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

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

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3 changes: 3 additions & 0 deletions clang/include/clang/Basic/DiagnosticSemaKinds.td
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Original file line number Diff line number Diff line change
Expand Up @@ -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">>;
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Not really a fan of this wording. Perhaps we could use a %select here to decide whether this is a false or null (or perhaps even 0?). Or perhaps we should just say false always? What do others think?

Also, a note pointing to the condition-var is probably a good idea too.

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@arrowten arrowten Oct 3, 2025 •
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Not really a fan of this wording. Perhaps we could use a %select here to decide whether this is a false or null (or perhaps even 0?). Or perhaps we should just say false always? What do others think?

I will try that.

Also, a note pointing to the condition-var is probably a good idea too.

Its pointing to the "used" condition variable though.

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ah, hmm... it should point to the DeclRefExpr not to the condition variable then. We want to say "this use right here is wrong".

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@erichkeane Correct me if I am wrong.
In here, this is the current scenario

if (int *ptr = get_something()) {  
  return ptr[0];                        
}
else if (int *ptr2 = get_something_else()) {  
  return ptr[0] * ptr2[0];          //warns for ptr
}
else if (int *ptr3 = get_something_else_again()) { 
  return ptr[0] * ptr2[0] * ptr3[0];               // warns for ptr and ptr2
}
else if (int *ptr4 = get_something_else_again_now()) { 
  return ptr[0] * ptr2[0] * ptr3[0] * ptr4[0];          // warns for ptr, ptr2 and ptr3
}

It should warn for something else too?


// AMDGCN builtins diagnostics
def err_amdgcn_load_lds_size_invalid_value : Error<"invalid size value">;
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9 changes: 9 additions & 0 deletions clang/include/clang/Sema/Scope.h
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Expand Up @@ -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:
Expand All @@ -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; }

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4 changes: 4 additions & 0 deletions clang/include/clang/Sema/Sema.h
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Expand Up @@ -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);
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4 changes: 4 additions & 0 deletions clang/lib/Parse/ParseStmt.cpp
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Expand Up @@ -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();
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22 changes: 22 additions & 0 deletions clang/lib/Sema/SemaExpr.cpp
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Expand Up @@ -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)
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I think we should still use isSameEntity, and we should probably figure out why that doesn't work. If it doesn't, I have some concerns about this.

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@arrowten arrowten Oct 3, 2025 •
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In the 5th testcase, I could see that it is returning true for VarX->getType() == VarY->getType() in isSameEntity.

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can you give a better example? They should be different entities...

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  if (bool x = get_something()) {}
  else {
    {   
      bool x = get_something_else();
      if (x) {} //throws warning here
    }   
  }

It is throwing here since it compares the local x to the if condition x and since both have the same type, it eventually throws a warning.

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hmm... those shouldn't be the same entity, but I see the ASTContext function is doing a lot of assuming for the purposes of deserialization, so that is probably why. It just doesn't do what I think it should be doing.

I think we actually want Sema::DeclaresSameEntity which'll handle it correctly.

return true;
}

return false;
}

DeclRefExpr *
Sema::BuildDeclRefExpr(ValueDecl *D, QualType Ty, ExprValueKind VK,
const DeclarationNameInfo &NameInfo,
Expand Down Expand Up @@ -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;
}

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146 changes: 146 additions & 0 deletions clang/test/Sema/warn-conditional-scope.cpp
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@@ -0,0 +1,146 @@
// RUN: %clang_cc1 -fsyntax-only -verify -Wconditional-scope -DTEST_1 %s
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These tests aren't particularly great or varied, try your situations but also try to make them 'flow' a bit to show you've covered everything. Make sure you cover more than just pointer cases.

Also, try some tests in a dependent case, where the variable itself is dependent.

also-also: What about an unevaluated context? Or a situation where you don't actually take the variable? Should THAT warn?

Consider:

if (auto x = bar()) {
}
else {
   decltype(x) f = g; // should this warn?
}

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If anyone can help with different types of testcases, I would appreciate that.

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I gave you some ideas, dependence, non-evaluated contexts, etc.

// 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
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Don't use macros like this in the test, just do all of these in the same 'run' line.

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Do you mean to use one single function and all of the testcases together?

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Not necessarily. But you can capture multiple diagnostics in the same run, so don't do macros, just do multiple checks.


int test() {
if (int *ptr = get_something()) {
return ptr[0] * ptr[0];
}
// expected-warning@+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@+3{{variable 'ptr' declared in 'if' block is always false or null here}}
// expected-warning@+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@+4{{variable 'ptr' declared in 'if' block is always false or null here}}
// expected-warning@+3{{variable 'ptr2' declared in 'if' block is always false or null here}}
// expected-warning@+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@+3{{variable 'ptr' declared in 'if' block is always false or null here}}
// expected-warning@+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@+3{{variable 'ptr2' declared in 'if' block is always false or null here}}
// expected-warning@+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@+3{{variable 'ptr' declared in 'if' block is always false or null here}}
// expected-warning@+2{{variable 'ptr' declared in 'if' block is always false or null here}}
else if (x == 20) {
return ptr[0] * ptr[0];
}
// expected-warning@+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@+3{{variable 'ptr' declared in 'if' block is always false or null here}}
// expected-warning@+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@+3{{variable 'ptr' declared in 'if' block is always false or null here}}
// expected-warning@+2{{variable 'ptr' declared in 'if' block is always false or null here}}
else {
return ptr[0] * ptr[0];
}
}

#endif