[clang] [Sema] Diagnose use of if/else-if condition variable inside else-if/else branch(s) (PR #156436)
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Fri Oct 3 12:08:06 PDT 2025
https://github.com/arrowten updated https://github.com/llvm/llvm-project/pull/156436
>From 6cfcc64ef626024438858d244834c7febca75b94 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 condition variable inside else
branch(s)
---
.../clang/Basic/DiagnosticSemaKinds.td | 3 +
clang/include/clang/Sema/Scope.h | 9 ++
clang/include/clang/Sema/Sema.h | 4 +
clang/lib/Parse/ParseStmt.cpp | 4 +
clang/lib/Sema/SemaExpr.cpp | 22 +++
clang/test/Sema/warn-conditional-scope.cpp | 146 ++++++++++++++++++
6 files changed, 188 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..cd7c0e6827eeb 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 declared in 'if' block is always false or null here">,
+ InGroup<DiagGroup<"conditional-scope">>;
// AMDGCN builtins diagnostics
def err_amdgcn_load_lds_size_invalid_value : Error<"invalid size value">;
diff --git a/clang/include/clang/Sema/Scope.h b/clang/include/clang/Sema/Scope.h
index 757f3dcc3fe8d..b542680df4df7 100644
--- a/clang/include/clang/Sema/Scope.h
+++ b/clang/include/clang/Sema/Scope.h
@@ -255,6 +255,9 @@ class Scope {
/// available for this variable in the current scope.
llvm::SmallPtrSet<VarDecl *, 8> ReturnSlots;
+ // Condition variable for if
+ VarDecl *ConditionVar = nullptr;
+
void setFlags(Scope *Parent, unsigned F);
public:
@@ -263,6 +266,12 @@ class Scope {
Init(Parent, ScopeFlags);
}
+ // getConditionVar - Gets the condition variable of the scope.
+ VarDecl *getConditionVar() const { return ConditionVar; }
+
+ // setConditionVar - Setter for condition variable of this scope.
+ void setConditionVar(VarDecl *D) { ConditionVar = D; }
+
/// getFlags - Return the flags for this scope.
unsigned getFlags() const { return Flags; }
diff --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
index c3fb57774c8dc..938509962fdb9 100644
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -7171,6 +7171,10 @@ class Sema final : public SemaBase {
/// return a reason explaining why. Otherwise, return NOUR_None.
NonOdrUseReason getNonOdrUseReasonInCurrentContext(ValueDecl *D);
+ /// Checks if the given DeclRefExpr refers to a variable declared in an
+ /// enclosing 'if' condition.
+ bool isConditionVarReference(const DeclRefExpr *DRE);
+
DeclRefExpr *BuildDeclRefExpr(ValueDecl *D, QualType Ty, ExprValueKind VK,
SourceLocation Loc,
const CXXScopeSpec *SS = nullptr);
diff --git a/clang/lib/Parse/ParseStmt.cpp b/clang/lib/Parse/ParseStmt.cpp
index bf1978c22ee9f..f791e30c34359 100644
--- a/clang/lib/Parse/ParseStmt.cpp
+++ b/clang/lib/Parse/ParseStmt.cpp
@@ -1526,6 +1526,10 @@ StmtResult Parser::ParseIfStatement(SourceLocation *TrailingElseLoc) {
SourceLocation ElseStmtLoc;
StmtResult ElseStmt;
+ if (VarDecl *VD = dyn_cast_or_null<VarDecl>(Cond.get().first);
+ !Cond.isInvalid() && VD)
+ getCurScope()->setConditionVar(VD->getCanonicalDecl());
+
if (Tok.is(tok::kw_else)) {
if (TrailingElseLoc)
*TrailingElseLoc = Tok.getLocation();
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index d383544e54329..7345f277011a0 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -2371,6 +2371,24 @@ NonOdrUseReason Sema::getNonOdrUseReasonInCurrentContext(ValueDecl *D) {
return NOUR_None;
}
+bool Sema::isConditionVarReference(const DeclRefExpr *DRE) {
+ if (!DRE)
+ return false;
+
+ const VarDecl *VD = dyn_cast<VarDecl>(DRE->getDecl());
+
+ if (!VD)
+ return false;
+
+ for (Scope *S = getCurScope(); S; S = S->getParent()) {
+ if (VarDecl *CV = S->getConditionVar())
+ if (VD == CV)
+ return true;
+ }
+
+ return false;
+}
+
DeclRefExpr *
Sema::BuildDeclRefExpr(ValueDecl *D, QualType Ty, ExprValueKind VK,
const DeclarationNameInfo &NameInfo,
@@ -2425,6 +2443,10 @@ Sema::BuildDeclRefExpr(ValueDecl *D, QualType Ty, ExprValueKind VK,
if (const auto *BE = BD->getBinding())
E->setObjectKind(BE->getObjectKind());
+ if (isConditionVarReference(E))
+ Diag(E->getBeginLoc(), diag::warn_out_of_scope_var_usage)
+ << D->getDeclName();
+
return E;
}
diff --git a/clang/test/Sema/warn-conditional-scope.cpp b/clang/test/Sema/warn-conditional-scope.cpp
new file mode 100644
index 0000000000000..0c65c578d1fb5
--- /dev/null
+++ b/clang/test/Sema/warn-conditional-scope.cpp
@@ -0,0 +1,146 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -Wconditional-scope -DTEST_1 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -Wconditional-scope -DTEST_2 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -Wconditional-scope -DTEST_3 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -Wconditional-scope -DTEST_4 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -Wconditional-scope -DTEST_5 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -Wconditional-scope -DTEST_6 %s
+
+int *get_something();
+int *get_something_else();
+int *get_something_else_again();
+int *get_something_else_again_now();
+
+#ifdef TEST_1
+
+int test() {
+ if (int *ptr = get_something()) {
+ return ptr[0] * ptr[0];
+ }
+ // expected-warning at +2{{variable 'ptr' declared in 'if' block is always false or null here}}
+ else if (int *ptr2 = get_something_else()) {
+ return ptr[0] * ptr2[0];
+ }
+ // expected-warning at +3{{variable 'ptr' declared in 'if' block is always false or null here}}
+ // expected-warning at +2{{variable 'ptr2' declared in 'if' block is always false or null here}}
+ else if (int* ptr3 = get_something_else_again()) {
+ return ptr[0] * ptr2[0] * ptr3[0];
+ }
+ // expected-warning at +4{{variable 'ptr' declared in 'if' block is always false or null here}}
+ // expected-warning at +3{{variable 'ptr2' declared in 'if' block is always false or null here}}
+ // expected-warning at +2{{variable 'ptr3' declared in 'if' block is always false or null here}}
+ else if (int *ptr4 = get_something_else_again_now()) {
+ return ptr[0] * ptr2[0] * ptr3[0] * ptr4[0];
+ }
+ else {
+ return -1;
+ }
+}
+
+#endif
+
+#ifdef TEST_2
+
+int test() {
+ if (int *ptr = get_something()) {
+ return ptr[0] * ptr[0];
+ }
+ else if (int *ptr2 = get_something_else()) {
+ return ptr2[0] * ptr2[0];
+ }
+ // expected-warning at +3{{variable 'ptr' declared in 'if' block is always false or null here}}
+ // expected-warning at +2{{variable 'ptr2' declared in 'if' block is always false or null here}}
+ else if (int* ptr3 = get_something_else_again()) {
+ return ptr[0] * ptr2[0] * ptr3[0];
+ }
+ else {
+ return -1;
+ }
+}
+
+#endif
+
+#ifdef TEST_3
+
+int test() {
+ if (int *ptr = get_something()) {
+ return ptr[0] * ptr[0];
+ }
+ else if (int *ptr2 = get_something_else()) {
+ return ptr2[0] * ptr2[0];
+ }
+ // expected-warning at +3{{variable 'ptr2' declared in 'if' block is always false or null here}}
+ // expected-warning at +2{{variable 'ptr2' declared in 'if' block is always false or null here}}
+ else if (int* ptr3 = get_something_else_again()) {
+ return ptr2[0] * ptr2[0] * ptr3[0];
+ }
+ else {
+ return -1;
+ }
+}
+
+#endif
+
+#ifdef TEST_4
+
+int test() {
+ int x = 10;
+
+ if (x == 30) {
+ return x;
+ }
+ else if (int *ptr = get_something()) {
+ return ptr[0];
+ }
+ // expected-warning at +3{{variable 'ptr' declared in 'if' block is always false or null here}}
+ // expected-warning at +2{{variable 'ptr' declared in 'if' block is always false or null here}}
+ else if (x == 20) {
+ return ptr[0] * ptr[0];
+ }
+ // expected-warning at +2{{variable 'ptr' declared in 'if' block is always false or null here}}
+ else if (int* ptr2 = get_something_else()) {
+ return ptr[0] * ptr2[0];
+ }
+ // expected-warning at +3{{variable 'ptr' declared in 'if' block is always false or null here}}
+ // expected-warning at +2{{variable 'ptr2' declared in 'if' block is always false or null here}}
+ else if (int *ptr3 = get_something_else_again()) {
+ return ptr[0] * ptr2[0] * ptr3[0];
+ }
+ else {
+ return -1;
+ }
+}
+
+#endif
+
+#ifdef TEST_5
+
+void test() {
+ // expected-no-diagnostics
+ if (bool x = get_something()) {}
+ else {
+ {
+ bool x = get_something_else();
+ if (x) {}
+ }
+ }
+}
+
+#endif
+
+#ifdef TEST_6
+
+int test() {
+ if (int *ptr = get_something()) {
+ return ptr[0];
+ }
+ else if (int *ptr = get_something_else()) {
+ return ptr[0] * ptr[0];
+ }
+ // expected-warning at +3{{variable 'ptr' declared in 'if' block is always false or null here}}
+ // expected-warning at +2{{variable 'ptr' declared in 'if' block is always false or null here}}
+ else {
+ return ptr[0] * ptr[0];
+ }
+}
+
+#endif
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