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[clang][bytecode] Only implicitly start lifetime of trivially-default-constructible union members #149835

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Merged
merged 1 commit into from
Jul 23, 2025
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[clang][bytecode] Only implicitly start lifetime of trivially-default-constructible union members #149835

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9 changes: 8 additions & 1 deletion clang/lib/AST/ByteCode/Compiler.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -25,11 +25,18 @@ using APSInt = llvm::APSInt;
namespace clang {
namespace interp {

static bool hasTrivialDefaultCtorParent(const FieldDecl *FD) {
assert(FD);
assert(FD->getParent()->isUnion());
const auto *CXXRD = dyn_cast<CXXRecordDecl>(FD->getParent());
return !CXXRD || CXXRD->hasTrivialDefaultConstructor();
}

static bool refersToUnion(const Expr *E) {
for (;;) {
if (const auto *ME = dyn_cast<MemberExpr>(E)) {
if (const auto *FD = dyn_cast<FieldDecl>(ME->getMemberDecl());
FD && FD->getParent()->isUnion())
FD && FD->getParent()->isUnion() && hasTrivialDefaultCtorParent(FD))
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Do you handle things like that?

struct s{
    union {
    union {
        int m;
    };
    };
};

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Handle what exactly? https://godbolt.org/z/581onze1G The diagnostics aren't a perfect 1:1 match but otherwise yes

return true;
E = ME->getBase();
continue;
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55 changes: 55 additions & 0 deletions clang/test/AST/ByteCode/unions.cpp
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Expand Up @@ -861,6 +861,61 @@ namespace CopyCtorMutable {
// both-note {{in call}}
}


namespace NonTrivialCtor {
struct A { int x = 1; constexpr int f() { return 1; } };
struct B : A { int y = 1; constexpr int g() { return 2; } };
struct C {
int x;
constexpr virtual int f() = 0;
};
struct D : C {
int y;
constexpr virtual int f() override { return 3; }
};

union U {
int n;
B b;
D d;
};

consteval int test(int which) {
if (which == 0) {}

U u{.n = 5};
assert_active(u);
assert_active(u.n);
assert_inactive(u.b);

switch (which) {
case 0:
u.b.x = 10; // both-note {{assignment to member 'b' of union with active member 'n'}}
return u.b.f();
case 1:
u.b.y = 10; // both-note {{assignment to member 'b' of union with active member 'n'}}
return u.b.g();
case 2:
u.d.x = 10; // both-note {{assignment to member 'd' of union with active member 'n'}}
return u.d.f();
case 3:
u.d.y = 10; // both-note {{assignment to member 'd' of union with active member 'n'}}
return u.d.f();
}

return 1;
}
static_assert(test(0)); // both-error {{not an integral constant expression}} \
// both-note {{in call}}
static_assert(test(1)); // both-error {{not an integral constant expression}} \
// both-note {{in call}}
static_assert(test(2)); // both-error {{not an integral constant expression}} \
// both-note {{in call}}
static_assert(test(3)); // both-error {{not an integral constant expression}} \
// both-note {{in call}}

}

#endif

namespace AddressComparison {
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