[clang] [Clang][Sema] Warn unused cxx var declared in condition of if-statement (PR #87348)

[Youngsuk Kim via cfe-commits]

https://github.com/JOE1994 created 
https://github.com/llvm/llvm-project/pull/87348

Fixes #41447 

>From 90834d7250d18c9389c84966279365638dce80a9 Mon Sep 17 00:00:00 2001
From: Youngsuk Kim <youngsuk....@hpe.com>
Date: Tue, 2 Apr 2024 08:17:36 -0500
Subject: [PATCH] [Clang][Sema] Warn unused cxx var declared in condition of
 if-statement

---
 clang/include/clang/AST/Decl.h                       | 12 ++++++++++++
 clang/lib/Sema/SemaDecl.cpp                          |  8 ++++++--
 clang/lib/Sema/SemaDeclCXX.cpp                       |  3 +++
 .../SemaCXX/warn-unused-but-set-variables-cpp.cpp    |  6 ++++++
 4 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/clang/include/clang/AST/Decl.h b/clang/include/clang/AST/Decl.h
index a5879591f4c659..5f1f83bb00282f 100644
--- a/clang/include/clang/AST/Decl.h
+++ b/clang/include/clang/AST/Decl.h
@@ -1100,6 +1100,9 @@ class VarDecl : public DeclaratorDecl, public 
Redeclarable<VarDecl> {
 
     LLVM_PREFERRED_TYPE(bool)
     unsigned EscapingByref : 1;
+
+    LLVM_PREFERRED_TYPE(bool)
+    unsigned IsCXXCondDecl : 1;
   };
 
   union {
@@ -1589,6 +1592,15 @@ class VarDecl : public DeclaratorDecl, public 
Redeclarable<VarDecl> {
     NonParmVarDeclBits.EscapingByref = true;
   }
 
+  bool isCXXCondDecl() const {
+    return isa<ParmVarDecl>(this) ? false : NonParmVarDeclBits.IsCXXCondDecl;
+  }
+
+  void setCXXCondDecl() {
+    assert(!isa<ParmVarDecl>(this));
+    NonParmVarDeclBits.IsCXXCondDecl = true;
+  }
+
   /// Determines if this variable's alignment is dependent.
   bool hasDependentAlignment() const;
 
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 8b44d24f5273aa..62f71658fa3b5a 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -2188,8 +2188,12 @@ void Sema::DiagnoseUnusedButSetDecl(const VarDecl *VD,
 
   assert(iter->getSecond() >= 0 &&
          "Found a negative number of references to a VarDecl");
-  if (iter->getSecond() != 0)
-    return;
+  if (iter->getSecond() != 0) {
+    bool UnusedCXXCondDecl = VD->isCXXCondDecl() && (iter->getSecond() == 1);
+    if (!UnusedCXXCondDecl)
+      return;
+  }
+
   unsigned DiagID = isa<ParmVarDecl>(VD) ? diag::warn_unused_but_set_parameter
                                          : diag::warn_unused_but_set_variable;
   DiagReceiver(VD->getLocation(), PDiag(DiagID) << VD);
diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index e9fecaea84b021..d4d69beeba5a05 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -18563,6 +18563,9 @@ DeclResult Sema::ActOnCXXConditionDeclaration(Scope *S, 
Declarator &D) {
     return true;
   }
 
+  if (auto *VD = dyn_cast<VarDecl>(Dcl))
+    VD->setCXXCondDecl();
+
   return Dcl;
 }
 
diff --git a/clang/test/SemaCXX/warn-unused-but-set-variables-cpp.cpp 
b/clang/test/SemaCXX/warn-unused-but-set-variables-cpp.cpp
index 418baa78aa964b..cb066e209cb453 100644
--- a/clang/test/SemaCXX/warn-unused-but-set-variables-cpp.cpp
+++ b/clang/test/SemaCXX/warn-unused-but-set-variables-cpp.cpp
@@ -69,3 +69,9 @@ template <typename T> void f5() {
   SWarnUnused swu;
   ++swu;
 }
+
+void f6() {
+  if (int x = 123) { // expected-warning{{variable 'x' set but not used}}
+    ;
+  }
+}

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