https://github.com/snarang181 updated
https://github.com/llvm/llvm-project/pull/144161
>From 10b723750e120420970f864ebda05e0495d4632f Mon Sep 17 00:00:00 2001
From: Samarth Narang <snar...@umass.edu>
Date: Fri, 13 Jun 2025 23:22:18 +0200
Subject: [PATCH 1/2] Implement diagnostics for why `std::is_standard_layout`
is false
---
.../clang/Basic/DiagnosticSemaKinds.td | 9 +-
clang/lib/Sema/SemaTypeTraits.cpp | 132 ++++++++++++++++++
.../type-traits-unsatisfied-diags-std.cpp | 50 +++++++
.../SemaCXX/type-traits-unsatisfied-diags.cpp | 80 +++++++++++
4 files changed, 270 insertions(+), 1 deletion(-)
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td
b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 8fe7ad6138aa0..8660ba6231998 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -1767,7 +1767,8 @@ def note_unsatisfied_trait
: Note<"%0 is not %enum_select<TraitName>{"
"%TriviallyRelocatable{trivially relocatable}|"
"%Replaceable{replaceable}|"
- "%TriviallyCopyable{trivially copyable}"
+ "%TriviallyCopyable{trivially copyable}|"
+ "%StandardLayout{standard-layout}"
"}1">;
def note_unsatisfied_trait_reason
@@ -1787,6 +1788,12 @@ def note_unsatisfied_trait_reason
"%NonReplaceableField{has a non-replaceable member %1 of type %2}|"
"%NTCBase{has a non-trivially-copyable base %1}|"
"%NTCField{has a non-trivially-copyable member %1 of type %2}|"
+ "%NonStdLayoutBase{has a non-standard-layout base %1}|"
+ "%MixedAccess{has mixed access specifiers}|"
+ "%MultipleDataBase{has multiple base classes with data members}|"
+ "%VirtualFunction{has virtual functions}|"
+ "%NonStdLayoutMember{has a non-standard-layout member %1 of type
%2}|"
+ "%IndirectBaseWithFields{has an indirect base %1 with data
members}|"
"%DeletedDtr{has a %select{deleted|user-provided}1 destructor}|"
"%UserProvidedCtr{has a user provided %select{copy|move}1 "
"constructor}|"
diff --git a/clang/lib/Sema/SemaTypeTraits.cpp
b/clang/lib/Sema/SemaTypeTraits.cpp
index 1738ab4466001..ef9f48918f5dc 100644
--- a/clang/lib/Sema/SemaTypeTraits.cpp
+++ b/clang/lib/Sema/SemaTypeTraits.cpp
@@ -1947,6 +1947,7 @@ static std::optional<TypeTrait>
StdNameToTypeTrait(StringRef Name) {
TypeTrait::UTT_IsCppTriviallyRelocatable)
.Case("is_replaceable", TypeTrait::UTT_IsReplaceable)
.Case("is_trivially_copyable", TypeTrait::UTT_IsTriviallyCopyable)
+ .Case("is_standard_layout", TypeTrait::UTT_IsStandardLayout)
.Default(std::nullopt);
}
@@ -2276,6 +2277,134 @@ static void DiagnoseNonTriviallyCopyableReason(Sema
&SemaRef,
SemaRef.Diag(D->getLocation(), diag::note_defined_here) << D;
}
+static bool hasMixedAccessSpecifier(const CXXRecordDecl *D) {
+ AccessSpecifier FirstAccess = AS_none;
+ for (const FieldDecl *Field : D->fields()) {
+
+ if (Field->isUnnamedBitField())
+ continue;
+ AccessSpecifier FieldAccess = Field->getAccess();
+ if (FirstAccess == AS_none) {
+ FirstAccess = FieldAccess;
+ } else if (FieldAccess != FirstAccess) {
+ return true;
+ }
+ }
+ return false;
+}
+
+static bool hasMultipleDataBaseClassesWithFields(const CXXRecordDecl *D) {
+ int NumBasesWithFields = 0;
+ for (const CXXBaseSpecifier &Base : D->bases()) {
+ const CXXRecordDecl *BaseRD = Base.getType()->getAsCXXRecordDecl();
+ if (!BaseRD || BaseRD->isInvalidDecl())
+ continue;
+
+ for (const FieldDecl *Field : BaseRD->fields()) {
+ if (!Field->isUnnamedBitField()) {
+ ++NumBasesWithFields;
+ break; // Only count the base once.
+ }
+ }
+ }
+ return NumBasesWithFields > 1;
+}
+
+static void DiagnoseNonStandardLayoutReason(Sema &SemaRef, SourceLocation Loc,
+ const CXXRecordDecl *D) {
+ for (const CXXBaseSpecifier &B : D->bases()) {
+ assert(B.getType()->getAsCXXRecordDecl() && "invalid base?");
+ if (B.isVirtual()) {
+ SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+ << diag::TraitNotSatisfiedReason::VBase << B.getType()
+ << B.getSourceRange();
+ }
+ if (!B.getType()->isStandardLayoutType()) {
+ SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+ << diag::TraitNotSatisfiedReason::NonStdLayoutBase << B.getType()
+ << B.getSourceRange();
+ }
+ }
+ if (hasMixedAccessSpecifier(D)) {
+ SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+ << diag::TraitNotSatisfiedReason::MixedAccess;
+ }
+ if (hasMultipleDataBaseClassesWithFields(D)) {
+ SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+ << diag::TraitNotSatisfiedReason::MultipleDataBase;
+ }
+ if (D->isPolymorphic()) {
+ SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+ << diag::TraitNotSatisfiedReason::VirtualFunction;
+ }
+ for (const FieldDecl *Field : D->fields()) {
+ if (!Field->getType()->isStandardLayoutType()) {
+ SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+ << diag::TraitNotSatisfiedReason::NonStdLayoutMember << Field
+ << Field->getType() << Field->getSourceRange();
+ }
+ }
+
+ // if this class and an indirect base
+ // both have non-static data members, grab the first such base.
+ if (D->hasDirectFields()) {
+ SmallVector<const CXXRecordDecl *, 4> Records;
+
+ // Recursive lambda to collect all bases that declare fields
+ std::function<void(const CXXRecordDecl *)> collect =
+ [&](const CXXRecordDecl *R) {
+ for (const CXXBaseSpecifier &B : R->bases()) {
+ const auto *BR = B.getType()->getAsCXXRecordDecl();
+ if (!BR || !BR->hasDefinition())
+ continue;
+ if (BR->hasDirectFields())
+ Records.push_back(BR);
+ // Recurse into the base class.
+ collect(BR);
+ }
+ };
+
+ // Collect all bases that declare fields.
+ collect(D);
+
+ // If more than one record has fields, then the layout is non-standard.
+ if (!Records.empty()) {
+ const CXXRecordDecl *Indirect = Records.front();
+ SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+ << diag::TraitNotSatisfiedReason::IndirectBaseWithFields << Indirect
+ << Indirect->getSourceRange();
+ }
+ }
+}
+
+static void DiagnoseNonStandardLayoutReason(Sema &SemaRef, SourceLocation Loc,
+ QualType T) {
+ SemaRef.Diag(Loc, diag::note_unsatisfied_trait)
+ << T << diag::TraitName::StandardLayout;
+
+ // Check type-level exclusion first
+ if (T->isVariablyModifiedType()) {
+ SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+ << diag::TraitNotSatisfiedReason::VLA;
+ return;
+ }
+
+ if (T->isReferenceType()) {
+ SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+ << diag::TraitNotSatisfiedReason::Ref;
+ return;
+ }
+ T = T.getNonReferenceType();
+ const CXXRecordDecl *D = T->getAsCXXRecordDecl();
+ if (!D || D->isInvalidDecl())
+ return;
+
+ if (D->hasDefinition())
+ DiagnoseNonStandardLayoutReason(SemaRef, Loc, D);
+
+ SemaRef.Diag(D->getLocation(), diag::note_defined_here) << D;
+}
+
void Sema::DiagnoseTypeTraitDetails(const Expr *E) {
E = E->IgnoreParenImpCasts();
if (E->containsErrors())
@@ -2296,6 +2425,9 @@ void Sema::DiagnoseTypeTraitDetails(const Expr *E) {
case UTT_IsTriviallyCopyable:
DiagnoseNonTriviallyCopyableReason(*this, E->getBeginLoc(), Args[0]);
break;
+ case UTT_IsStandardLayout:
+ DiagnoseNonStandardLayoutReason(*this, E->getBeginLoc(), Args[0]);
+ break;
default:
break;
}
diff --git a/clang/test/SemaCXX/type-traits-unsatisfied-diags-std.cpp
b/clang/test/SemaCXX/type-traits-unsatisfied-diags-std.cpp
index 329b611110c1d..59f3d95a4d215 100644
--- a/clang/test/SemaCXX/type-traits-unsatisfied-diags-std.cpp
+++ b/clang/test/SemaCXX/type-traits-unsatisfied-diags-std.cpp
@@ -20,6 +20,13 @@ struct is_trivially_copyable {
template <typename T>
constexpr bool is_trivially_copyable_v = __is_trivially_copyable(T);
+
+template <typename T>
+struct is_standard_layout {
+static constexpr bool value = __is_standard_layout(T);
+};
+template <typename T>
+constexpr bool is_standard_layout_v = __is_standard_layout(T);
#endif
#ifdef STD2
@@ -44,6 +51,17 @@ using is_trivially_copyable =
__details_is_trivially_copyable<T>;
template <typename T>
constexpr bool is_trivially_copyable_v = __is_trivially_copyable(T);
+
+template <typename T>
+struct __details_is_standard_layout {
+static constexpr bool value = __is_standard_layout(T);
+
+
+};
+template <typename T>
+using is_standard_layout = __details_is_standard_layout<T>;
+template <typename T>
+constexpr bool is_standard_layout_v = __is_standard_layout(T);
#endif
@@ -73,6 +91,13 @@ using is_trivially_copyable =
__details_is_trivially_copyable<T>;
template <typename T>
constexpr bool is_trivially_copyable_v = is_trivially_copyable<T>::value;
+
+template <typename T>
+struct __details_is_standard_layout : bool_constant<__is_standard_layout(T)>
{};
+template <typename T>
+using is_standard_layout = __details_is_standard_layout<T>;
+template <typename T>
+constexpr bool is_standard_layout_v = is_standard_layout<T>::value;
#endif
}
@@ -100,6 +125,21 @@ static_assert(std::is_trivially_copyable_v<int&>);
// expected-note@-1 {{because it is a reference type}}
+ // Direct tests
+ static_assert(std::is_standard_layout<int>::value);
+ static_assert(std::is_standard_layout_v<int>);
+
+ static_assert(std::is_standard_layout<int&>::value);
+ // expected-error-re@-1 {{static assertion failed due to requirement
'std::{{.*}}is_standard_layout<int &>::value'}} \
+ // expected-note@-1 {{'int &' is not standard-layout}} \
+ // expected-note@-1 {{because it is a reference type}}
+
+ static_assert(std::is_standard_layout_v<int&>);
+ // expected-error@-1 {{static assertion failed due to requirement
'std::is_standard_layout_v<int &>'}} \
+ // expected-note@-1 {{'int &' is not standard-layout}} \
+ // expected-note@-1 {{because it is a reference type}}
+
+
namespace test_namespace {
using namespace std;
static_assert(is_trivially_relocatable<int&>::value);
@@ -119,6 +159,16 @@ namespace test_namespace {
// expected-error@-1 {{static assertion failed due to requirement
'is_trivially_copyable_v<int &>'}} \
// expected-note@-1 {{'int &' is not trivially copyable}} \
// expected-note@-1 {{because it is a reference type}}
+
+ static_assert(is_standard_layout<int&>::value);
+ // expected-error-re@-1 {{static assertion failed due to requirement
'{{.*}}is_standard_layout<int &>::value'}} \
+ // expected-note@-1 {{'int &' is not standard-layout}} \
+ // expected-note@-1 {{because it is a reference type}}
+
+ static_assert(is_standard_layout_v<int&>);
+ // expected-error@-1 {{static assertion failed due to requirement
'is_standard_layout_v<int &>'}} \
+ // expected-note@-1 {{'int &' is not standard-layout}} \
+ // expected-note@-1 {{because it is a reference type}}
}
diff --git a/clang/test/SemaCXX/type-traits-unsatisfied-diags.cpp
b/clang/test/SemaCXX/type-traits-unsatisfied-diags.cpp
index a8c78f6304ca9..404f6e914b3f6 100644
--- a/clang/test/SemaCXX/type-traits-unsatisfied-diags.cpp
+++ b/clang/test/SemaCXX/type-traits-unsatisfied-diags.cpp
@@ -488,3 +488,83 @@ static_assert(__is_trivially_copyable(S12));
// expected-note@-1 {{'S12' is not trivially copyable}} \
// expected-note@#tc-S12 {{'S12' defined here}}
}
+
+namespace standard_layout_tests {
+struct WithVirtual { // #sl-Virtual
+ virtual void foo();
+};
+static_assert(__is_standard_layout(WithVirtual));
+// expected-error@-1 {{static assertion failed due to requirement
'__is_standard_layout(standard_layout_tests::WithVirtual)'}} \
+// expected-note@-1 {{'WithVirtual' is not standard-layout}} \
+// expected-note@-1 {{because it has virtual functions}} \
+// expected-note@#sl-Virtual {{'WithVirtual' defined here}}
+
+struct MixedAccess { // #sl-Mixed
+public:
+ int a;
+private:
+ int b;
+};
+static_assert(__is_standard_layout(MixedAccess));
+// expected-error@-1 {{static assertion failed due to requirement
'__is_standard_layout(standard_layout_tests::MixedAccess)'}} \
+// expected-note@-1 {{'MixedAccess' is not standard-layout}} \
+// expected-note@-1 {{because it has mixed access specifiers}} \
+// expected-note@#sl-Mixed {{'MixedAccess' defined here}}
+
+struct VirtualBase { virtual ~VirtualBase(); }; //
#sl-VirtualBase
+struct VB : virtual VirtualBase {}; // #sl-VB
+static_assert(__is_standard_layout(VB));
+// expected-error@-1 {{static assertion failed due to requirement
'__is_standard_layout(standard_layout_tests::VB)'}} \
+// expected-note@-1 {{'VB' is not standard-layout}} \
+// expected-note@-1 {{because it has a virtual base 'VirtualBase'}} \
+// expected-note@-1 {{because it has a non-standard-layout base
'VirtualBase'}} \
+// expected-note@-1 {{because it has virtual functions}}
+// expected-note@#sl-VB {{'VB' defined here}}
+
+union U { // #sl-U
+public:
+ int x;
+private:
+ int y;
+};
+static_assert(__is_standard_layout(U));
+// expected-error@-1 {{static assertion failed due to requirement
'__is_standard_layout(standard_layout_tests::U)'}} \
+// expected-note@-1 {{'U' is not standard-layout}} \
+// expected-note@-1 {{because it has mixed access specifiers}}
+// expected-note@#sl-U {{'U' defined here}}
+
+// Single base class is OK
+struct BaseClass{ int a; }; // #sl-BaseClass
+struct DerivedOK : BaseClass {}; //
#sl-DerivedOK
+static_assert(__is_standard_layout(DerivedOK));
+
+// Primitive types should be standard layout
+static_assert(__is_standard_layout(int)); // #sl-Int
+static_assert(__is_standard_layout(float)); // #sl-Float
+
+// Multi-level inheritance: Non-standard layout
+struct Base1 { int a; }; // #sl-Base1
+struct Base2 { int b; }; // #sl-Base2
+struct DerivedClass : Base1, Base2 {}; //
#sl-DerivedClass
+static_assert(__is_standard_layout(DerivedClass));
+// expected-error@-1 {{static assertion failed due to requirement
'__is_standard_layout(standard_layout_tests::DerivedClass)'}} \
+// expected-note@-1 {{'DerivedClass' is not standard-layout}} \
+// expected-note@-1 {{because it has multiple base classes with data members}}
\
+// expected-note@#sl-DerivedClass {{'DerivedClass' defined here}}
+
+// Inheritance hierarchy with multiple classes having data members
+struct BaseA { int a; }; // #sl-BaseA
+struct BaseB : BaseA {}; // inherits
BaseA, has no new members
+struct BaseC: BaseB { int c; }; // #sl-BaseC
+static_assert(__is_standard_layout(BaseC));
+// expected-error@-1 {{static assertion failed due to requirement
'__is_standard_layout(standard_layout_tests::BaseC)'}} \
+// expected-note@-1 {{'BaseC' is not standard-layout}} \
+// expected-note@-1 {{because it has an indirect base 'BaseA' with data
members}} \
+// expected-note@#sl-BaseC {{'BaseC' defined here}} \
+// Multiple direct base classes with no data members --> standard layout
+struct BaseX {}; // #sl-BaseX
+struct BaseY {}; // #sl-BaseY
+struct MultiBase : BaseX, BaseY {}; // #sl-MultiBase
+static_assert(__is_standard_layout(MultiBase));
+
+ }
>From 46f2a26b05a4cef58de1102c03842af92f62f3dc Mon Sep 17 00:00:00 2001
From: Samarth Narang <snar...@umass.edu>
Date: Sat, 14 Jun 2025 08:56:44 +0200
Subject: [PATCH 2/2] Replace hand-written recursion with forAllBases
---
clang/lib/Sema/SemaTypeTraits.cpp | 35 +++++++++----------------------
1 file changed, 10 insertions(+), 25 deletions(-)
diff --git a/clang/lib/Sema/SemaTypeTraits.cpp
b/clang/lib/Sema/SemaTypeTraits.cpp
index ef9f48918f5dc..08682cc09f350 100644
--- a/clang/lib/Sema/SemaTypeTraits.cpp
+++ b/clang/lib/Sema/SemaTypeTraits.cpp
@@ -2344,32 +2344,17 @@ static void DiagnoseNonStandardLayoutReason(Sema
&SemaRef, SourceLocation Loc,
<< Field->getType() << Field->getSourceRange();
}
}
-
- // if this class and an indirect base
- // both have non-static data members, grab the first such base.
+ // find any indirect base classes that have fields
if (D->hasDirectFields()) {
- SmallVector<const CXXRecordDecl *, 4> Records;
-
- // Recursive lambda to collect all bases that declare fields
- std::function<void(const CXXRecordDecl *)> collect =
- [&](const CXXRecordDecl *R) {
- for (const CXXBaseSpecifier &B : R->bases()) {
- const auto *BR = B.getType()->getAsCXXRecordDecl();
- if (!BR || !BR->hasDefinition())
- continue;
- if (BR->hasDirectFields())
- Records.push_back(BR);
- // Recurse into the base class.
- collect(BR);
- }
- };
-
- // Collect all bases that declare fields.
- collect(D);
-
- // If more than one record has fields, then the layout is non-standard.
- if (!Records.empty()) {
- const CXXRecordDecl *Indirect = Records.front();
+ const CXXRecordDecl *Indirect = nullptr;
+ D->forallBases([&](const CXXRecordDecl *BaseDef) {
+ if (BaseDef->hasDirectFields()) {
+ Indirect = BaseDef;
+ return false; // stop traversal
+ }
+ return true; // continue to the next base
+ });
+ if (Indirect) {
SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
<< diag::TraitNotSatisfiedReason::IndirectBaseWithFields << Indirect
<< Indirect->getSourceRange();
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