Implement the likely resolution of core issue 253.

C++11 requires const objects to have a user-provided constructor, even for
classes without any fields. DR 253 relaxes this to say "If the implicit default
constructor initializes all subobjects, no initializer should be required."

clang is currently the only compiler that implements this C++11 rule, and e.g.
libstdc++ relies on something like DR 253 to compile in newer versions.  This
change  makes it possible to build code that says `const vector<int> v;' again
when using libstdc++5.2 and _GLIBCXX_DEBUG
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60284).

Fixes PR23381.

http://reviews.llvm.org/D16552


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@261297 91177308-0d34-0410-b5e6-96231b3b80d8

14 files changed, 135 insertions, 42 deletions

diff --git a/include/clang/AST/DeclCXX.h b/include/clang/AST/DeclCXX.h
index 7c54901be3..dde0270ce3 100644
--- a/include/clang/AST/DeclCXX.h
+++ b/include/clang/AST/DeclCXX.h
@@ -378,6 +378,10 @@ class CXXRecordDecl : public RecordDecl {
     /// even if the class has a trivial default constructor.
     bool HasUninitializedReferenceMember : 1;
 
+    /// \brief True if any non-mutable field whose type doesn't have a user-
+    /// provided default ctor also doesn't have an in-class initializer.
+    bool HasUninitializedFields : 1;
+
     /// \brief These flags are \c true if a defaulted corresponding special
     /// member can't be fully analyzed without performing overload resolution.
     /// @{
@@ -1270,6 +1274,13 @@ public:
     return !(data().HasTrivialSpecialMembers & SMF_Destructor);
   }
 
+  /// \brief Determine whether declaring a const variable with this type is ok
+  /// per core issue 253.
+  bool allowConstDefaultInit() const {
+    return !data().HasUninitializedFields ||
+           hasUserProvidedDefaultConstructor();
+  }
+
   /// \brief Determine whether this class has a destructor which has no
   /// semantic effect.
   ///
diff --git a/lib/AST/ASTImporter.cpp b/lib/AST/ASTImporter.cpp
index 039ea20eed..1ea44fe600 100644
--- a/lib/AST/ASTImporter.cpp
+++ b/lib/AST/ASTImporter.cpp
@@ -2024,6 +2024,7 @@ bool ASTNodeImporter::ImportDefinition(RecordDecl *From, RecordDecl *To,
     ToData.HasInClassInitializer = FromData.HasInClassInitializer;
     ToData.HasUninitializedReferenceMember
       = FromData.HasUninitializedReferenceMember;
+    ToData.HasUninitializedFields = FromData.HasUninitializedFields;
     ToData.NeedOverloadResolutionForMoveConstructor
       = FromData.NeedOverloadResolutionForMoveConstructor;
     ToData.NeedOverloadResolutionForMoveAssignment
diff --git a/lib/AST/DeclCXX.cpp b/lib/AST/DeclCXX.cpp
index 4f24fdc28f..c8edd5a1b4 100644
--- a/lib/AST/DeclCXX.cpp
+++ b/lib/AST/DeclCXX.cpp
@@ -46,34 +46,31 @@ void LazyASTUnresolvedSet::getFromExternalSource(ASTContext &C) const {
 }
 
 CXXRecordDecl::DefinitionData::DefinitionData(CXXRecordDecl *D)
-  : UserDeclaredConstructor(false), UserDeclaredSpecialMembers(0),
-    Aggregate(true), PlainOldData(true), Empty(true), Polymorphic(false),
-    Abstract(false), IsStandardLayout(true), HasNoNonEmptyBases(true),
-    HasPrivateFields(false), HasProtectedFields(false), HasPublicFields(false),
-    HasMutableFields(false), HasVariantMembers(false), HasOnlyCMembers(true),
-    HasInClassInitializer(false), HasUninitializedReferenceMember(false),
-    NeedOverloadResolutionForMoveConstructor(false),
-    NeedOverloadResolutionForMoveAssignment(false),
-    NeedOverloadResolutionForDestructor(false),
-    DefaultedMoveConstructorIsDeleted(false),
-    DefaultedMoveAssignmentIsDeleted(false),
-    DefaultedDestructorIsDeleted(false),
-    HasTrivialSpecialMembers(SMF_All),
-    DeclaredNonTrivialSpecialMembers(0),
-    HasIrrelevantDestructor(true),
-    HasConstexprNonCopyMoveConstructor(false),
-    DefaultedDefaultConstructorIsConstexpr(true),
-    HasConstexprDefaultConstructor(false),
-    HasNonLiteralTypeFieldsOrBases(false), ComputedVisibleConversions(false),
-    UserProvidedDefaultConstructor(false), DeclaredSpecialMembers(0),
-    ImplicitCopyConstructorHasConstParam(true),
-    ImplicitCopyAssignmentHasConstParam(true),
-    HasDeclaredCopyConstructorWithConstParam(false),
-    HasDeclaredCopyAssignmentWithConstParam(false),
-    IsLambda(false), IsParsingBaseSpecifiers(false), NumBases(0), NumVBases(0),
-    Bases(), VBases(),
-    Definition(D), FirstFriend() {
-}
+    : UserDeclaredConstructor(false), UserDeclaredSpecialMembers(0),
+      Aggregate(true), PlainOldData(true), Empty(true), Polymorphic(false),
+      Abstract(false), IsStandardLayout(true), HasNoNonEmptyBases(true),
+      HasPrivateFields(false), HasProtectedFields(false),
+      HasPublicFields(false), HasMutableFields(false), HasVariantMembers(false),
+      HasOnlyCMembers(true), HasInClassInitializer(false),
+      HasUninitializedReferenceMember(false), HasUninitializedFields(false),
+      NeedOverloadResolutionForMoveConstructor(false),
+      NeedOverloadResolutionForMoveAssignment(false),
+      NeedOverloadResolutionForDestructor(false),
+      DefaultedMoveConstructorIsDeleted(false),
+      DefaultedMoveAssignmentIsDeleted(false),
+      DefaultedDestructorIsDeleted(false), HasTrivialSpecialMembers(SMF_All),
+      DeclaredNonTrivialSpecialMembers(0), HasIrrelevantDestructor(true),
+      HasConstexprNonCopyMoveConstructor(false),
+      DefaultedDefaultConstructorIsConstexpr(true),
+      HasConstexprDefaultConstructor(false),
+      HasNonLiteralTypeFieldsOrBases(false), ComputedVisibleConversions(false),
+      UserProvidedDefaultConstructor(false), DeclaredSpecialMembers(0),
+      ImplicitCopyConstructorHasConstParam(true),
+      ImplicitCopyAssignmentHasConstParam(true),
+      HasDeclaredCopyConstructorWithConstParam(false),
+      HasDeclaredCopyAssignmentWithConstParam(false), IsLambda(false),
+      IsParsingBaseSpecifiers(false), NumBases(0), NumVBases(0), Bases(),
+      VBases(), Definition(D), FirstFriend() {}
 
 CXXBaseSpecifier *CXXRecordDecl::DefinitionData::getBasesSlowCase() const {
   return Bases.get(Definition->getASTContext().getExternalSource());
@@ -332,6 +329,9 @@ CXXRecordDecl::setBases(CXXBaseSpecifier const * const *Bases,
     if (BaseClassDecl->hasUninitializedReferenceMember())
       data().HasUninitializedReferenceMember = true;
 
+    if (!BaseClassDecl->allowConstDefaultInit())
+      data().HasUninitializedFields = true;
+
     addedClassSubobject(BaseClassDecl);
   }
   
@@ -702,6 +702,15 @@ void CXXRecordDecl::addedMember(Decl *D) {
       data().IsStandardLayout = false;
     }
 
+    if (!Field->hasInClassInitializer() && !Field->isMutable()) {
+      if (CXXRecordDecl *FieldType = Field->getType()->getAsCXXRecordDecl()) {
+        if (!FieldType->allowConstDefaultInit())
+          data().HasUninitializedFields = true;
+      } else {
+        data().HasUninitializedFields = true;
+      }
+    }
+
     // Record if this field is the first non-literal or volatile field or base.
     if (!T->isLiteralType(Context) || T.isVolatileQualified())
       data().HasNonLiteralTypeFieldsOrBases = true;
diff --git a/lib/Sema/SemaInit.cpp b/lib/Sema/SemaInit.cpp
index c34107d54c..afa681453d 100644
--- a/lib/Sema/SemaInit.cpp
+++ b/lib/Sema/SemaInit.cpp
@@ -3515,18 +3515,23 @@ static void TryConstructorInitialization(Sema &S,
   //   If a program calls for the default initialization of an object
   //   of a const-qualified type T, T shall be a class type with a
   //   user-provided default constructor.
+  // C++ core issue 253 proposal:
+  //   If the implicit default constructor initializes all subobjects, no
+  //   initializer should be required.
+  // The 253 proposal is for example needed to process libstdc++ headers in 5.x.
+  CXXConstructorDecl *CtorDecl = cast<CXXConstructorDecl>(Best->Function);
   if (Kind.getKind() == InitializationKind::IK_Default &&
-      Entity.getType().isConstQualified() &&
-      !cast<CXXConstructorDecl>(Best->Function)->isUserProvided()) {
-    if (!maybeRecoverWithZeroInitialization(S, Sequence, Entity))
-      Sequence.SetFailed(InitializationSequence::FK_DefaultInitOfConst);
-    return;
+      Entity.getType().isConstQualified()) {
+    if (!CtorDecl->getParent()->allowConstDefaultInit()) {
+      if (!maybeRecoverWithZeroInitialization(S, Sequence, Entity))
+        Sequence.SetFailed(InitializationSequence::FK_DefaultInitOfConst);
+      return;
+    }
   }
 
   // C++11 [over.match.list]p1:
   //   In copy-list-initialization, if an explicit constructor is chosen, the
   //   initializer is ill-formed.
-  CXXConstructorDecl *CtorDecl = cast<CXXConstructorDecl>(Best->Function);
   if (IsListInit && !Kind.AllowExplicit() && CtorDecl->isExplicit()) {
     Sequence.SetFailed(InitializationSequence::FK_ExplicitConstructor);
     return;
diff --git a/lib/Serialization/ASTReaderDecl.cpp b/lib/Serialization/ASTReaderDecl.cpp
index 7aa6d5236c..bf60ba9478 100644
--- a/lib/Serialization/ASTReaderDecl.cpp
+++ b/lib/Serialization/ASTReaderDecl.cpp
@@ -1412,6 +1412,7 @@ void ASTDeclReader::ReadCXXDefinitionData(
   Data.HasOnlyCMembers = Record[Idx++];
   Data.HasInClassInitializer = Record[Idx++];
   Data.HasUninitializedReferenceMember = Record[Idx++];
+  Data.HasUninitializedFields = Record[Idx++];
   Data.NeedOverloadResolutionForMoveConstructor = Record[Idx++];
   Data.NeedOverloadResolutionForMoveAssignment = Record[Idx++];
   Data.NeedOverloadResolutionForDestructor = Record[Idx++];
@@ -1536,6 +1537,7 @@ void ASTDeclReader::MergeDefinitionData(
   MATCH_FIELD(HasOnlyCMembers)
   MATCH_FIELD(HasInClassInitializer)
   MATCH_FIELD(HasUninitializedReferenceMember)
+  MATCH_FIELD(HasUninitializedFields)
   MATCH_FIELD(NeedOverloadResolutionForMoveConstructor)
   MATCH_FIELD(NeedOverloadResolutionForMoveAssignment)
   MATCH_FIELD(NeedOverloadResolutionForDestructor)
diff --git a/lib/Serialization/ASTWriter.cpp b/lib/Serialization/ASTWriter.cpp
index 985bcb05b4..1d1c99874b 100644
--- a/lib/Serialization/ASTWriter.cpp
+++ b/lib/Serialization/ASTWriter.cpp
@@ -5547,6 +5547,7 @@ void ASTWriter::AddCXXDefinitionData(const CXXRecordDecl *D, RecordDataImpl &Rec
   Record.push_back(Data.HasOnlyCMembers);
   Record.push_back(Data.HasInClassInitializer);
   Record.push_back(Data.HasUninitializedReferenceMember);
+  Record.push_back(Data.HasUninitializedFields);
   Record.push_back(Data.NeedOverloadResolutionForMoveConstructor);
   Record.push_back(Data.NeedOverloadResolutionForMoveAssignment);
   Record.push_back(Data.NeedOverloadResolutionForDestructor);
diff --git a/test/CXX/dcl.decl/dcl.fct.def/dcl.fct.def.default/p2.cpp b/test/CXX/dcl.decl/dcl.fct.def/dcl.fct.def.default/p2.cpp
index 5cf281c185..c2f3b5a045 100644
--- a/test/CXX/dcl.decl/dcl.fct.def/dcl.fct.def.default/p2.cpp
+++ b/test/CXX/dcl.decl/dcl.fct.def/dcl.fct.def.default/p2.cpp
@@ -116,6 +116,7 @@ static_assert(!noexcept(e5 = e5), "");
 namespace PR13492 {
   struct B {
     B() = default;
+    int field;
   };
 
   void f() {
diff --git a/test/CXX/dcl.decl/dcl.init/p6.cpp b/test/CXX/dcl.decl/dcl.init/p6.cpp
index e404a1ebc1..b646ba776a 100644
--- a/test/CXX/dcl.decl/dcl.init/p6.cpp
+++ b/test/CXX/dcl.decl/dcl.init/p6.cpp
@@ -4,9 +4,9 @@
 
 // If a program calls for the default initialization of an object of a
 // const-qualified type T, T shall be a class type with a
-// user-provided default constructor.
+// user-provided default constructor, except if T has no uninitialized fields.
 struct MakeNonPOD { MakeNonPOD(); };
-struct NoUserDefault : public MakeNonPOD { };
+struct NoUserDefault : public MakeNonPOD { int field; };
 struct HasUserDefault { HasUserDefault(); };
 
 void test_const_default_init() {
@@ -16,7 +16,7 @@ void test_const_default_init() {
 }
 
 // rdar://8501008
-struct s0 {};
+struct s0 { int field; };
 struct s1 { static const s0 foo; };
 const struct s0 s1::foo; // expected-error{{default initialization of an object of const type 'const struct s0' without a user-provided default constructor}}
 
diff --git a/test/CXX/drs/dr4xx.cpp b/test/CXX/drs/dr4xx.cpp
index bceea793fa..09298cb010 100644
--- a/test/CXX/drs/dr4xx.cpp
+++ b/test/CXX/drs/dr4xx.cpp
@@ -1197,12 +1197,12 @@ namespace dr496 { // dr496: no
   int check6[ __is_trivially_assignable(B, const B&) ? 1 : -1];
 }
 
-namespace dr497 { // dr497: yes
+namespace dr497 { // dr497: sup 253
   void before() {
     struct S {
       mutable int i;
     };
-    const S cs; // expected-error {{default initialization}}
+    const S cs;
     int S::*pm = &S::i;
     cs.*pm = 88; // expected-error {{not assignable}}
   }
diff --git a/test/SemaCXX/attr-selectany.cpp b/test/SemaCXX/attr-selectany.cpp
index 058f2fcb84..9dc14b3c38 100644
--- a/test/SemaCXX/attr-selectany.cpp
+++ b/test/SemaCXX/attr-selectany.cpp
@@ -39,7 +39,9 @@ __declspec(selectany) auto x8 = Internal(); // expected-error {{'selectany' can
 // The D3D11 headers do something like this.  MSVC doesn't error on this at
 // all, even without the __declspec(selectany), in violation of the standard.
 // We fall back to a warning for selectany to accept headers.
-struct SomeStruct {};
+struct SomeStruct {
+  int foo;
+};
 extern const __declspec(selectany) SomeStruct some_struct; // expected-warning {{default initialization of an object of const type 'const SomeStruct' without a user-provided default constructor is a Microsoft extension}}
 
 // It should be possible to redeclare variables that were defined
diff --git a/test/SemaCXX/constexpr-value-init.cpp b/test/SemaCXX/constexpr-value-init.cpp
index 0651111335..3528fdcd88 100644
--- a/test/SemaCXX/constexpr-value-init.cpp
+++ b/test/SemaCXX/constexpr-value-init.cpp
@@ -14,7 +14,7 @@ void f() {
   constexpr A a; // expected-error {{constant expression}} expected-note {{in call to 'A()'}}
 }
 
-constexpr B b1; // expected-error {{without a user-provided default constructor}}
+constexpr B b1; // ok
 constexpr B b2 = B(); // ok
 static_assert(b2.a.a == 1, "");
 static_assert(b2.a.b == 2, "");
diff --git a/test/SemaCXX/cxx0x-cursory-default-delete.cpp b/test/SemaCXX/cxx0x-cursory-default-delete.cpp
index dfca17ad7c..f671bd190e 100644
--- a/test/SemaCXX/cxx0x-cursory-default-delete.cpp
+++ b/test/SemaCXX/cxx0x-cursory-default-delete.cpp
@@ -11,6 +11,7 @@ struct non_const_copy {
   non_const_copy& operator = (non_const_copy&) &;
   non_const_copy& operator = (non_const_copy&) &&;
   non_const_copy() = default; // expected-note {{not viable}}
+  int uninit_field;
 };
 non_const_copy::non_const_copy(non_const_copy&) = default; // expected-note {{not viable}}
 non_const_copy& non_const_copy::operator = (non_const_copy&) & = default; // expected-note {{not viable}}
@@ -30,6 +31,65 @@ void fn1 () {
   ncc = cncc; // expected-error {{no viable overloaded}}
 };
 
+struct no_fields { };
+struct all_init {
+  int a = 0;
+  int b = 0;
+};
+struct some_init {
+  int a = 0;
+  int b;
+  int c = 0;
+};
+struct some_init_mutable {
+  int a = 0;
+  mutable int b;
+  int c = 0;
+};
+struct some_init_def {
+  some_init_def() = default;
+  int a = 0;
+  int b;
+  int c = 0;
+};
+struct some_init_ctor {
+  some_init_ctor();
+  int a = 0;
+  int b;
+  int c = 0;
+};
+struct sub_some_init : public some_init_def { };
+struct sub_some_init_ctor : public some_init_def {
+  sub_some_init_ctor();
+};
+struct sub_some_init_ctor2 : public some_init_ctor {
+};
+struct some_init_container {
+  some_init_def sid;
+};
+struct some_init_container_ctor {
+  some_init_container_ctor();
+  some_init_def sid;
+};
+struct no_fields_container {
+  no_fields nf;
+};
+
+void constobjs() {
+  const no_fields nf; // ok
+  const all_init ai; // ok
+  const some_init si; // expected-error {{default initialization of an object of const type 'const some_init' without a user-provided default constructor}}
+  const some_init_mutable sim; // ok
+  const some_init_def sid; // expected-error {{default initialization of an object of const type 'const some_init_def' without a user-provided default constructor}}
+  const some_init_ctor sic; // ok
+  const sub_some_init ssi; // expected-error {{default initialization of an object of const type 'const sub_some_init' without a user-provided default constructor}}
+  const sub_some_init_ctor ssic; // ok
+  const sub_some_init_ctor2 ssic2; // ok
+  const some_init_container sicon; // expected-error {{default initialization of an object of const type 'const some_init_container' without a user-provided default constructor}}
+  const some_init_container_ctor siconc; // ok
+  const no_fields_container nfc; // ok
+}
+
 struct non_const_derived : non_const_copy {
   non_const_derived(const non_const_derived&) = default; // expected-error {{requires it to be non-const}}
   non_const_derived& operator =(non_const_derived&) = default;
diff --git a/test/SemaCXX/illegal-member-initialization.cpp b/test/SemaCXX/illegal-member-initialization.cpp
index 87069efaac..17faed7eff 100644
--- a/test/SemaCXX/illegal-member-initialization.cpp
+++ b/test/SemaCXX/illegal-member-initialization.cpp
@@ -7,6 +7,7 @@ struct A {
 };
 
 struct B {
+  int field;
 };
 
 struct X {
diff --git a/www/cxx_dr_status.html b/www/cxx_dr_status.html
index 21bee9d030..28af0749a5 100644
--- a/www/cxx_dr_status.html
+++ b/www/cxx_dr_status.html
@@ -3023,7 +3023,7 @@ of class templates</td>
     <td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#497">497</a></td>
     <td>CD1</td>
     <td>Missing required initialization in example</td>
-    <td class="full" align="center">Yes</td>
+    <td class="none" align="center">Superseded by <a href="#253">253</a></td>
   </tr>
   <tr class="open" id="498">
     <td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#498">498</a></td>