[P3074] Partial implementation of support for trivial unions #146815
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@llvm/pr-subscribers-clang Author: Barry Revzin (brevzin) ChangesThis commit makes union construction/destruction trivial by default, but does not add the lifetime aspects of the paper — am holding out until P3726R0 will be discussed which alters that approach somewhat based on feedback from Richard Smith. But the constructor/destructor part is common regardless and also simpler. Full diff: https://github.com/llvm/llvm-project/pull/146815.diff 6 Files Affected:
diff --git a/clang/lib/AST/DeclCXX.cpp b/clang/lib/AST/DeclCXX.cpp
index ccb308e103253..49adbdf1c393d 100644
--- a/clang/lib/AST/DeclCXX.cpp
+++ b/clang/lib/AST/DeclCXX.cpp
@@ -1217,6 +1217,9 @@ void CXXRecordDecl::addedMember(Decl *D) {
// those because they are always unnamed.
bool IsZeroSize = Field->isZeroSize(Context);
+ // P3074
+ const bool TrivialUnion = Context.getLangOpts().CPlusPlus26 && isUnion();
+
if (const auto *RecordTy = T->getAs<RecordType>()) {
auto *FieldRec = cast<CXXRecordDecl>(RecordTy->getDecl());
if (FieldRec->getDefinition()) {
@@ -1277,7 +1280,7 @@ void CXXRecordDecl::addedMember(Decl *D) {
// -- for all the non-static data members of its class that are of
// class type (or array thereof), each such class has a trivial
// default constructor.
- if (!FieldRec->hasTrivialDefaultConstructor())
+ if (!FieldRec->hasTrivialDefaultConstructor() && !TrivialUnion)
data().HasTrivialSpecialMembers &= ~SMF_DefaultConstructor;
// C++0x [class.copy]p13:
@@ -1315,9 +1318,9 @@ void CXXRecordDecl::addedMember(Decl *D) {
if (!FieldRec->hasTrivialMoveAssignment())
data().HasTrivialSpecialMembers &= ~SMF_MoveAssignment;
- if (!FieldRec->hasTrivialDestructor())
+ if (!FieldRec->hasTrivialDestructor() && !TrivialUnion)
data().HasTrivialSpecialMembers &= ~SMF_Destructor;
- if (!FieldRec->hasTrivialDestructorForCall())
+ if (!FieldRec->hasTrivialDestructorForCall() && !TrivialUnion)
data().HasTrivialSpecialMembersForCall &= ~SMF_Destructor;
if (!FieldRec->hasIrrelevantDestructor())
data().HasIrrelevantDestructor = false;
diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index e8c65025bfe6d..e9b45bf99620c 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -9511,6 +9511,15 @@ bool SpecialMemberDeletionInfo::shouldDeleteForSubobjectCall(
CXXMethodDecl *Decl = SMOR.getMethod();
FieldDecl *Field = Subobj.dyn_cast<FieldDecl*>();
+ // P3074: default ctor and dtor for unions are not deleted, regardless of
+ // whether the underlying fields have non-trivial or deleted versions of those
+ // members
+ if (S.Context.getLangOpts().CPlusPlus26)
+ if (Field && Field->getParent()->isUnion() &&
+ (CSM == CXXSpecialMemberKind::DefaultConstructor ||
+ CSM == CXXSpecialMemberKind::Destructor))
+ return false;
+
int DiagKind = -1;
if (SMOR.getKind() == Sema::SpecialMemberOverloadResult::NoMemberOrDeleted)
@@ -9774,7 +9783,8 @@ bool SpecialMemberDeletionInfo::shouldDeleteForField(FieldDecl *FD) {
// At least one member in each anonymous union must be non-const
if (CSM == CXXSpecialMemberKind::DefaultConstructor &&
- AllVariantFieldsAreConst && !FieldRecord->field_empty()) {
+ AllVariantFieldsAreConst && !FieldRecord->field_empty() &&
+ !this->S.Context.getLangOpts().CPlusPlus26) {
if (Diagnose)
S.Diag(FieldRecord->getLocation(),
diag::note_deleted_default_ctor_all_const)
@@ -9804,6 +9814,10 @@ bool SpecialMemberDeletionInfo::shouldDeleteForAllConstMembers() {
// default constructor. Don't do that.
if (CSM == CXXSpecialMemberKind::DefaultConstructor && inUnion() &&
AllFieldsAreConst) {
+
+ if (S.Context.getLangOpts().CPlusPlus26)
+ return false;
+
bool AnyFields = false;
for (auto *F : MD->getParent()->fields())
if ((AnyFields = !F->isUnnamedBitField()))
diff --git a/clang/test/CXX/drs/cwg6xx.cpp b/clang/test/CXX/drs/cwg6xx.cpp
index e2eb009508b52..dd54bb5295b56 100644
--- a/clang/test/CXX/drs/cwg6xx.cpp
+++ b/clang/test/CXX/drs/cwg6xx.cpp
@@ -4,7 +4,7 @@
// RUN: %clang_cc1 -std=c++17 %s -verify=expected,cxx11-20,cxx98-17,cxx11-17,since-cxx11 -fexceptions -fcxx-exceptions -pedantic-errors
// RUN: %clang_cc1 -std=c++20 %s -verify=expected,cxx11-20,since-cxx11 -fexceptions -fcxx-exceptions -pedantic-errors
// RUN: %clang_cc1 -std=c++23 %s -verify=expected,since-cxx11 -fexceptions -fcxx-exceptions -pedantic-errors
-// RUN: %clang_cc1 -std=c++2c %s -verify=expected,since-cxx11 -fexceptions -fcxx-exceptions -pedantic-errors
+// RUN: %clang_cc1 -std=c++2c %s -verify=expected,since-cxx11,cxx26 -fexceptions -fcxx-exceptions -pedantic-errors
#if __cplusplus == 199711L
#define static_assert(...) __extension__ _Static_assert(__VA_ARGS__)
@@ -922,7 +922,7 @@ namespace cwg667 { // cwg667: 8
struct B { ~B() = delete; };
union C { B b; };
- static_assert(!__is_trivially_destructible(C), "");
+ static_assert(!__is_trivially_destructible(C), ""); // cxx26-error {{failed}}
struct D { D(const D&) = delete; };
struct E : D {};
diff --git a/clang/test/CXX/special/class.ctor/p5-0x.cpp b/clang/test/CXX/special/class.ctor/p5-0x.cpp
index e0c53058f892b..97a8b035a0705 100644
--- a/clang/test/CXX/special/class.ctor/p5-0x.cpp
+++ b/clang/test/CXX/special/class.ctor/p5-0x.cpp
@@ -1,4 +1,5 @@
-// RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++11 -Wno-deprecated-builtins -Wno-defaulted-function-deleted
+// RUN: %clang_cc1 -fsyntax-only -verify=expected,until26 %s -std=c++11 -Wno-deprecated-builtins -Wno-defaulted-function-deleted
+// RUN: %clang_cc1 -fsyntax-only -verify=expected %s -std=c++26 -Wno-deprecated-builtins -Wno-defaulted-function-deleted
struct DefaultedDefCtor1 {};
struct DefaultedDefCtor2 { DefaultedDefCtor2() = default; };
@@ -23,8 +24,8 @@ int n;
// - X is a union-like class that has a variant member with a non-trivial
// default constructor,
-union Deleted1a { UserProvidedDefCtor u; }; // expected-note {{default constructor of 'Deleted1a' is implicitly deleted because variant field 'u' has a non-trivial default constructor}}
-Deleted1a d1a; // expected-error {{implicitly-deleted default constructor}}
+union Deleted1a { UserProvidedDefCtor u; }; // until26-note {{default constructor of 'Deleted1a' is implicitly deleted because variant field 'u' has a non-trivial default constructor}}
+Deleted1a d1a; // until26-error {{implicitly-deleted default constructor}}
union NotDeleted1a { DefaultedDefCtor1 nu; };
NotDeleted1a nd1a;
union NotDeleted1b { DefaultedDefCtor2 nu; };
@@ -86,19 +87,19 @@ NotDeleted3i nd3i;
union Deleted4a {
const int a;
const int b;
- const UserProvidedDefCtor c; // expected-note {{because variant field 'c' has a non-trivial default constructor}}
+ const UserProvidedDefCtor c; // until26-note {{because variant field 'c' has a non-trivial default constructor}}
};
-Deleted4a d4a; // expected-error {{implicitly-deleted default constructor}}
+Deleted4a d4a; // until26-error {{implicitly-deleted default constructor}}
union NotDeleted4a { const int a; int b; };
NotDeleted4a nd4a;
// - X is a non-union class and all members of any anonymous union member are of
// const-qualified type (or array thereof),
struct Deleted5a {
- union { const int a; }; // expected-note {{because all data members of an anonymous union member are const-qualified}}
+ union { const int a; }; // until26-note {{because all data members of an anonymous union member are const-qualified}}
union { int b; };
};
-Deleted5a d5a; // expected-error {{implicitly-deleted default constructor}}
+Deleted5a d5a; // until26-error {{implicitly-deleted default constructor}}
struct NotDeleted5a { union { const int a; int b; }; union { const int c; int d; }; };
NotDeleted5a nd5a;
diff --git a/clang/test/CXX/special/class.ctor/p6-0x.cpp b/clang/test/CXX/special/class.ctor/p6-0x.cpp
index 156a2b20c6b52..6ce1b06841ab4 100644
--- a/clang/test/CXX/special/class.ctor/p6-0x.cpp
+++ b/clang/test/CXX/special/class.ctor/p6-0x.cpp
@@ -1,4 +1,5 @@
-// RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++11
+// RUN: %clang_cc1 -fsyntax-only -verify=expected,until26 %s -std=c++11
+// RUN: %clang_cc1 -fsyntax-only -verify=expected,cxx26 %s -std=c++26
// Implicitly-defined default constructors are constexpr if the implicit
// definition would be.
@@ -15,8 +16,9 @@ constexpr NonConstexpr2a nc2a = NonConstexpr2a(); // ok, does not call construct
constexpr int nc2_a = NonConstexpr2().nl.a; // ok
constexpr int nc2a_a = NonConstexpr2a().a; // ok
struct Helper {
- friend constexpr NonConstexpr1::NonConstexpr1(); // expected-error {{follows non-constexpr declaration}}
- friend constexpr NonConstexpr2::NonConstexpr2(); // expected-error {{follows non-constexpr declaration}}
+ friend constexpr NonConstexpr1::NonConstexpr1(); // until26-error {{follows non-constexpr declaration}} cxx26-error {{missing exception specification}}
+ friend constexpr NonConstexpr2::NonConstexpr2(); // until26-error {{follows non-constexpr declaration}} cxx26-error {{missing exception specification}}
+
};
struct Constexpr1 {};
@@ -31,14 +33,14 @@ constexpr Constexpr2 c2 = Constexpr2(); // ok
int n;
struct Member {
- Member() : a(n) {}
+ Member() : a(n) {} // cxx26-note {{here}}
constexpr Member(int&a) : a(a) {}
int &a;
};
-struct NonConstexpr4 { // expected-note {{here}}
+struct NonConstexpr4 { // until26-note {{here}} cxx26-note {{non-constexpr constructor}}
Member m;
};
-constexpr NonConstexpr4 nc4 = NonConstexpr4(); // expected-error {{constant expression}} expected-note {{non-constexpr constructor 'NonConstexpr4'}}
+constexpr NonConstexpr4 nc4 = NonConstexpr4(); // expected-error {{constant expression}} until26-note {{non-constexpr constructor 'NonConstexpr4'}} cxx26-note {{in call to}}
struct Constexpr3 {
constexpr Constexpr3() : m(n) {}
Member m;
@@ -53,11 +55,11 @@ constexpr Constexpr4 c4 = Constexpr4(); // ok
// This rule breaks some legal C++98 programs!
struct A {}; // expected-note {{here}}
struct B {
- friend A::A(); // expected-error {{non-constexpr declaration of 'A' follows constexpr declaration}}
+ friend A::A(); // until26-error {{non-constexpr declaration of 'A' follows constexpr declaration}} cxx26-error {{missing exception specification}}
};
namespace UnionCtors {
- union A { // expected-note {{here}}
+ union A { // until26-note {{here}}
int a;
int b;
};
@@ -79,7 +81,7 @@ namespace UnionCtors {
int d = 5;
};
};
- struct E { // expected-note {{here}}
+ struct E { // until26-note {{here}}
union {
int a;
int b;
@@ -87,11 +89,11 @@ namespace UnionCtors {
};
struct Test {
- friend constexpr A::A() noexcept; // expected-error {{follows non-constexpr declaration}}
+ friend constexpr A::A() noexcept; // until26-error {{follows non-constexpr declaration}}
friend constexpr B::B() noexcept;
friend constexpr C::C() noexcept;
friend constexpr D::D() noexcept;
- friend constexpr E::E() noexcept; // expected-error {{follows non-constexpr declaration}}
+ friend constexpr E::E() noexcept; // until26-error {{follows non-constexpr declaration}}
};
}
@@ -122,6 +124,6 @@ namespace PR48763 {
struct G { G(); };
struct H : D { using D::D; H(int); G g; };
- union V { H h; }; // expected-note {{field 'h' has a non-trivial default constructor}}
- V v; // expected-error {{deleted}}
+ union V { H h; }; // until26-note {{field 'h' has a non-trivial default constructor}}
+ V v; // until26-error {{deleted}}
}
diff --git a/clang/test/CXX/special/class.dtor/p5-0x.cpp b/clang/test/CXX/special/class.dtor/p5-0x.cpp
index ae14dcdaf102a..7616383515e0a 100644
--- a/clang/test/CXX/special/class.dtor/p5-0x.cpp
+++ b/clang/test/CXX/special/class.dtor/p5-0x.cpp
@@ -1,10 +1,11 @@
-// RUN: %clang_cc1 -verify -std=c++11 %s -Wno-defaulted-function-deleted -triple x86_64-linux-gnu
+// RUN: %clang_cc1 -verify=expected,until26 -std=c++11 %s -Wno-defaulted-function-deleted -triple x86_64-linux-gnu
+// RUN: %clang_cc1 -verify=expected -std=c++26 %s -Wno-defaulted-function-deleted -triple x86_64-linux-gnu
struct NonTrivDtor {
~NonTrivDtor();
};
struct DeletedDtor {
- ~DeletedDtor() = delete; // expected-note 5 {{deleted here}}
+ ~DeletedDtor() = delete; // expected-note 4+ {{deleted here}}
};
class InaccessibleDtor {
~InaccessibleDtor() = default;
@@ -16,28 +17,28 @@ class InaccessibleDtor {
// destructor.
union A1 {
A1();
- NonTrivDtor n; // expected-note {{destructor of 'A1' is implicitly deleted because variant field 'n' has a non-trivial destructor}}
+ NonTrivDtor n; // until26-note {{destructor of 'A1' is implicitly deleted because variant field 'n' has a non-trivial destructor}}
};
-A1 a1; // expected-error {{deleted function}}
+A1 a1; // until26-error {{deleted function}}
struct A2 {
A2();
union {
- NonTrivDtor n; // expected-note {{because variant field 'n' has a non-trivial destructor}}
+ NonTrivDtor n; // until26-note {{because variant field 'n' has a non-trivial destructor}}
};
};
-A2 a2; // expected-error {{deleted function}}
+A2 a2; // until26-error {{deleted function}}
union A3 {
A3();
- NonTrivDtor n[3]; // expected-note {{because variant field 'n' has a non-trivial destructor}}
+ NonTrivDtor n[3]; // until26-note {{because variant field 'n' has a non-trivial destructor}}
};
-A3 a3; // expected-error {{deleted function}}
+A3 a3; // until26-error {{deleted function}}
struct A4 {
A4();
union {
- NonTrivDtor n[3]; // expected-note {{because variant field 'n' has a non-trivial destructor}}
+ NonTrivDtor n[3]; // until26-note {{because variant field 'n' has a non-trivial destructor}}
};
};
-A4 a4; // expected-error {{deleted function}}
+A4 a4; // until26-error {{deleted function}}
// -- any of the non-static data members has class type M (or array thereof) and
// M has a deleted or inaccessible destructor.
@@ -63,18 +64,18 @@ struct B4 {
B4 b4; // expected-error {{deleted function}}
union B5 {
B5();
- union { // expected-note-re {{because field 'B5::(anonymous union at {{.+}})' has a deleted destructor}}
- DeletedDtor a; // expected-note {{because field 'a' has a deleted destructor}}
+ union { // until26-note-re {{because field 'B5::(anonymous union at {{.+}})' has a deleted destructor}}
+ DeletedDtor a; // until26-note {{because field 'a' has a deleted destructor}}
};
};
-B5 b5; // expected-error {{deleted function}}
+B5 b5; // until26-error {{deleted function}}
union B6 {
B6();
- union { // expected-note-re {{because field 'B6::(anonymous union at {{.+}})' has a deleted destructor}}
- InaccessibleDtor a; // expected-note {{because field 'a' has an inaccessible destructor}}
+ union { // until26-note-re {{because field 'B6::(anonymous union at {{.+}})' has a deleted destructor}}
+ InaccessibleDtor a; // until26-note {{because field 'a' has an inaccessible destructor}}
};
};
-B6 b6; // expected-error {{deleted function}}
+B6 b6; // until26-error {{deleted function}}
// -- any direct or virtual base class has a deleted or inaccessible destructor.
struct C1 : DeletedDtor { C1(); } c1; // expected-error {{deleted function}} expected-note {{base class 'DeletedDtor' has a deleted destructor}}
|
Endilll
reviewed
Jul 3, 2025
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This commit makes union construction/destruction trivial by default, but does not add the lifetime aspects of the paper — am holding out until P3726R0 will be discussed which alters that approach somewhat based on feedback from Richard Smith. But the constructor/destructor part is common regardless and also simpler.