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[Clang][Sema] Preserve bit-field info in conditional operator #169480

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[Clang][Sema] Preserve bit-field info in conditional operator #169480

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7 changes: 7 additions & 0 deletions clang/lib/AST/Expr.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -4247,6 +4247,13 @@ FieldDecl *Expr::getSourceBitField() {
if (UnOp->isPrefix() && UnOp->isIncrementDecrementOp())
return UnOp->getSubExpr()->getSourceBitField();

if (const ConditionalOperator *Cond = dyn_cast<ConditionalOperator>(E)) {
if (FieldDecl *FD = Cond->getTrueExpr()->getSourceBitField())
return FD;
if (FieldDecl *FD = Cond->getFalseExpr()->getSourceBitField())
return FD;
}
Comment on lines +4250 to +4255
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@Halalaluyafail3 Halalaluyafail3 Nov 25, 2025

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Wouldn't this mean the order of the second and third operands would matter? Also, it looks like it would make cond?bit-field:non-bit-field promote and check for narrowing conversions based off of the width of the bit-field. Therefore promotion could change the value if the non-bit-field had a value not representable in int but representable in unsigned, and narrowing could happen in contexts where narrowing conversions are disabled.

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@MitchBriles MitchBriles Nov 26, 2025

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I think if the operands differ in that way, it's handled in Sema::CXXCheckConditionalOperands. If folks can agree on the expected behavior, I'll make sure my patch is correct before marking it as ready.


return nullptr;
}

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4 changes: 2 additions & 2 deletions clang/test/CXX/dcl.decl/dcl.init/dcl.init.ref/p5.cpp
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Expand Up @@ -41,8 +41,8 @@ namespace PR6066 {

namespace test3 {
struct A {
unsigned bitX : 4; // expected-note 3 {{bit-field is declared here}}
unsigned bitY : 4; // expected-note {{bit-field is declared here}}
unsigned bitX : 4; // expected-note 6 {{bit-field is declared here}}
unsigned bitY : 4; // expected-note 2 {{bit-field is declared here}}
unsigned var;

void foo();
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1 change: 1 addition & 0 deletions clang/test/CXX/drs/cwg3xx.cpp
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Expand Up @@ -397,6 +397,7 @@ namespace cwg324 { // cwg324: 3.6
// expected-error@-1 {{non-const reference cannot bind to bit-field}}
int *f = &(true ? s.n : s.n);
// expected-error@-1 {{address of bit-field requested}}
// expected-note@#cwg324-n {{bit-field is declared here}}
int &g = (void(), s.n);
// expected-error@-1 {{non-const reference cannot bind to bit-field 'n'}}
// expected-note@#cwg324-n {{bit-field is declared here}}
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16 changes: 16 additions & 0 deletions clang/test/SemaCXX/bitfield-cond-promotion.cpp
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@@ -0,0 +1,16 @@
// RUN: %clang_cc1 -fcxx-exceptions -fexceptions -fsyntax-only -verify %s -std=c++11
// RUN: %clang_cc1 -fcxx-exceptions -fexceptions -fsyntax-only -verify %s -std=c++14
// RUN: %clang_cc1 -fcxx-exceptions -fexceptions -fsyntax-only -verify %s -std=c++17
// RUN: %clang_cc1 -fcxx-exceptions -fexceptions -fsyntax-only -verify %s -std=c++20
// RUN: %clang_cc1 -fcxx-exceptions -fexceptions -fsyntax-only -verify %s -std=c++23

void test_runtime_behavior() {
struct {
unsigned f : 1;
} constexpr s{};

constexpr int result = (0 ? throw 0 : s.f) - 1;
static_assert(result == -1, "Bit-field should promote to int"); // expected-no-diagnostics
constexpr int result2 = (1 ? s.f : s.f) - 1;
static_assert(result2 == -1, "Bit-field should promote to int"); // expected-no-diagnostics
}
6 changes: 5 additions & 1 deletion clang/test/SemaCXX/conditional-expr.cpp
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Expand Up @@ -42,7 +42,8 @@ struct BadBase { operator BadDerived&(); };
struct BadDerived : BadBase {};

struct Fields {
int i1, i2, b1 : 3, b2 : 3;
int i1, i2, b1 : 3, b2 : 3; // expected-note 2 {{bit-field is declared here}}
unsigned u1: 1;
};
struct MixedFields {
int i;
Expand Down Expand Up @@ -201,6 +202,9 @@ void test()
(void)&(i1 ? flds.b1 : flds.i1); // expected-error {{address of bit-field requested}}
(void)&(i1 ? flds.i1 : flds.b1); // expected-error {{address of bit-field requested}}

// shouldn't be considered narrowing
unsigned char uc1{0 ? throw 0 : flds.u1};
unsigned char uc2{1 ? flds.u1 : flds.u1};

unsigned long test0 = 5;
test0 = test0 ? (long) test0 : test0; // expected-warning {{operand of ? changes signedness: 'long' to 'unsigned long'}}
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