implement recent "standard-layout" changes

[rsmith]

Summary

DR1672, DR1813, DR1881, DR2120: Implement recent fixes to "standard layout" rules.

The new rules say that a standard-layout struct has its first non-static data member and all base classes at offset 0, and consider a class to not be standard-layout if that would result in multiple subobjects of a single type having the same address.

We separately track the results that the "standard-layout" computation would have produced strictly following the C++11 rules, because that is part of the ABI tail-padding-reuse decision for some targets.

[rsmith]

Patch updated to track "is C++11 standard-layout class" separately from "is standard-layout class" so that we can use the former for record layout in the targets that need it. This turned out to be less ugly than I'd feared.

[rjmccall]

AST/DeclCXX.cpp

1113

IsCXX11StandardLayout should be based on FieldRec->isCXX11StandardLayout(), I assume.

[rsmith]

AST/DeclCXX.cpp

1113

Oops. Yes.

[rjmccall]

Okay. LGTM. Thank you for putting the effort into maintaining both rules simultaneously.

AST/ASTImporter.cpp

if (CXXRecordDecl *ToCXX = dyn_cast<CXXRecordDecl>(To)) {
     ToData.UserDeclaredConstructor = FromData.UserDeclaredConstructor;
     ToData.UserDeclaredSpecialMembers = FromData.UserDeclaredSpecialMembers;
     ToData.Aggregate = FromData.Aggregate;
     ToData.PlainOldData = FromData.PlainOldData;
     ToData.Empty = FromData.Empty;
     ToData.Polymorphic = FromData.Polymorphic;
     ToData.Abstract = FromData.Abstract;
     ToData.IsStandardLayout = FromData.IsStandardLayout;
-    ToData.HasNoNonEmptyBases = FromData.HasNoNonEmptyBases;
+    ToData.HasBasesWithFields = FromData.HasBasesWithFields;
     ToData.HasPrivateFields = FromData.HasPrivateFields;
     ToData.HasProtectedFields = FromData.HasProtectedFields;
     ToData.HasPublicFields = FromData.HasPublicFields;
     ToData.HasMutableFields = FromData.HasMutableFields;
     ToData.HasVariantMembers = FromData.HasVariantMembers;
     ToData.HasOnlyCMembers = FromData.HasOnlyCMembers;
     ToData.HasInClassInitializer = FromData.HasInClassInitializer;
     ToData.HasUninitializedReferenceMember

AST/DeclCXX.cpp

for (ASTUnresolvedSet::iterator I = Impl.begin(); I != Impl.end(); ++I)
     I.setDecl(cast<NamedDecl>(Source->GetExternalDecl(
         reinterpret_cast<uintptr_t>(I.getDecl()) >> 2)));
   Impl.Decls.setLazy(false);
 }
 
 CXXRecordDecl::DefinitionData::DefinitionData(CXXRecordDecl *D)
     : UserDeclaredConstructor(false), UserDeclaredSpecialMembers(0),
       Aggregate(true), PlainOldData(true), Empty(true), Polymorphic(false),
-      Abstract(false), IsStandardLayout(true), HasNoNonEmptyBases(true),
+      Abstract(false), IsStandardLayout(true), HasBasesWithFields(false),
       HasPrivateFields(false), HasProtectedFields(false),
       HasPublicFields(false), HasMutableFields(false), HasVariantMembers(false),
       HasOnlyCMembers(true), HasInClassInitializer(false),
       HasUninitializedReferenceMember(false), HasUninitializedFields(false),
       HasInheritedConstructor(false), HasInheritedAssignment(false),
       NeedOverloadResolutionForCopyConstructor(false),
       NeedOverloadResolutionForMoveConstructor(false),
       NeedOverloadResolutionForMoveAssignment(false),

 CXXRecordDecl::CreateDeserialized(const ASTContext &C, unsigned ID) {
   CXXRecordDecl *R = new (C, ID) CXXRecordDecl(
       CXXRecord, TTK_Struct, C, nullptr, SourceLocation(), SourceLocation(),
       nullptr, nullptr);
   R->MayHaveOutOfDateDef = false;
   return R;
 }
 
+/// Determine whether a class has a repeated base class. This is intended for
+/// use when determining if a class is standard-layout, so makes no attempt to
+/// handle virtual bases.
+static bool hasRepeatedBaseClass(const CXXRecordDecl *StartRD) {
+  llvm::SmallPtrSet<const CXXRecordDecl*, 8> SeenBaseTypes;
+  SmallVector<const CXXRecordDecl*, 8> WorkList = {StartRD};
+  while (!WorkList.empty()) {
+    const CXXRecordDecl *RD = WorkList.pop_back_val();
+    for (const CXXBaseSpecifier &BaseSpec : RD->bases()) {
+      if (const CXXRecordDecl *B = BaseSpec.getType()->getAsCXXRecordDecl()) {
+        if (!SeenBaseTypes.insert(B).second)
+          return true;
+        WorkList.push_back(B);
+      }
+    }
+  }
+  return false;
+}
+
 void
 CXXRecordDecl::setBases(CXXBaseSpecifier const * const *Bases,
                         unsigned NumBases) {
   ASTContext &C = getASTContext();
 
   if (!data().Bases.isOffset() && data().NumBases > 0)
     C.Deallocate(data().getBases());
 
for (unsigned i = 0; i < NumBases; ++i) {
     const CXXBaseSpecifier *Base = Bases[i];
     QualType BaseType = Base->getType();
     // Skip dependent types; we can't do any checking on them now.
     if (BaseType->isDependentType())
       continue;
     CXXRecordDecl *BaseClassDecl
       = cast<CXXRecordDecl>(BaseType->getAs<RecordType>()->getDecl());
 
-    if (!BaseClassDecl->isEmpty()) {
-      if (!data().Empty) {
-        // C++0x [class]p7:
-        //   A standard-layout class is a class that:
-        //    [...]
-        //    -- either has no non-static data members in the most derived
-        //       class and at most one base class with non-static data members,
-        //       or has no base classes with non-static data members, and
-        // If this is the second non-empty base, then neither of these two
-        // clauses can be true.
+    // [DR1813] In Clang 6 and earlier, we allowed length 0 bit-fields in
+    // base classes of a standard-layout class.
+    if (BaseClassDecl->data().HasBasesWithFields ||
+        (C.getLangOpts().getClangABICompat() <= LangOptions::ClangABI::Ver6
+             ? !BaseClassDecl->data().Empty
+             : !BaseClassDecl->field_empty())) {
+      // C++2a [class]p7:
+      //    -- has all non-static data members and bit-fields in the class and
+      //       its base classes first declared in the same class
+      if (data().HasBasesWithFields)
+        // Two bases have members or bit-fields: not standard-layout.
         data().IsStandardLayout = false;
-      }
+      data().HasBasesWithFields = true;
 
+    if (!BaseClassDecl->isEmpty()) {
       // C++14 [meta.unary.prop]p4:
       //   T is a class type [...] with [...] no base class B for which
       //   is_empty<B>::value is false.
       data().Empty = false;
-      data().HasNoNonEmptyBases = false;
     }
-    
     // C++1z [dcl.init.agg]p1:
     //   An aggregate is a class with [...] no private or protected base classes
     if (Base->getAccessSpecifier() != AS_public)
       data().Aggregate = false;
 
     // C++ [class.virtual]p1:
     //   A class that declares or inherits a virtual function is called a 
     //   polymorphic class.
for (unsigned i = 0; i < NumBases; ++i) {
     if (BaseClassDecl->hasUninitializedReferenceMember())
       data().HasUninitializedReferenceMember = true;
 
     if (!BaseClassDecl->allowConstDefaultInit())
       data().HasUninitializedFields = true;
 
     addedClassSubobject(BaseClassDecl);
   }
-  
+  // C++2a [class]p7:
+  //   A class S is a standard-layout class if it:
+  //     -- has at most one base class subobject of any given type
+  //
+  // Note that we only need to check this for classes with more than one base
+  // class. If there's only one base class, and it's standard layout, then
+  // we know there are no repeated base classes.
+  if (data().IsStandardLayout && NumBases > 1 &&
+      (C.getLangOpts().getClangABICompat() > LangOptions::ClangABI::Ver6) &&
+      hasRepeatedBaseClass(this))
+    data().IsStandardLayout = false;
+  
   if (VBases.empty()) {
     data().IsParsingBaseSpecifiers = false;
     return;
   }
 
   // Create base specifier for any direct or indirect virtual bases.
   data().VBases = new (C) CXXBaseSpecifier[VBases.size()];
   data().NumVBases = VBases.size();

 }
 
 void CXXRecordDecl::markedVirtualFunctionPure() {
   // C++ [class.abstract]p2: 
   //   A class is abstract if it has at least one pure virtual function.
   data().Abstract = true;
 }
 
+bool CXXRecordDecl::hasSubobjectAtOffsetZeroOfEmptyBaseType(
+    ASTContext &Ctx, const CXXRecordDecl *XFirst) {
+  if (!getNumBases())
+    return false;
+
+  llvm::SmallPtrSet<const CXXRecordDecl*, 8> Bases;
+  llvm::SmallPtrSet<const CXXRecordDecl*, 8> M;
+  SmallVector<const CXXRecordDecl*, 8> WorkList;
+
+  // Visit a type that we have determined is an element of M(S).
+  auto Visit = [&](const CXXRecordDecl *RD) -> bool {
+    RD = RD->getCanonicalDecl();
+
+    // C++2a [class]p8:
+    //   A class S is a standard-layout class if it [...] has no element of the
+    //   set M(S) of types as a base class.
+    //
+    // If we find a subobject of an empty type, it might also be a base class,
+    // so we'll need to walk the base classes to check.
+    if (!RD->data().HasBasesWithFields) {
+      // Walk the bases the first time, stopping if we find the type. Build a
+      // set of them so we don't need to walk them again.
+      if (Bases.empty()) {
+        bool RDIsBase = !forallBases([&](const CXXRecordDecl *Base) -> bool {
+          Base = Base->getCanonicalDecl();
+          if (RD == Base)
+            return false;
+          Bases.insert(Base);
+          return true;
+        });
+        if (RDIsBase)
+          return true;
+      } else {
+        if (Bases.count(RD))
+          return true;
+      }
+    }
+
+    if (M.insert(RD).second)
+      WorkList.push_back(RD);
+    return false;
+  };
+
+  if (Visit(XFirst))
+    return true;
+
+  while (!WorkList.empty()) {
+    const CXXRecordDecl *X = WorkList.pop_back_val();
+
+    // FIXME: We don't check the bases of X. That matches the standard, but
+    // that sure looks like a wording bug.
+
+    //   -- If X is a non-union class type with a non-static data member
+    //      [recurse to] the first non-static data member of X
+    //   -- If X is a union type, [recurse to union members]
+    for (auto *FD : X->fields()) {
+      // FIXME: Should we really care about the type of the first non-static
+      // data member of a non-union if there are preceding unnamed bit-fields?
+      if (FD->isUnnamedBitfield())
+        continue;
+
+      //   -- If X is n array type, [visit the element type]
+      QualType T = Ctx.getBaseElementType(FD->getType());
+      if (auto *RD = T->getAsCXXRecordDecl())
+        if (Visit(RD))
+          return true;
+
+      if (!X->isUnion())
+        break;
+    }
+  }
+
+  return false;
+}
+
 void CXXRecordDecl::addedMember(Decl *D) {
   if (!D->isImplicit() &&
       !isa<FieldDecl>(D) &&
       !isa<IndirectFieldDecl>(D) &&
       (!isa<TagDecl>(D) || cast<TagDecl>(D)->getTagKind() == TTK_Class ||
         cast<TagDecl>(D)->getTagKind() == TTK_Interface))
     data().HasOnlyCMembers = false;
 
if (SMKind) {
         // This is an extension in C++03.
         data().PlainOldData = false;
       }
     }
 
     return;
   }
 
+  ASTContext &Context = getASTContext();
+
   // Handle non-static data members.
   if (FieldDecl *Field = dyn_cast<FieldDecl>(D)) {
+    // C++2a [class]p7:
+    //   A standard-layout class is a class that:
+    //    [...]
+    //    -- has all non-static data members and bit-fields in the class and
+    //       its base classes first declared in the same class
+    if (data().HasBasesWithFields &&
+        (Context.getLangOpts().getClangABICompat() >
+             LangOptions::ClangABI::Ver6 ||
+         !Field->isUnnamedBitfield()))
+      data().IsStandardLayout = false;
+
     // C++ [class.bit]p2:
     //   A declaration for a bit-field that omits the identifier declares an 
     //   unnamed bit-field. Unnamed bit-fields are not members and cannot be 
     //   initialized.
     if (Field->isUnnamedBitfield())
       return;
     
     // C++ [dcl.init.aggr]p1:
     //   An aggregate is an array or a class (clause 9) with [...] no
     //   private or protected non-static data members (clause 11).
     //
     // A POD must be an aggregate.    
     if (D->getAccess() == AS_private || D->getAccess() == AS_protected) {
       data().Aggregate = false;
       data().PlainOldData = false;
     }
 
+    // Track whether this is the first field. We use this when checking
+    // whether the class is standard-layout below.
+    bool IsFirstField = !data().HasPrivateFields &&
+                        !data().HasProtectedFields && !data().HasPublicFields;
+
     // C++0x [class]p7:
     //   A standard-layout class is a class that:
     //    [...]
     //    -- has the same access control for all non-static data members,
     switch (D->getAccess()) {
     case AS_private:    data().HasPrivateFields = true;   break;
     case AS_protected:  data().HasProtectedFields = true; break;
     case AS_public:     data().HasPublicFields = true;    break;
if (FieldDecl *Field = dyn_cast<FieldDecl>(D)) {
 
     // C++0x [class]p9:
     //   A POD struct is a class that is both a trivial class and a 
     //   standard-layout class, and has no non-static data members of type 
     //   non-POD struct, non-POD union (or array of such types).
     //
     // Automatic Reference Counting: the presence of a member of Objective-C pointer type
     // that does not explicitly have no lifetime makes the class a non-POD.
-    ASTContext &Context = getASTContext();
     QualType T = Context.getBaseElementType(Field->getType());
     if (T->isObjCRetainableType() || T.isObjCGCStrong()) {
       if (T.hasNonTrivialObjCLifetime()) {
         // Objective-C Automatic Reference Counting:
         //   If a class has a non-static data member of Objective-C pointer
         //   type (or array thereof), it is a non-POD type and its
         //   default constructor (if any), copy constructor, move constructor,
         //   copy assignment operator, move assignment operator, and destructor are
if (const RecordType *RecordTy = T->getAs<RecordType>()) {
           setHasVolatileMember(true);
 
         // C++0x [class]p7:
         //   A standard-layout class is a class that:
         //    -- has no non-static data members of type non-standard-layout
         //       class (or array of such types) [...]
         if (!FieldRec->isStandardLayout())
           data().IsStandardLayout = false;
 
-        // C++0x [class]p7:
+        // C++2a [class]p7:
         //   A standard-layout class is a class that:
         //    [...]
-        //    -- has no base classes of the same type as the first non-static
-        //       data member.
-        // We don't want to expend bits in the state of the record decl
-        // tracking whether this is the first non-static data member so we
-        // cheat a bit and use some of the existing state: the empty bit.
-        // Virtual bases and virtual methods make a class non-empty, but they
-        // also make it non-standard-layout so we needn't check here.
-        // A non-empty base class may leave the class standard-layout, but not
-        // if we have arrived here, and have at least one non-static data
-        // member. If IsStandardLayout remains true, then the first non-static
-        // data member must come through here with Empty still true, and Empty
-        // will subsequently be set to false below.
-        if (data().IsStandardLayout && data().Empty) {
+        //    -- has no element of the set M(S) of types as a base class.
+        if (data().IsStandardLayout) {
+          if (Context.getLangOpts().getClangABICompat() >
+              LangOptions::ClangABI::Ver6) {
+            if ((isUnion() || IsFirstField) &&
+                hasSubobjectAtOffsetZeroOfEmptyBaseType(Context, FieldRec))
+              data().IsStandardLayout = false;
+          } else {
+            // Clang 6 compatibility: ignore all but the first field of unions,
+            // don't check for subobject collisions.
+            if (data().Empty) {
               for (const auto &BI : bases()) {
                 if (Context.hasSameUnqualifiedType(BI.getType(), T)) {
                   data().IsStandardLayout = false;
                   break;
                 }
               }
             }
-        
+          }
+        }
         // Keep track of the presence of mutable fields.
         if (FieldRec->hasMutableFields()) {
           data().HasMutableFields = true;
           data().NeedOverloadResolutionForCopyConstructor = true;
         }
 
         // C++11 [class.copy]p13:
         //   If the implicitly-defined constructor would satisfy the
if (const RecordType *RecordTy = T->getAs<RecordType>()) {
       //   A defaulted copy/move assignment operator for a class X is defined
       //   as deleted if X has:
       //    -- a non-static data member of const non-class type (or array
       //       thereof)
       if (T.isConstQualified())
         data().DefaultedMoveAssignmentIsDeleted = true;
     }
 
-    // C++0x [class]p7:
-    //   A standard-layout class is a class that:
-    //    [...]
-    //    -- either has no non-static data members in the most derived
-    //       class and at most one base class with non-static data members,
-    //       or has no base classes with non-static data members, and
-    // At this point we know that we have a non-static data member, so the last
-    // clause holds.
-    if (!data().HasNoNonEmptyBases)
-      data().IsStandardLayout = false;
-
     // C++14 [meta.unary.prop]p4:
     //   T is a class type [...] with [...] no non-static data members other
     //   than bit-fields of length 0...
     if (data().Empty) {
       if (!Field->isZeroLengthBitField(Context))
         data().Empty = false;
     }
   }

CXX/drs/dr14xx.cpp

 // RUN: %clang_cc1 -std=c++98 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++11 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++14 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++1z %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 
 #if __cplusplus < 201103L
 // expected-no-diagnostics
 #endif
 
+// dr1425: na abi
+
 namespace dr1460 { // dr1460: 3.5
 #if __cplusplus >= 201103L
   namespace DRExample {
     union A {
       union {};
       union {};
       constexpr A() {}
     };

CXX/drs/dr16xx.cpp

 // RUN: %clang_cc1 -std=c++98 -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++11 -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++14 -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++1z -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 
+#if __cplusplus < 201103L
+// expected-error@+1 {{variadic macro}}
+#define static_assert(...) __extension__ _Static_assert(__VA_ARGS__)
+#endif
+
 namespace dr1611 { // dr1611: dup 1658
   struct A { A(int); };
   struct B : virtual A { virtual void f() = 0; };
   struct C : B { C() : A(0) {} void f(); };
   C c;
 }
 
 namespace dr1684 { // dr1684: 3.6

 #else
     F::F(const F&) = default; // expected-error {{would delete}} expected-note@-5{{inaccessible copy constructor}}
     F::F(F&&) = default; // expected-error {{would delete}} expected-note@-6{{inaccessible move constructor}}
 #endif
   }
 
   // assignment case is superseded by dr2180
 }
+
+namespace dr1672 { // dr1672: 7
+  struct Empty {};
+  struct A : Empty {};
+  struct B { Empty e; };
+  struct C : A { B b; int n; };
+  struct D : A { int n; B b; };
+
+  static_assert(!__is_standard_layout(C), "");
+  static_assert(__is_standard_layout(D), "");
+
+  struct E { B b; int n; };
+  struct F { int n; B b; };
+  union G { B b; int n; };
+  union H { int n; B b; };
+
+  struct X {};
+  template<typename T> struct Y : X, A { T t; };
+
+  static_assert(!__is_standard_layout(Y<E>), "");
+  static_assert(__is_standard_layout(Y<F>), "");
+  static_assert(!__is_standard_layout(Y<G>), "");
+  static_assert(!__is_standard_layout(Y<H>), "");
+  static_assert(!__is_standard_layout(Y<X>), "");
+}

CXX/drs/dr18xx.cpp

 // RUN: %clang_cc1 -std=c++98 -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++11 -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++14 -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++1z -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 
 #if __cplusplus < 201103L
-// expected-no-diagnostics
+// expected-error@+1 {{variadic macro}}
+#define static_assert(...) __extension__ _Static_assert(__VA_ARGS__)
 #endif
 
+namespace dr1813 { // dr1813: 7
+  struct B { int i; };
+  struct C : B {};
+  struct D : C {};
+  struct E : D { char : 4; };
+
+  static_assert(__is_standard_layout(B), "");
+  static_assert(__is_standard_layout(C), "");
+  static_assert(__is_standard_layout(D), "");
+  static_assert(!__is_standard_layout(E), "");
+
+  struct Q {};
+  struct S : Q {};
+  struct T : Q {};
+  struct U : S, T {};
+
+  static_assert(__is_standard_layout(Q), "");
+  static_assert(__is_standard_layout(S), "");
+  static_assert(__is_standard_layout(T), "");
+  static_assert(!__is_standard_layout(U), "");
+}
+
+namespace dr1881 { // dr1881: 7
+  struct A { int a : 4; };
+  struct B : A { int b : 3; };
+  static_assert(__is_standard_layout(A), "");
+  static_assert(!__is_standard_layout(B), "");
+
+  struct C { int : 0; };
+  struct D : C { int : 0; };
+  static_assert(__is_standard_layout(C), "");
+  static_assert(!__is_standard_layout(D), "");
+}
+
 void dr1891() { // dr1891: 4
 #if __cplusplus >= 201103L
   int n;
   auto a = []{}; // expected-note 2{{candidate}} expected-note 2{{here}}
   auto b = [=]{ return n; }; // expected-note 2{{candidate}} expected-note 2{{here}}
   typedef decltype(a) A;
   typedef decltype(b) B;
 

CXX/drs/dr21xx.cpp

 // RUN: %clang_cc1 -std=c++98 -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++11 -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++14 -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++1z -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 
+#if __cplusplus < 201103L
+// expected-error@+1 {{variadic macro}}
+#define static_assert(...) __extension__ _Static_assert(__VA_ARGS__)
+#endif
+
+namespace dr2120 { // dr2120: 7
+  struct A {};
+  struct B : A {};
+  struct C { A a; };
+  struct D { C c[5]; };
+  struct E : B { D d; };
+  static_assert(__is_standard_layout(B), "");
+  static_assert(__is_standard_layout(D), "");
+  static_assert(!__is_standard_layout(E), "");
+}
+
 namespace dr2180 { // dr2180: yes
   class A {
     A &operator=(const A &); // expected-note 0-2{{here}}
     A &operator=(A &&); // expected-note 0-2{{here}} expected-error 0-1{{extension}}
   };
 
   struct B : virtual A {
     B &operator=(const B &);

CXX/drs/dr22xx.cpp

 // RUN: %clang_cc1 -std=c++98 -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++11 -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++14 -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++1z -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 
-namespace dr2229 { // dr2229: yes
+namespace dr2229 { // dr2229: 7
 struct AnonBitfieldQualifiers {
   const unsigned : 1; // expected-error {{anonymous bit-field cannot have qualifiers}}
   volatile unsigned : 1; // expected-error {{anonymous bit-field cannot have qualifiers}}
   const volatile unsigned : 1; // expected-error {{anonymous bit-field cannot have qualifiers}}
 
   unsigned : 1;
   const unsigned i1 : 1;
   volatile unsigned i2 : 1;
   const volatile unsigned i3 : 1;
 };
 }

Layout/v6-standard-layout.cpp

+// RUN: %clang_cc1 -fsyntax-only -fclang-abi-compat=6 -triple armv7k-apple-darwin-watchos -fdump-record-layouts %s | FileCheck %s --check-prefix=CHECK --check-prefix=CHECK-V6
+// RUN: %clang_cc1 -fsyntax-only -fclang-abi-compat=7 -triple armv7k-apple-darwin-watchos -fdump-record-layouts %s | FileCheck %s --check-prefix=CHECK --check-prefix=CHECK-V7
+
+// In Clang 6 and before, we determined that B was standard-layout, so we
+// didn't pack C::d into its tail padding.
+struct A { int : 4; };
+struct B : A { int n; char c[3]; };
+struct C : B { char d; };
+int c = sizeof(C);
+
+// CHECK:*** Dumping AST Record Layout
+// CHECK:             0 | struct C
+// CHECK-NEXT:        0 |   struct B (base)
+// CHECK-NEXT:        0 |     struct A (base) (empty)
+// CHECK-NEXT:    0:0-3 |       int 
+// CHECK-NEXT:        0 |     int n
+// CHECK-NEXT:        4 |     char [3] c
+// CHECK-V6-NEXT:     8 |   char d
+// CHECK-V6-NEXT:       | [sizeof=12, dsize=9, align=4,
+// CHECK-V6-NEXT:       |  nvsize=9, nvalign=4]
+// CHECK-V7-NEXT:     7 |   char d
+// CHECK-V7-NEXT:       | [sizeof=8, dsize=8, align=4,
+// CHECK-V7-NEXT:       |  nvsize=8, nvalign=4]
+
+// In Clang 6 and before, we determined that F was standard-layout despite it
+// having multiple empty base classes of the same type.
+struct D {};
+struct E : D {};
+struct F : D, E { int n; char c[3]; };
+struct G : F { G(const G&); char d; };
+int g = sizeof(G);
+
+// CHECK:*** Dumping AST Record Layout
+// CHECK:             0 | struct G
+// CHECK-NEXT:        0 |   struct F (base)
+// CHECK-NEXT:        0 |     struct D (base) (empty)
+// CHECK-NEXT:        1 |     struct E (base) (empty)
+// CHECK-NEXT:        1 |       struct D (base) (empty)
+// CHECK-NEXT:        0 |     int n
+// CHECK-NEXT:        4 |     char [3] c
+// CHECK-V6-NEXT:     8 |   char d
+// CHECK-V6-NEXT:       | [sizeof=12, dsize=9, align=4,
+// CHECK-V6-NEXT:       |  nvsize=9, nvalign=4]
+// CHECK-V7-NEXT:     7 |   char d
+// CHECK-V7-NEXT:       | [sizeof=8, dsize=8, align=4,
+// CHECK-V7-NEXT:       |  nvsize=8, nvalign=4]

ReleaseNotes.rst

- ``-Wextra-semi`` now also controls ``-Wc++98-compat-extra-semi``.
   to explicitly re-enable it (e.g. by appending ``-Wextra-semi``).
 
 Non-comprehensive list of changes in this release
 -------------------------------------------------
 
 - clang binary and libraries have been renamed from 7.0 to 7.
   For example, the clang binary will be called clang-7 instead of clang-7.0.
 
+- Clang implements a collection of recent fixes to the C++ standard's definition
+  of "standard-layout". In particular, a class is only considered to be
+  standard-layout if all base classes and the first data member (or bit-field)
+  can be laid out at offset zero. This unfortunately results in an ABI break
+  for ABIs that base their choice to reuse base class tail padding on whether
+  the base class is standard-layout (64-bit iOS, WatchOS, and WebAssembly).
+  The old behavior can be restored by setting ``-fclang-abi-compat`` to ``6``
+  or lower.
+
 - ...
 
 New Compiler Flags
 ------------------
 
 - ...
 
 Deprecated Compiler Flags

SemaCXX/type-traits.cpp

void is_standard_layout()
   int t17[F(__is_standard_layout(CppStructNonStandardBy2ndVirtBase))];
 
   int t40[T(__is_standard_layout(ACompleteType))];
   int t41[F(__is_standard_layout(AnIncompleteType))]; // expected-error {{incomplete type}}
   int t42[F(__is_standard_layout(AnIncompleteType[]))]; // expected-error {{incomplete type}}
   int t43[F(__is_standard_layout(AnIncompleteType[1]))]; // expected-error {{incomplete type}}
   int t44[F(__is_standard_layout(void))];
   int t45[F(__is_standard_layout(const volatile void))];
+
+  struct HasAnonEmptyBitfield { int : 0; };
+  struct HasAnonBitfield { int : 4; };
+  struct DerivesFromBitfield : HasAnonBitfield {};
+  struct DerivesFromBitfieldWithBitfield : HasAnonBitfield { int : 5; };
+  struct DerivesFromBitfieldTwice : DerivesFromBitfield, HasAnonEmptyBitfield {};
+
+  int t50[T(__is_standard_layout(HasAnonEmptyBitfield))];
+  int t51[T(__is_standard_layout(HasAnonBitfield))];
+  int t52[T(__is_standard_layout(DerivesFromBitfield))];
+  int t53[F(__is_standard_layout(DerivesFromBitfieldWithBitfield))];
+  int t54[F(__is_standard_layout(DerivesFromBitfieldTwice))];
+
+  struct Empty {};
+  struct HasEmptyBase : Empty {};
+  struct HoldsEmptyBase { Empty e; };
+  struct HasRepeatedEmptyBase : Empty, HasEmptyBase {}; // expected-warning {{inaccessible}}
+  struct HasEmptyBaseAsMember : Empty { Empty e; };
+  struct HasEmptyBaseAsSubobjectOfMember1 : Empty { HoldsEmptyBase e; };
+  struct HasEmptyBaseAsSubobjectOfMember2 : Empty { HasEmptyBase e; };
+  struct HasEmptyBaseAsSubobjectOfMember3 : Empty { HoldsEmptyBase e[2]; };
+  struct HasEmptyIndirectBaseAsMember : HasEmptyBase { Empty e; };
+  struct HasEmptyIndirectBaseAsSecondMember : HasEmptyBase { int n; Empty e; };
+  struct HasEmptyIndirectBaseAfterBitfield : HasEmptyBase { int : 4; Empty e; };
+
+  int t60[T(__is_standard_layout(Empty))];
+  int t61[T(__is_standard_layout(HasEmptyBase))];
+  int t62[F(__is_standard_layout(HasRepeatedEmptyBase))];
+  int t63[F(__is_standard_layout(HasEmptyBaseAsMember))];
+  int t64[F(__is_standard_layout(HasEmptyBaseAsSubobjectOfMember1))];
+  int t65[T(__is_standard_layout(HasEmptyBaseAsSubobjectOfMember2))]; // FIXME: standard bug?
+  int t66[F(__is_standard_layout(HasEmptyBaseAsSubobjectOfMember3))];
+  int t67[F(__is_standard_layout(HasEmptyIndirectBaseAsMember))];
+  int t68[T(__is_standard_layout(HasEmptyIndirectBaseAsSecondMember))];
+  int t69[F(__is_standard_layout(HasEmptyIndirectBaseAfterBitfield))]; // FIXME: standard bug?
+
+  struct StructWithEmptyFields {
+    int n;
+    HoldsEmptyBase e[3];
+  };
+  union UnionWithEmptyFields {
+    int n;
+    HoldsEmptyBase e[3];
+  };
+  struct HasEmptyIndirectBaseAsSecondStructMember : HasEmptyBase {
+    StructWithEmptyFields u;
+  };
+  struct HasEmptyIndirectBaseAsSecondUnionMember : HasEmptyBase {
+    UnionWithEmptyFields u;
+  };
+
+  int t70[T(__is_standard_layout(HasEmptyIndirectBaseAsSecondStructMember))];
+  int t71[F(__is_standard_layout(HasEmptyIndirectBaseAsSecondUnionMember))];
 }
 
 void is_signed()
 {
   //int t01[T(__is_signed(char))];
   int t02[T(__is_signed(int))];
   int t03[T(__is_signed(long))];
   int t04[T(__is_signed(short))];

Serialization/ASTReaderDecl.cpp

void ASTDeclReader::ReadCXXDefinitionData(
   Data.UserDeclaredConstructor = Record.readInt();
   Data.UserDeclaredSpecialMembers = Record.readInt();
   Data.Aggregate = Record.readInt();
   Data.PlainOldData = Record.readInt();
   Data.Empty = Record.readInt();
   Data.Polymorphic = Record.readInt();
   Data.Abstract = Record.readInt();
   Data.IsStandardLayout = Record.readInt();
-  Data.HasNoNonEmptyBases = Record.readInt();
+  Data.HasBasesWithFields = Record.readInt();
   Data.HasPrivateFields = Record.readInt();
   Data.HasProtectedFields = Record.readInt();
   Data.HasPublicFields = Record.readInt();
   Data.HasMutableFields = Record.readInt();
   Data.HasVariantMembers = Record.readInt();
   Data.HasOnlyCMembers = Record.readInt();
   Data.HasInClassInitializer = Record.readInt();
   Data.HasUninitializedReferenceMember = Record.readInt();
#define MATCH_FIELD(Field) \
   MATCH_FIELD(UserDeclaredConstructor)
   MATCH_FIELD(UserDeclaredSpecialMembers)
   MATCH_FIELD(Aggregate)
   MATCH_FIELD(PlainOldData)
   MATCH_FIELD(Empty)
   MATCH_FIELD(Polymorphic)
   MATCH_FIELD(Abstract)
   MATCH_FIELD(IsStandardLayout)
-  MATCH_FIELD(HasNoNonEmptyBases)
+  MATCH_FIELD(HasBasesWithFields)
   MATCH_FIELD(HasPrivateFields)
   MATCH_FIELD(HasProtectedFields)
   MATCH_FIELD(HasPublicFields)
   MATCH_FIELD(HasMutableFields)
   MATCH_FIELD(HasVariantMembers)
   MATCH_FIELD(HasOnlyCMembers)
   MATCH_FIELD(HasInClassInitializer)
   MATCH_FIELD(HasUninitializedReferenceMember)

Serialization/ASTWriter.cpp

void ASTRecordWriter::AddCXXDefinitionData(const CXXRecordDecl *D) {
   Record->push_back(Data.UserDeclaredConstructor);
   Record->push_back(Data.UserDeclaredSpecialMembers);
   Record->push_back(Data.Aggregate);
   Record->push_back(Data.PlainOldData);
   Record->push_back(Data.Empty);
   Record->push_back(Data.Polymorphic);
   Record->push_back(Data.Abstract);
   Record->push_back(Data.IsStandardLayout);
-  Record->push_back(Data.HasNoNonEmptyBases);
+  Record->push_back(Data.HasBasesWithFields);
   Record->push_back(Data.HasPrivateFields);
   Record->push_back(Data.HasProtectedFields);
   Record->push_back(Data.HasPublicFields);
   Record->push_back(Data.HasMutableFields);
   Record->push_back(Data.HasVariantMembers);
   Record->push_back(Data.HasOnlyCMembers);
   Record->push_back(Data.HasInClassInitializer);
   Record->push_back(Data.HasUninitializedReferenceMember);

clang/AST/DeclCXX.h

struct DefinitionData {
     /// * has no non-standard-layout base classes,
     /// * either has no non-static data members in the most derived class and at
     ///   most one base class with non-static data members, or has no base
     ///   classes with non-static data members, and
     /// * has no base classes of the same type as the first non-static data
     ///   member.
     unsigned IsStandardLayout : 1;
 
-    /// \brief True when there are no non-empty base classes.
-    ///
+    /// \brief True when any base class has any declared non-static data
+    /// members or bit-fields.
     /// This is a helper bit of state used to implement IsStandardLayout more
     /// efficiently.
-    unsigned HasNoNonEmptyBases : 1;
+    unsigned HasBasesWithFields : 1;
 
     /// \brief True when there are private non-static data members.
     unsigned HasPrivateFields : 1;
 
     /// \brief True when there are protected non-static data members.
     unsigned HasProtectedFields : 1;
 
     /// \brief True when there are private non-static data members.
class CXXRecordDecl : public RecordDecl {
   void addedMember(Decl *D);
 
   void markedVirtualFunctionPure();
 
   /// \brief Get the head of our list of friend declarations, possibly
   /// deserializing the friends from an external AST source.
   FriendDecl *getFirstFriend() const;
 
+  /// Determine whether this class has an empty base class subobject of type X
+  /// or of one of the types that might be at offset 0 within X (per the C++
+  /// "standard layout" rules).
+  bool hasSubobjectAtOffsetZeroOfEmptyBaseType(ASTContext &Ctx,
+                                               const CXXRecordDecl *X);
+
 protected:
   CXXRecordDecl(Kind K, TagKind TK, const ASTContext &C, DeclContext *DC,
                 SourceLocation StartLoc, SourceLocation IdLoc,
                 IdentifierInfo *Id, CXXRecordDecl *PrevDecl);
 
 public:
   /// \brief Iterator that traverses the base classes of a class.
   using base_class_iterator = CXXBaseSpecifier *;

cxx_dr_status.html

<tr id="1424">
     <td>C++14</td>
     <td>When must sub-object destructors be accessible?</td>
     <td class="none" align="center">Unknown</td>
   </tr>
   <tr id="1425">
     <td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1425">1425</a></td>
     <td>CD3</td>
     <td>Base-class subobjects of standard-layout structs</td>
-    <td class="none" align="center">Unknown</td>
+    <td class="na" align="center">N/A (ABI constraint)</td>
   </tr>
   <tr class="open" id="1426">
     <td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_closed.html#1426">1426</a></td>
     <td>extension</td>
     <td>Allowing additional parameter types in defaulted functions</td>
     <td align="center">Not resolved</td>
   </tr>
   <tr id="1427">
<tr id="1671">
     <td>NAD</td>
     <td>Unclear rules for deduction with cv-qualification</td>
     <td class="none" align="center">Unknown</td>
   </tr>
   <tr id="1672">
     <td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1672">1672</a></td>
     <td>CD4</td>
     <td>Layout compatibility with multiple empty bases</td>
-    <td class="none" align="center">Unknown</td>
+    <td class="svn" align="center">SVN</td>
   </tr>
   <tr id="1673">
     <td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1673">1673</a></td>
     <td>C++14</td>
     <td>Clarifying overload resolution for the second step of copy-initialization</td>
     <td class="none" align="center">Unknown</td>
   </tr>
   <tr id="1674">
<tr id="1812">
     <td>C++17</td>
     <td>Omission of <TT>template</TT> in a <I>typename-specifier</I></td>
     <td class="none" align="center">Unknown</td>
   </tr>
   <tr id="1813">
     <td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1813">1813</a></td>
     <td>CD4</td>
     <td>Direct vs indirect bases in standard-layout classes</td>
-    <td class="none" align="center">Unknown</td>
+    <td class="svn" align="center">SVN</td>
   </tr>
   <tr id="1814">
     <td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1814">1814</a></td>
     <td>CD4</td>
     <td>Default arguments in <I>lambda-expression</I>s</td>
     <td class="none" align="center">Unknown</td>
   </tr>
   <tr id="1815">
<tr class="open" id="1880">
     <td>drafting</td>
     <td>When are parameter objects destroyed?</td>
     <td align="center">Not resolved</td>
   </tr>
   <tr id="1881">
     <td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1881">1881</a></td>
     <td>CD4</td>
     <td>Standard-layout classes and unnamed bit-fields</td>
-    <td class="none" align="center">Unknown</td>
+    <td class="svn" align="center">SVN</td>
   </tr>
   <tr id="1882">
     <td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1882">1882</a></td>
     <td>CD4</td>
     <td>Reserved names without library use</td>
     <td class="none" align="center">Unknown</td>
   </tr>
   <tr class="open" id="1883">
<tr id="2119">
     <td>NAD</td>
     <td>Disambiguation of multi-level covariant return type</td>
     <td class="none" align="center">Unknown</td>
   </tr>
   <tr id="2120">
     <td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#2120">2120</a></td>
     <td>CD4</td>
     <td>Array as first non-static data member in standard-layout class</td>
-    <td class="none" align="center">Unknown</td>
+    <td class="svn" align="center">SVN</td>
   </tr>
   <tr class="open" id="2121">
     <td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#2121">2121</a></td>
     <td>drafting</td>
     <td>More flexible lambda syntax</td>
     <td align="center">Not resolved</td>
   </tr>
   <tr id="2122">
<tr class="open" id="2228">
     <td>drafting</td>
     <td>Ambiguity resolution for cast to function type</td>
     <td align="center">Not resolved</td>
   </tr>
   <tr id="2229">
     <td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#2229">2229</a></td>
     <td>tentatively ready</td>
     <td>Volatile unnamed bit-fields</td>
-    <td class="none" align="center">Unknown</td>
+    <td class="svn" align="center">SVN</td>
   </tr>
   <tr id="2230">
     <td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_closed.html#2230">2230</a></td>
     <td>NAD</td>
     <td>Linkage of <TT>extern "C"</TT> function in unnamed namespace</td>
     <td class="none" align="center">Unknown</td>
   </tr>
   <tr id="2231">

make_cxx_dr_status

elif status == 'no':
     avail = 'No'
     avail_style = ' class="none"'
   elif status == 'na':
     avail = 'N/A'
     avail_style = ' class="na"'
   elif status == 'na lib':
     avail = 'N/A (Library DR)'
     avail_style = ' class="na"'
+  elif status == 'na abi':
+    avail = 'N/A (ABI constraint)'
+    avail_style = ' class="na"'
   elif status.startswith('sup '):
     dup = status.split(' ', 1)[1]
     avail = 'Superseded by <a href="#%s">%s</a>' % (dup, dup)
     try:
       _, avail_style = availability(int(dup))
     except:
       print >>sys.stderr, "issue %s marked as sup %s" % (issue, dup)
       avail_style = ' class="none"'