clang-tools-extra/clang-tidy/misc/RedundantExpressionCheck.cpp
+++ b/clang-tools-extra/clang-tidy/misc/RedundantExpressionCheck.cpp
@@ -19,6 +19,7 @@
 #include "llvm/ADT/FoldingSet.h"
 #include "llvm/ADT/SmallBitVector.h"
 #include "llvm/Support/FormatVariadic.h"
+#include "llvm/Support/raw_ostream.h"
 #include <algorithm>
 #include <cassert>
 #include <cstdint>
@@ -97,9 +98,22 @@ static bool areEquivalentExpr(const Expr *Left, const Expr *Right) {
       return false;
     return cast<DependentScopeDeclRefExpr>(Left)->getQualifier() ==
            cast<DependentScopeDeclRefExpr>(Right)->getQualifier();
-  case Stmt::DeclRefExprClass:
-    return cast<DeclRefExpr>(Left)->getDecl() ==
-           cast<DeclRefExpr>(Right)->getDecl();
+  case Stmt::DeclRefExprClass: {
+    const auto *L = cast<DeclRefExpr>(Left);
+    const auto *R = cast<DeclRefExpr>(Right);
+
+    if (L->getDecl() != R->getDecl())
+      return false;
+
+    // Compare source-level spellings to safely differentiate sugared types.
+    std::string LStr, RStr;
+    llvm::raw_string_ostream LOS(LStr), ROS(RStr);
+    
+    L->printPretty(LOS, nullptr, L->getDecl()->getASTContext().getPrintingPolicy());
+    R->printPretty(ROS, nullptr, R->getDecl()->getASTContext().getPrintingPolicy());
+
+    return LStr == RStr; 
+  }
   case Stmt::MemberExprClass:
     return cast<MemberExpr>(Left)->getMemberDecl() ==
            cast<MemberExpr>(Right)->getMemberDecl();
diff --git a/clang-tools-extra/test/clang-tidy/checkers/misc/redundant-expression-type-aliases.cpp b/clang-tools-extra/test/clang-tidy/checkers/misc/redundant-expression-type-aliases.cpp
new file mode 100644
index 0000000000000..2df04809d5b58
--- /dev/null
+++ b/clang-tools-extra/test/clang-tidy/checkers/misc/redundant-expression-type-aliases.cpp
@@ -0,0 +1,24 @@
+// RUN: %check_clang_tidy %s misc-redundant-expression %t -- -- -std=c++17
+
+namespace std {
+  template<class T, int N> struct array { };
+  template<class T> struct tuple_size;
+  template<class T, int N> struct tuple_size<array<T, N>> { static constexpr int value = N; };
+  template<class T> constexpr int tuple_size_v = tuple_size<T>::value;
+}
+
+using MonthArray = std::array<int, 12>;
+using ZodiacArray = std::array<int, 12>;
+
+void test() {
+  // False positive cases (Should NOT warn):
+  bool b1 = std::tuple_size<MonthArray>::value == std::tuple_size<ZodiacArray>::value;
+  bool b2 = std::tuple_size_v<MonthArray> == std::tuple_size_v<ZodiacArray>;
+
+  // True positive cases (Should warn):
+  bool b3 = std::tuple_size<MonthArray>::value == std::tuple_size<MonthArray>::value;
+  // CHECK-MESSAGES: :[[@LINE-1]]:{{[0-9]+}}: warning: both sides of operator are equivalent [misc-redundant-expression]
+  
+  bool b4 = std::tuple_size_v<MonthArray> == std::tuple_size_v<MonthArray>;
+  // CHECK-MESSAGES: :[[@LINE-1]]:{{[0-9]+}}: warning: both sides of operator are equivalent [misc-redundant-expression]
+}
\ No newline at end of file

``````````

</details>


https://github.com/llvm/llvm-project/pull/198085