[clang] [Sema] Diagnose use of if/else-if condition variable inside else-if/else branch(s) (PR #156436)

[via cfe-commits]

https://github.com/arrowten created https://github.com/llvm/llvm-project/pull/156436

This patch adds a warning when variable(s) declared in if/else-if scope are used in else-if/else branch(s)

>From 232cb40aecf07eeac0c9bf9a1aa302f3a55a5ee1 Mon Sep 17 00:00:00 2001
From: arrowten <ajaywakodikarsocial at gmail.com>
Date: Tue, 2 Sep 2025 15:33:52 +0530
Subject: [PATCH] [Sema] Diagnose use of if/else-if condition variable inside
 else-if/else branch(s)

---
 .../clang/Basic/DiagnosticSemaKinds.td        |  3 ++
 clang/lib/Sema/SemaStmt.cpp                   | 29 +++++++++++++++++++
 clang/test/Sema/warn-conditional-scope.cpp    | 23 +++++++++++++++
 3 files changed, 55 insertions(+)
 create mode 100644 clang/test/Sema/warn-conditional-scope.cpp

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index c934fed2c7462..3d370d46480d5 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -13594,6 +13594,9 @@ def warn_acc_var_referenced_lacks_op
               "reference has no effect">,
       InGroup<DiagGroup<"openacc-var-lacks-operation">>,
       DefaultError;
+def warn_out_of_scope_var_usage
+    : Warning<"variable %0 used in else/else if block is out of scope">,
+      InGroup<DiagGroup<"conditional-scope">>;
 
 // AMDGCN builtins diagnostics
 def err_amdgcn_load_lds_size_invalid_value : Error<"invalid size value">;
diff --git a/clang/lib/Sema/SemaStmt.cpp b/clang/lib/Sema/SemaStmt.cpp
index 5625fb359807a..f0f5c62380f30 100644
--- a/clang/lib/Sema/SemaStmt.cpp
+++ b/clang/lib/Sema/SemaStmt.cpp
@@ -25,6 +25,7 @@
 #include "clang/AST/StmtObjC.h"
 #include "clang/AST/TypeLoc.h"
 #include "clang/AST/TypeOrdering.h"
+#include "clang/AST/RecursiveASTVisitor.h"
 #include "clang/Basic/TargetInfo.h"
 #include "clang/Lex/Preprocessor.h"
 #include "clang/Sema/EnterExpressionEvaluationContext.h"
@@ -971,6 +972,34 @@ StmtResult Sema::ActOnIfStmt(SourceLocation IfLoc,
   if (!ConstevalOrNegatedConsteval && !elseStmt)
     DiagnoseEmptyStmtBody(RParenLoc, thenStmt, diag::warn_empty_if_body);
 
+  // Checks for if/else-if condition variable usage in else-if/else scope
+  if (elseStmt) {
+    VarDecl *ConditionVar = nullptr;
+
+    if (auto *CondVar = Cond.get().first) {
+      ConditionVar = dyn_cast<VarDecl>(CondVar);
+    }    
+
+    if (ConditionVar) {
+      struct ElseVariableUsageChecker
+        : public RecursiveASTVisitor<ElseVariableUsageChecker> {
+        VarDecl *TargetVar;
+        Sema &SemaRef;
+        ElseVariableUsageChecker(VarDecl *Var, Sema &S) : TargetVar(Var), SemaRef(S) {}
+
+        bool VisitDeclRefExpr(DeclRefExpr *DRE) {
+          if (DRE->getDecl() == TargetVar) {
+            SemaRef.Diag(DRE->getBeginLoc(), diag::warn_out_of_scope_var_usage) << TargetVar->getName();
+          }
+          return true;
+        }
+      };   
+
+      ElseVariableUsageChecker Checker(ConditionVar, *this);
+      Checker.TraverseStmt(elseStmt);
+    }    
+  }
+
   if (ConstevalOrNegatedConsteval ||
       StatementKind == IfStatementKind::Constexpr) {
     auto DiagnoseLikelihood = [&](const Stmt *S) {
diff --git a/clang/test/Sema/warn-conditional-scope.cpp b/clang/test/Sema/warn-conditional-scope.cpp
new file mode 100644
index 0000000000000..3d7d1ad522fdd
--- /dev/null
+++ b/clang/test/Sema/warn-conditional-scope.cpp
@@ -0,0 +1,23 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -Wconditional-scope %s
+
+int *get_something();
+int *get_something_else();
+int *get_something_else_again();
+
+int test() {
+  if (int *ptr = get_something()) {
+    return ptr[0] * ptr[0];
+  }
+  else if (int *ptr2 = get_something_else()) {
+    // expected-warning at +1{{variable ptr used in else/else if block is out of scope}}
+    return ptr[0] * ptr2[0];
+  }
+  else if (int* ptr3 = get_something_else_again()) {
+    // expected-warning at +2{{variable ptr used in else/else if block is out of scope}}
+    // expected-warning at +1{{variable ptr2 used in else/else if block is out of scope}}
+    return ptr[0] * ptr2[0] * ptr3[0];	
+  }
+  else {
+    return -1;
+  }
+}