P0136R1, DR1573, DR1645, DR1715, DR1736, DR1903, DR1941, DR1959, DR1991:

Replace inheriting constructors implementation with new approach, voted into
C++ last year as a DR against C++11.

Instead of synthesizing a set of derived class constructors for each inherited
base class constructor, we make the constructors of the base class visible to
constructor lookup in the derived class, using the normal rules for
using-declarations.

For constructors, UsingShadowDecl now has a ConstructorUsingShadowDecl derived
class that tracks the requisite additional information. We create shadow
constructors (not found by name lookup) in the derived class to model the
actual initialization, and have a new expression node,
CXXInheritedCtorInitExpr, to model the initialization of a base class from such
a constructor. (This initialization is special because it performs real perfect
forwarding of arguments.)

In cases where argument forwarding is not possible (for inalloca calls,
variadic calls, and calls with callee parameter cleanup), the shadow inheriting
constructor is not emitted and instead we directly emit the initialization code
into the caller of the inherited constructor.

Note that this new model is not perfectly compatible with the old model in some
corner cases. In particular:
 * if B inherits a private constructor from A, and C uses that constructor to
   construct a B, then we previously required that A befriends B and B
   befriends C, but the new rules require A to befriend C directly, and
 * if a derived class has its own constructors (and so its implicit default
   constructor is suppressed), it may still inherit a default constructor from
   a base class

llvm-svn: 274049

Author: zygoloid

clang/include/clang/AST/ASTMutationListener.h

@@ -17,6 +17,7 @@ namespace clang {
   class Attr;
   class ClassTemplateDecl;
   class ClassTemplateSpecializationDecl;
+  class ConstructorUsingShadowDecl;
   class CXXDestructorDecl;
   class CXXRecordDecl;
   class Decl;

clang/include/clang/AST/Decl.h

@@ -387,6 +387,7 @@ class NamedDecl : public Decl {
   NamedDecl *getUnderlyingDecl() {
     // Fast-path the common case.
     if (this->getKind() != UsingShadow &&
+        this->getKind() != ConstructorUsingShadow &&
         this->getKind() != ObjCCompatibleAlias &&
         this->getKind() != NamespaceAlias)
       return this;

clang/include/clang/AST/DeclCXX.h

@@ -29,6 +29,7 @@ namespace clang {
 
 class ClassTemplateDecl;
 class ClassTemplateSpecializationDecl;
+class ConstructorUsingShadowDecl;
 class CXXBasePath;
 class CXXBasePaths;
 class CXXConstructorDecl;
@@ -1298,7 +1299,7 @@ class CXXRecordDecl : public RecordDecl {
   }
 
   /// \brief Determine whether this class has a using-declaration that names
-  /// a base class constructor.
+  /// a user-declared base class constructor.
   bool hasInheritedConstructor() const {
     return data().HasInheritedConstructor;
   }
@@ -2153,6 +2154,23 @@ class CXXCtorInitializer final
   friend TrailingObjects;
 };
 
+/// Description of a constructor that was inherited from a base class.
+class InheritedConstructor {
+  ConstructorUsingShadowDecl *Shadow;
+  CXXConstructorDecl *BaseCtor;
+
+public:
+  InheritedConstructor() : Shadow(), BaseCtor() {}
+  InheritedConstructor(ConstructorUsingShadowDecl *Shadow,
+                       CXXConstructorDecl *BaseCtor)
+      : Shadow(Shadow), BaseCtor(BaseCtor) {}
+
+  explicit operator bool() const { return Shadow; }
+
+  ConstructorUsingShadowDecl *getShadowDecl() const { return Shadow; }
+  CXXConstructorDecl *getConstructor() const { return BaseCtor; }
+};
+
 /// \brief Represents a C++ constructor within a class.
 ///
 /// For example:
@@ -2163,41 +2181,51 @@ class CXXCtorInitializer final
 ///   explicit X(int); // represented by a CXXConstructorDecl.
 /// };
 /// \endcode
-class CXXConstructorDecl : public CXXMethodDecl {
+class CXXConstructorDecl final
+    : public CXXMethodDecl,
+      private llvm::TrailingObjects<CXXConstructorDecl, InheritedConstructor> {
   void anchor() override;
 
   /// \name Support for base and member initializers.
   /// \{
   /// \brief The arguments used to initialize the base or member.
   LazyCXXCtorInitializersPtr CtorInitializers;
-  unsigned NumCtorInitializers : 31;
+  unsigned NumCtorInitializers : 30;
   /// \}
 
   /// \brief Whether this constructor declaration has the \c explicit keyword
   /// specified.
   unsigned IsExplicitSpecified : 1;
 
+  /// \brief Whether this constructor declaration is an implicitly-declared
+  /// inheriting constructor.
+  unsigned IsInheritingConstructor : 1;
+
   CXXConstructorDecl(ASTContext &C, CXXRecordDecl *RD, SourceLocation StartLoc,
                      const DeclarationNameInfo &NameInfo,
                      QualType T, TypeSourceInfo *TInfo,
                      bool isExplicitSpecified, bool isInline,
-                     bool isImplicitlyDeclared, bool isConstexpr)
+                     bool isImplicitlyDeclared, bool isConstexpr,
+                     InheritedConstructor Inherited)
     : CXXMethodDecl(CXXConstructor, C, RD, StartLoc, NameInfo, T, TInfo,
                     SC_None, isInline, isConstexpr, SourceLocation()),
       CtorInitializers(nullptr), NumCtorInitializers(0),
-      IsExplicitSpecified(isExplicitSpecified) {
+      IsExplicitSpecified(isExplicitSpecified),
+      IsInheritingConstructor((bool)Inherited) {
     setImplicit(isImplicitlyDeclared);
+    if (Inherited)
+      *getTrailingObjects<InheritedConstructor>() = Inherited;
   }
 
 public:
-  static CXXConstructorDecl *CreateDeserialized(ASTContext &C, unsigned ID);
-  static CXXConstructorDecl *Create(ASTContext &C, CXXRecordDecl *RD,
-                                    SourceLocation StartLoc,
-                                    const DeclarationNameInfo &NameInfo,
-                                    QualType T, TypeSourceInfo *TInfo,
-                                    bool isExplicit,
-                                    bool isInline, bool isImplicitlyDeclared,
-                                    bool isConstexpr);
+  static CXXConstructorDecl *CreateDeserialized(ASTContext &C, unsigned ID,
+                                                bool InheritsConstructor);
+  static CXXConstructorDecl *
+  Create(ASTContext &C, CXXRecordDecl *RD, SourceLocation StartLoc,
+         const DeclarationNameInfo &NameInfo, QualType T, TypeSourceInfo *TInfo,
+         bool isExplicit, bool isInline, bool isImplicitlyDeclared,
+         bool isConstexpr,
+         InheritedConstructor Inherited = InheritedConstructor());
 
   /// \brief Determine whether this constructor declaration has the
   /// \c explicit keyword specified.
@@ -2344,11 +2372,15 @@ class CXXConstructorDecl : public CXXMethodDecl {
   /// an object.
   bool isSpecializationCopyingObject() const;
 
-  /// \brief Get the constructor that this inheriting constructor is based on.
-  const CXXConstructorDecl *getInheritedConstructor() const;
+  /// \brief Determine whether this is an implicit constructor synthesized to
+  /// model a call to a constructor inherited from a base class.
+  bool isInheritingConstructor() const { return IsInheritingConstructor; }
 
-  /// \brief Set the constructor that this inheriting constructor is based on.
-  void setInheritedConstructor(const CXXConstructorDecl *BaseCtor);
+  /// \brief Get the constructor that this inheriting constructor is based on.
+  InheritedConstructor getInheritedConstructor() const {
+    return IsInheritingConstructor ? *getTrailingObjects<InheritedConstructor>()
+                                   : InheritedConstructor();
+  }
 
   CXXConstructorDecl *getCanonicalDecl() override {
     return cast<CXXConstructorDecl>(FunctionDecl::getCanonicalDecl());
@@ -2363,6 +2395,7 @@ class CXXConstructorDecl : public CXXMethodDecl {
 
   friend class ASTDeclReader;
   friend class ASTDeclWriter;
+  friend TrailingObjects;
 };
 
 /// \brief Represents a C++ destructor within a class.
@@ -2807,18 +2840,6 @@ class UsingShadowDecl : public NamedDecl, public Redeclarable<UsingShadowDecl> {
   NamedDecl *UsingOrNextShadow;
   friend class UsingDecl;
 
-  UsingShadowDecl(ASTContext &C, DeclContext *DC, SourceLocation Loc,
-                  UsingDecl *Using, NamedDecl *Target)
-    : NamedDecl(UsingShadow, DC, Loc, DeclarationName()),
-      redeclarable_base(C), Underlying(Target),
-      UsingOrNextShadow(reinterpret_cast<NamedDecl *>(Using)) {
-    if (Target) {
-      setDeclName(Target->getDeclName());
-      IdentifierNamespace = Target->getIdentifierNamespace();
-    }
-    setImplicit();
-  }
-
   typedef Redeclarable<UsingShadowDecl> redeclarable_base;
   UsingShadowDecl *getNextRedeclarationImpl() override {
     return getNextRedeclaration();
@@ -2830,11 +2851,16 @@ class UsingShadowDecl : public NamedDecl, public Redeclarable<UsingShadowDecl> {
     return getMostRecentDecl();
   }
 
+protected:
+  UsingShadowDecl(Kind K, ASTContext &C, DeclContext *DC, SourceLocation Loc,
+                  UsingDecl *Using, NamedDecl *Target);
+  UsingShadowDecl(Kind K, ASTContext &C, EmptyShell);
+
 public:
   static UsingShadowDecl *Create(ASTContext &C, DeclContext *DC,
                                  SourceLocation Loc, UsingDecl *Using,
                                  NamedDecl *Target) {
-    return new (C, DC) UsingShadowDecl(C, DC, Loc, Using, Target);
+    return new (C, DC) UsingShadowDecl(UsingShadow, C, DC, Loc, Using, Target);
   }
 
   static UsingShadowDecl *CreateDeserialized(ASTContext &C, unsigned ID);
@@ -2846,6 +2872,7 @@ class UsingShadowDecl : public NamedDecl, public Redeclarable<UsingShadowDecl> {
   using redeclarable_base::redecls;
   using redeclarable_base::getPreviousDecl;
   using redeclarable_base::getMostRecentDecl;
+  using redeclarable_base::isFirstDecl;
 
   UsingShadowDecl *getCanonicalDecl() override {
     return getFirstDecl();
@@ -2876,7 +2903,125 @@ class UsingShadowDecl : public NamedDecl, public Redeclarable<UsingShadowDecl> {
   }
 
   static bool classof(const Decl *D) { return classofKind(D->getKind()); }
-  static bool classofKind(Kind K) { return K == Decl::UsingShadow; }
+  static bool classofKind(Kind K) {
+    return K == Decl::UsingShadow || K == Decl::ConstructorUsingShadow;
+  }
+
+  friend class ASTDeclReader;
+  friend class ASTDeclWriter;
+};
+
+/// \brief Represents a shadow constructor declaration introduced into a
+/// class by a C++11 using-declaration that names a constructor.
+///
+/// For example:
+/// \code
+/// struct Base { Base(int); };
+/// struct Derived {
+///    using Base::Base; // creates a UsingDecl and a ConstructorUsingShadowDecl
+/// };
+/// \endcode
+class ConstructorUsingShadowDecl final : public UsingShadowDecl {
+  void anchor() override;
+
+  /// \brief If this constructor using declaration inherted the constructor
+  /// from an indirect base class, this is the ConstructorUsingShadowDecl
+  /// in the named direct base class from which the declaration was inherited.
+  ConstructorUsingShadowDecl *NominatedBaseClassShadowDecl;
+
+  /// \brief If this constructor using declaration inherted the constructor
+  /// from an indirect base class, this is the ConstructorUsingShadowDecl
+  /// that will be used to construct the unique direct or virtual base class
+  /// that receives the constructor arguments.
+  ConstructorUsingShadowDecl *ConstructedBaseClassShadowDecl;
+
+  /// \brief \c true if the constructor ultimately named by this using shadow
+  /// declaration is within a virtual base class subobject of the class that
+  /// contains this declaration.
+  unsigned IsVirtual : 1;
+
+  ConstructorUsingShadowDecl(ASTContext &C, DeclContext *DC, SourceLocation Loc,
+                             UsingDecl *Using, NamedDecl *Target,
+                             bool TargetInVirtualBase)
+      : UsingShadowDecl(ConstructorUsingShadow, C, DC, Loc, Using,
+                        Target->getUnderlyingDecl()),
+        NominatedBaseClassShadowDecl(
+            dyn_cast<ConstructorUsingShadowDecl>(Target)),
+        ConstructedBaseClassShadowDecl(NominatedBaseClassShadowDecl),
+        IsVirtual(TargetInVirtualBase) {
+    // If we found a constructor for a non-virtual base class, but it chains to
+    // a constructor for a virtual base, we should directly call the virtual
+    // base constructor instead.
+    // FIXME: This logic belongs in Sema.
+    if (!TargetInVirtualBase && NominatedBaseClassShadowDecl &&
+        NominatedBaseClassShadowDecl->constructsVirtualBase()) {
+      ConstructedBaseClassShadowDecl =
+          NominatedBaseClassShadowDecl->ConstructedBaseClassShadowDecl;
+      IsVirtual = true;
+    }
+  }
+  ConstructorUsingShadowDecl(ASTContext &C, EmptyShell Empty)
+      : UsingShadowDecl(ConstructorUsingShadow, C, Empty) {}
+
+public:
+  static ConstructorUsingShadowDecl *Create(ASTContext &C, DeclContext *DC,
+                                            SourceLocation Loc,
+                                            UsingDecl *Using, NamedDecl *Target,
+                                            bool IsVirtual);
+  static ConstructorUsingShadowDecl *CreateDeserialized(ASTContext &C,
+                                                        unsigned ID);
+
+  /// Returns the parent of this using shadow declaration, which
+  /// is the class in which this is declared.
+  //@{
+  const CXXRecordDecl *getParent() const {
+    return cast<CXXRecordDecl>(getDeclContext());
+  }
+  CXXRecordDecl *getParent() {
+    return cast<CXXRecordDecl>(getDeclContext());
+  }
+  //@}
+
+  /// \brief Get the inheriting constructor declaration for the direct base
+  /// class from which this using shadow declaration was inherited, if there is
+  /// one. This can be different for each redeclaration of the same shadow decl.
+  ConstructorUsingShadowDecl *getNominatedBaseClassShadowDecl() const {
+    return NominatedBaseClassShadowDecl;
+  }
+
+  /// \brief Get the inheriting constructor declaration for the base class
+  /// for which we don't have an explicit initializer, if there is one.
+  ConstructorUsingShadowDecl *getConstructedBaseClassShadowDecl() const {
+    return ConstructedBaseClassShadowDecl;
+  }
+
+  /// \brief Get the base class that was named in the using declaration. This
+  /// can be different for each redeclaration of this same shadow decl.
+  CXXRecordDecl *getNominatedBaseClass() const;
+
+  /// \brief Get the base class whose constructor or constructor shadow
+  /// declaration is passed the constructor arguments.
+  CXXRecordDecl *getConstructedBaseClass() const {
+    return cast<CXXRecordDecl>((ConstructedBaseClassShadowDecl
+                                    ? ConstructedBaseClassShadowDecl
+                                    : getTargetDecl())
+                                   ->getDeclContext());
+  }
+
+  /// \brief Returns \c true if the constructed base class is a virtual base
+  /// class subobject of this declaration's class.
+  bool constructsVirtualBase() const {
+    return IsVirtual;
+  }
+
+  /// \brief Get the constructor or constructor template in the derived class
+  /// correspnding to this using shadow declaration, if it has been implicitly
+  /// declared already.
+  CXXConstructorDecl *getConstructor() const;
+  void setConstructor(NamedDecl *Ctor);
+
+  static bool classof(const Decl *D) { return classofKind(D->getKind()); }
+  static bool classofKind(Kind K) { return K == ConstructorUsingShadow; }
 
   friend class ASTDeclReader;
   friend class ASTDeclWriter;

clang/include/clang/AST/ExprCXX.h

@@ -1318,6 +1318,73 @@ class CXXConstructExpr : public Expr {
   friend class ASTStmtReader;
 };
 
+/// \brief Represents a call to an inherited base class constructor from an
+/// inheriting constructor. This call implicitly forwards the arguments from
+/// the enclosing context (an inheriting constructor) to the specified inherited
+/// base class constructor.
+class CXXInheritedCtorInitExpr : public Expr {
+private:
+  CXXConstructorDecl *Constructor;
+
+  /// The location of the using declaration.
+  SourceLocation Loc;
+
+  /// Whether this is the construction of a virtual base.
+  unsigned ConstructsVirtualBase : 1;
+
+  /// Whether the constructor is inherited from a virtual base class of the
+  /// class that we construct.
+  unsigned InheritedFromVirtualBase : 1;
+
+public:
+  /// \brief Construct a C++ inheriting construction expression.
+  CXXInheritedCtorInitExpr(SourceLocation Loc, QualType T,
+                           CXXConstructorDecl *Ctor, bool ConstructsVirtualBase,
+                           bool InheritedFromVirtualBase)
+      : Expr(CXXInheritedCtorInitExprClass, T, VK_RValue, OK_Ordinary, false,
+             false, false, false),
+        Constructor(Ctor), Loc(Loc),
+        ConstructsVirtualBase(ConstructsVirtualBase),
+        InheritedFromVirtualBase(InheritedFromVirtualBase) {
+    assert(!T->isDependentType());
+  }
+
+  /// \brief Construct an empty C++ inheriting construction expression.
+  explicit CXXInheritedCtorInitExpr(EmptyShell Empty)
+      : Expr(CXXInheritedCtorInitExprClass, Empty), Constructor(nullptr),
+        ConstructsVirtualBase(false), InheritedFromVirtualBase(false) {}
+
+  /// \brief Get the constructor that this expression will call.
+  CXXConstructorDecl *getConstructor() const { return Constructor; }
+
+  /// \brief Determine whether this constructor is actually constructing
+  /// a base class (rather than a complete object).
+  bool constructsVBase() const { return ConstructsVirtualBase; }
+  CXXConstructExpr::ConstructionKind getConstructionKind() const {
+    return ConstructsVirtualBase ? CXXConstructExpr::CK_VirtualBase
+                                 : CXXConstructExpr::CK_NonVirtualBase;
+  }
+
+  /// \brief Determine whether the inherited constructor is inherited from a
+  /// virtual base of the object we construct. If so, we are not responsible
+  /// for calling the inherited constructor (the complete object constructor
+  /// does that), and so we don't need to pass any arguments.
+  bool inheritedFromVBase() const { return InheritedFromVirtualBase; }
+
+  SourceLocation getLocation() const LLVM_READONLY { return Loc; }
+  SourceLocation getLocStart() const LLVM_READONLY { return Loc; }
+  SourceLocation getLocEnd() const LLVM_READONLY { return Loc; }
+
+  static bool classof(const Stmt *T) {
+    return T->getStmtClass() == CXXInheritedCtorInitExprClass;
+  }
+  child_range children() {
+    return child_range(child_iterator(), child_iterator());
+  }
+
+  friend class ASTStmtReader;
+};
+
 /// \brief Represents an explicit C++ type conversion that uses "functional"
 /// notation (C++ [expr.type.conv]).
 ///