lib/Sema/SemaInherit.cpp - clang - Git at Google

 //===---- SemaInherit.cpp - C++ Inheritance ---------------------*- C++ -*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This file provides Sema routines for C++ inheritance semantics,
 // including searching the inheritance hierarchy.
 //
 //===----------------------------------------------------------------------===//

 #include "SemaInherit.h"
 #include "Sema.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/DeclCXX.h"
 #include "clang/AST/Type.h"
 #include "clang/AST/TypeOrdering.h"
 #include <algorithm>
 #include <memory>
 #include <set>
 #include <string>

 using namespace clang;

 /// \brief Computes the set of declarations referenced by these base
 /// paths.
 void BasePaths::ComputeDeclsFound() {
   assert(NumDeclsFound == 0 && !DeclsFound &&
          "Already computed the set of declarations");

   std::set<NamedDecl *> Decls;
   for (BasePaths::paths_iterator Path = begin(), PathEnd = end();
        Path != PathEnd; ++Path)
     Decls.insert(*Path->Decls.first);

   NumDeclsFound = Decls.size();
   DeclsFound = new NamedDecl * [NumDeclsFound];
   std::copy(Decls.begin(), Decls.end(), DeclsFound);
 }

 BasePaths::decl_iterator BasePaths::found_decls_begin() {
   if (NumDeclsFound == 0)
     ComputeDeclsFound();
   return DeclsFound;
 }

 BasePaths::decl_iterator BasePaths::found_decls_end() {
   if (NumDeclsFound == 0)
     ComputeDeclsFound();
   return DeclsFound + NumDeclsFound;
 }

 /// isAmbiguous - Determines whether the set of paths provided is
 /// ambiguous, i.e., there are two or more paths that refer to
 /// different base class subobjects of the same type. BaseType must be
 /// an unqualified, canonical class type.
 bool BasePaths::isAmbiguous(QualType BaseType) {
   assert(BaseType->isCanonical() && "Base type must be the canonical type");
   assert(BaseType.getCVRQualifiers() == 0 && "Base type must be unqualified");
   std::pair<bool, unsigned>& Subobjects = ClassSubobjects[BaseType];
   return Subobjects.second + (Subobjects.first? 1 : 0) > 1;
 }

 /// clear - Clear out all prior path information.
 void BasePaths::clear() {
   Paths.clear();
   ClassSubobjects.clear();
   ScratchPath.clear();
   DetectedVirtual = 0;
 }

 /// @brief Swaps the contents of this BasePaths structure with the
 /// contents of Other.
 void BasePaths::swap(BasePaths &Other) {
   std::swap(Origin, Other.Origin);
   Paths.swap(Other.Paths);
   ClassSubobjects.swap(Other.ClassSubobjects);
   std::swap(FindAmbiguities, Other.FindAmbiguities);
   std::swap(RecordPaths, Other.RecordPaths);
   std::swap(DetectVirtual, Other.DetectVirtual);
   std::swap(DetectedVirtual, Other.DetectedVirtual);
 }

 /// IsDerivedFrom - Determine whether the type Derived is derived from
 /// the type Base, ignoring qualifiers on Base and Derived. This
 /// routine does not assess whether an actual conversion from a
 /// Derived* to a Base* is legal, because it does not account for
 /// ambiguous conversions or conversions to private/protected bases.
 bool Sema::IsDerivedFrom(QualType Derived, QualType Base) {
   BasePaths Paths(/*FindAmbiguities=*/false, /*RecordPaths=*/false,
                   /*DetectVirtual=*/false);
   return IsDerivedFrom(Derived, Base, Paths);
 }

 /// IsDerivedFrom - Determine whether the type Derived is derived from
 /// the type Base, ignoring qualifiers on Base and Derived. This
 /// routine does not assess whether an actual conversion from a
 /// Derived* to a Base* is legal, because it does not account for
 /// ambiguous conversions or conversions to private/protected
 /// bases. This routine will use Paths to determine if there are
 /// ambiguous paths (if @c Paths.isFindingAmbiguities()) and record
 /// information about all of the paths (if @c Paths.isRecordingPaths()).
 bool Sema::IsDerivedFrom(QualType Derived, QualType Base, BasePaths &Paths) {
   Derived = Context.getCanonicalType(Derived).getUnqualifiedType();
   Base = Context.getCanonicalType(Base).getUnqualifiedType();

   if (!Derived->isRecordType() || !Base->isRecordType())
     return false;

   if (Derived == Base)
     return false;

   Paths.setOrigin(Derived);
   return LookupInBases(cast<CXXRecordDecl>(Derived->getAs<RecordType>()->getDecl()),
                        MemberLookupCriteria(Base), Paths);
 }

 /// LookupInBases - Look for something that meets the specified
 /// Criteria within the base classes of Class (or any of its base
 /// classes, transitively). This routine populates BasePaths with the
 /// list of paths that one can take to find the entity that meets the
 /// search criteria, and returns true if any such entity is found. The
 /// various options passed to the BasePath constructor will affect the
 /// behavior of this lookup, e.g., whether it finds ambiguities,
 /// records paths, or attempts to detect the use of virtual base
 /// classes.
 bool Sema::LookupInBases(CXXRecordDecl *Class,
                          const MemberLookupCriteria& Criteria,
                          BasePaths &Paths) {
   bool FoundPath = false;

   for (CXXRecordDecl::base_class_const_iterator BaseSpec = Class->bases_begin(),
                                              BaseSpecEnd = Class->bases_end();
        BaseSpec != BaseSpecEnd; ++BaseSpec) {
     // Find the record of the base class subobjects for this type.
     QualType BaseType = Context.getCanonicalType(BaseSpec->getType());
     BaseType = BaseType.getUnqualifiedType();

     // If a base class of the class template depends on a template-parameter,
     // the base class scope is not examined during unqualified name lookup.
     // [temp.dep]p3.
     if (BaseType->isDependentType())
       continue;

     // Determine whether we need to visit this base class at all,
     // updating the count of subobjects appropriately.
     std::pair<bool, unsigned>& Subobjects = Paths.ClassSubobjects[BaseType];
     bool VisitBase = true;
     bool SetVirtual = false;
     if (BaseSpec->isVirtual()) {
       VisitBase = !Subobjects.first;
       Subobjects.first = true;
       if (Paths.isDetectingVirtual() && Paths.DetectedVirtual == 0) {
         // If this is the first virtual we find, remember it. If it turns out
         // there is no base path here, we'll reset it later.
         Paths.DetectedVirtual = BaseType->getAs<RecordType>();
         SetVirtual = true;
       }
     } else
       ++Subobjects.second;

     if (Paths.isRecordingPaths()) {
       // Add this base specifier to the current path.
       BasePathElement Element;
       Element.Base = &*BaseSpec;
       Element.Class = Class;
       if (BaseSpec->isVirtual())
         Element.SubobjectNumber = 0;
       else
         Element.SubobjectNumber = Subobjects.second;
       Paths.ScratchPath.push_back(Element);
     }

     CXXRecordDecl *BaseRecord
       = cast<CXXRecordDecl>(BaseSpec->getType()->getAs<RecordType>()->getDecl());

     // Either look at the base class type or look into the base class
     // type to see if we've found a member that meets the search
     // criteria.
     bool FoundPathToThisBase = false;
     switch (Criteria.Kind) {
     case MemberLookupCriteria::LK_Base:
       FoundPathToThisBase
         = (Context.getCanonicalType(BaseSpec->getType()) == Criteria.Base);
       break;
     case MemberLookupCriteria::LK_NamedMember:
       Paths.ScratchPath.Decls = BaseRecord->lookup(Criteria.Name);
       while (Paths.ScratchPath.Decls.first != Paths.ScratchPath.Decls.second) {
         if (isAcceptableLookupResult(*Paths.ScratchPath.Decls.first,
                                      Criteria.NameKind, Criteria.IDNS)) {
           FoundPathToThisBase = true;
           break;
         }
         ++Paths.ScratchPath.Decls.first;
       }
       break;
     case MemberLookupCriteria::LK_OverriddenMember:
       Paths.ScratchPath.Decls =
         BaseRecord->lookup(Criteria.Method->getDeclName());
       while (Paths.ScratchPath.Decls.first != Paths.ScratchPath.Decls.second) {
         if (CXXMethodDecl *MD =
               dyn_cast<CXXMethodDecl>(*Paths.ScratchPath.Decls.first)) {
           OverloadedFunctionDecl::function_iterator MatchedDecl;
           if (MD->isVirtual() &&
               !IsOverload(Criteria.Method, MD, MatchedDecl)) {
             FoundPathToThisBase = true;
             break;
           }
         }

         ++Paths.ScratchPath.Decls.first;
       }
       break;
     }

     if (FoundPathToThisBase) {
       // We've found a path that terminates that this base.
       FoundPath = true;
       if (Paths.isRecordingPaths()) {
         // We have a path. Make a copy of it before moving on.
         Paths.Paths.push_back(Paths.ScratchPath);
       } else if (!Paths.isFindingAmbiguities()) {
         // We found a path and we don't care about ambiguities;
         // return immediately.
         return FoundPath;
       }
     } else if (VisitBase && LookupInBases(BaseRecord, Criteria, Paths)) {
       // C++ [class.member.lookup]p2:
       //   A member name f in one sub-object B hides a member name f in
       //   a sub-object A if A is a base class sub-object of B. Any
       //   declarations that are so hidden are eliminated from
       //   consideration.

       // There is a path to a base class that meets the criteria. If we're not
       // collecting paths or finding ambiguities, we're done.
       FoundPath = true;
       if (!Paths.isFindingAmbiguities())
         return FoundPath;
     }

     // Pop this base specifier off the current path (if we're
     // collecting paths).
     if (Paths.isRecordingPaths())
       Paths.ScratchPath.pop_back();
     // If we set a virtual earlier, and this isn't a path, forget it again.
     if (SetVirtual && !FoundPath) {
       Paths.DetectedVirtual = 0;
     }
   }

   return FoundPath;
 }

 /// CheckDerivedToBaseConversion - Check whether the Derived-to-Base
 /// conversion (where Derived and Base are class types) is
 /// well-formed, meaning that the conversion is unambiguous (and
 /// that all of the base classes are accessible). Returns true
 /// and emits a diagnostic if the code is ill-formed, returns false
 /// otherwise. Loc is the location where this routine should point to
 /// if there is an error, and Range is the source range to highlight
 /// if there is an error.
 bool
 Sema::CheckDerivedToBaseConversion(QualType Derived, QualType Base,
                                    unsigned InaccessibleBaseID,
                                    unsigned AmbigiousBaseConvID,
                                    SourceLocation Loc, SourceRange Range,
                                    DeclarationName Name) {
   // First, determine whether the path from Derived to Base is
   // ambiguous. This is slightly more expensive than checking whether
   // the Derived to Base conversion exists, because here we need to
   // explore multiple paths to determine if there is an ambiguity.
   BasePaths Paths(/*FindAmbiguities=*/true, /*RecordPaths=*/true,
                   /*DetectVirtual=*/false);
   bool DerivationOkay = IsDerivedFrom(Derived, Base, Paths);
   assert(DerivationOkay &&
          "Can only be used with a derived-to-base conversion");
   (void)DerivationOkay;

   if (!Paths.isAmbiguous(Context.getCanonicalType(Base).getUnqualifiedType())) {
     // Check that the base class can be accessed.
     return CheckBaseClassAccess(Derived, Base, InaccessibleBaseID, Paths, Loc,
                                 Name);
   }

   // We know that the derived-to-base conversion is ambiguous, and
   // we're going to produce a diagnostic. Perform the derived-to-base
   // search just one more time to compute all of the possible paths so
   // that we can print them out. This is more expensive than any of
   // the previous derived-to-base checks we've done, but at this point
   // performance isn't as much of an issue.
   Paths.clear();
   Paths.setRecordingPaths(true);
   bool StillOkay = IsDerivedFrom(Derived, Base, Paths);
   assert(StillOkay && "Can only be used with a derived-to-base conversion");
   (void)StillOkay;

   // Build up a textual representation of the ambiguous paths, e.g.,
   // D -> B -> A, that will be used to illustrate the ambiguous
   // conversions in the diagnostic. We only print one of the paths
   // to each base class subobject.
   std::string PathDisplayStr = getAmbiguousPathsDisplayString(Paths);

   Diag(Loc, AmbigiousBaseConvID)
     << Derived << Base << PathDisplayStr << Range << Name;
   return true;
 }

 bool
 Sema::CheckDerivedToBaseConversion(QualType Derived, QualType Base,
                                    SourceLocation Loc, SourceRange Range) {
   return CheckDerivedToBaseConversion(Derived, Base,
                                       diag::err_conv_to_inaccessible_base,
                                       diag::err_ambiguous_derived_to_base_conv,
                                       Loc, Range, DeclarationName());
 }


 /// @brief Builds a string representing ambiguous paths from a
 /// specific derived class to different subobjects of the same base
 /// class.
 ///
 /// This function builds a string that can be used in error messages
 /// to show the different paths that one can take through the
 /// inheritance hierarchy to go from the derived class to different
 /// subobjects of a base class. The result looks something like this:
 /// @code
 /// struct D -> struct B -> struct A
 /// struct D -> struct C -> struct A
 /// @endcode
 std::string Sema::getAmbiguousPathsDisplayString(BasePaths &Paths) {
   std::string PathDisplayStr;
   std::set<unsigned> DisplayedPaths;
   for (BasePaths::paths_iterator Path = Paths.begin();
        Path != Paths.end(); ++Path) {
     if (DisplayedPaths.insert(Path->back().SubobjectNumber).second) {
       // We haven't displayed a path to this particular base
       // class subobject yet.
       PathDisplayStr += "\n    ";
       PathDisplayStr += Paths.getOrigin().getAsString();
       for (BasePath::const_iterator Element = Path->begin();
            Element != Path->end(); ++Element)
         PathDisplayStr += " -> " + Element->Base->getType().getAsString();
     }
   }

   return PathDisplayStr;
 }
	//===---- SemaInherit.cpp - C++ Inheritance ---------------------- C++ --===//
	//
	// The LLVM Compiler Infrastructure
	//
	// This file is distributed under the University of Illinois Open Source
	// License. See LICENSE.TXT for details.
	//
	//===----------------------------------------------------------------------===//
	//
	// This file provides Sema routines for C++ inheritance semantics,
	// including searching the inheritance hierarchy.
	//
	//===----------------------------------------------------------------------===//

	#include "SemaInherit.h"
	#include "Sema.h"
	#include "clang/AST/ASTContext.h"
	#include "clang/AST/DeclCXX.h"
	#include "clang/AST/Type.h"
	#include "clang/AST/TypeOrdering.h"
	#include <algorithm>
	#include <memory>
	#include <set>
	#include <string>

	using namespace clang;

	/// \brief Computes the set of declarations referenced by these base
	/// paths.
	void BasePaths::ComputeDeclsFound() {
	assert(NumDeclsFound == 0 && !DeclsFound &&
	"Already computed the set of declarations");

	std::set<NamedDecl *> Decls;
	for (BasePaths::paths_iterator Path = begin(), PathEnd = end();
	Path != PathEnd; ++Path)
	Decls.insert(*Path->Decls.first);

	NumDeclsFound = Decls.size();
	DeclsFound = new NamedDecl * [NumDeclsFound];
	std::copy(Decls.begin(), Decls.end(), DeclsFound);
	}

	BasePaths::decl_iterator BasePaths::found_decls_begin() {
	if (NumDeclsFound == 0)
	ComputeDeclsFound();
	return DeclsFound;
	}

	BasePaths::decl_iterator BasePaths::found_decls_end() {
	if (NumDeclsFound == 0)
	ComputeDeclsFound();
	return DeclsFound + NumDeclsFound;
	}

	/// isAmbiguous - Determines whether the set of paths provided is
	/// ambiguous, i.e., there are two or more paths that refer to
	/// different base class subobjects of the same type. BaseType must be
	/// an unqualified, canonical class type.
	bool BasePaths::isAmbiguous(QualType BaseType) {
	assert(BaseType->isCanonical() && "Base type must be the canonical type");
	assert(BaseType.getCVRQualifiers() == 0 && "Base type must be unqualified");
	std::pair<bool, unsigned>& Subobjects = ClassSubobjects[BaseType];
	return Subobjects.second + (Subobjects.first? 1 : 0) > 1;
	}

	/// clear - Clear out all prior path information.
	void BasePaths::clear() {
	Paths.clear();
	ClassSubobjects.clear();
	ScratchPath.clear();
	DetectedVirtual = 0;
	}

	/// @brief Swaps the contents of this BasePaths structure with the
	/// contents of Other.
	void BasePaths::swap(BasePaths &Other) {
	std::swap(Origin, Other.Origin);
	Paths.swap(Other.Paths);
	ClassSubobjects.swap(Other.ClassSubobjects);
	std::swap(FindAmbiguities, Other.FindAmbiguities);
	std::swap(RecordPaths, Other.RecordPaths);
	std::swap(DetectVirtual, Other.DetectVirtual);
	std::swap(DetectedVirtual, Other.DetectedVirtual);
	}

	/// IsDerivedFrom - Determine whether the type Derived is derived from
	/// the type Base, ignoring qualifiers on Base and Derived. This
	/// routine does not assess whether an actual conversion from a
	/// Derived* to a Base* is legal, because it does not account for
	/// ambiguous conversions or conversions to private/protected bases.
	bool Sema::IsDerivedFrom(QualType Derived, QualType Base) {
	BasePaths Paths(/FindAmbiguities=/false, /RecordPaths=/false,
	/DetectVirtual=/false);
	return IsDerivedFrom(Derived, Base, Paths);
	}

	/// IsDerivedFrom - Determine whether the type Derived is derived from
	/// the type Base, ignoring qualifiers on Base and Derived. This
	/// routine does not assess whether an actual conversion from a
	/// Derived* to a Base* is legal, because it does not account for
	/// ambiguous conversions or conversions to private/protected
	/// bases. This routine will use Paths to determine if there are
	/// ambiguous paths (if @c Paths.isFindingAmbiguities()) and record
	/// information about all of the paths (if @c Paths.isRecordingPaths()).
	bool Sema::IsDerivedFrom(QualType Derived, QualType Base, BasePaths &Paths) {
	Derived = Context.getCanonicalType(Derived).getUnqualifiedType();
	Base = Context.getCanonicalType(Base).getUnqualifiedType();

	if (!Derived->isRecordType() \|\| !Base->isRecordType())
	return false;

	if (Derived == Base)
	return false;

	Paths.setOrigin(Derived);
	return LookupInBases(cast<CXXRecordDecl>(Derived->getAs<RecordType>()->getDecl()),
	MemberLookupCriteria(Base), Paths);
	}

	/// LookupInBases - Look for something that meets the specified
	/// Criteria within the base classes of Class (or any of its base
	/// classes, transitively). This routine populates BasePaths with the
	/// list of paths that one can take to find the entity that meets the
	/// search criteria, and returns true if any such entity is found. The
	/// various options passed to the BasePath constructor will affect the
	/// behavior of this lookup, e.g., whether it finds ambiguities,
	/// records paths, or attempts to detect the use of virtual base
	/// classes.
	bool Sema::LookupInBases(CXXRecordDecl *Class,
	const MemberLookupCriteria& Criteria,
	BasePaths &Paths) {
	bool FoundPath = false;

	for (CXXRecordDecl::base_class_const_iterator BaseSpec = Class->bases_begin(),
	BaseSpecEnd = Class->bases_end();
	BaseSpec != BaseSpecEnd; ++BaseSpec) {
	// Find the record of the base class subobjects for this type.
	QualType BaseType = Context.getCanonicalType(BaseSpec->getType());
	BaseType = BaseType.getUnqualifiedType();

	// If a base class of the class template depends on a template-parameter,
	// the base class scope is not examined during unqualified name lookup.
	// [temp.dep]p3.
	if (BaseType->isDependentType())
	continue;

	// Determine whether we need to visit this base class at all,
	// updating the count of subobjects appropriately.
	std::pair<bool, unsigned>& Subobjects = Paths.ClassSubobjects[BaseType];
	bool VisitBase = true;
	bool SetVirtual = false;
	if (BaseSpec->isVirtual()) {
	VisitBase = !Subobjects.first;
	Subobjects.first = true;
	if (Paths.isDetectingVirtual() && Paths.DetectedVirtual == 0) {
	// If this is the first virtual we find, remember it. If it turns out
	// there is no base path here, we'll reset it later.
	Paths.DetectedVirtual = BaseType->getAs<RecordType>();
	SetVirtual = true;
	}
	} else
	++Subobjects.second;

	if (Paths.isRecordingPaths()) {
	// Add this base specifier to the current path.
	BasePathElement Element;
	Element.Base = &*BaseSpec;
	Element.Class = Class;
	if (BaseSpec->isVirtual())
	Element.SubobjectNumber = 0;
	else
	Element.SubobjectNumber = Subobjects.second;
	Paths.ScratchPath.push_back(Element);
	}

	CXXRecordDecl *BaseRecord
	= cast<CXXRecordDecl>(BaseSpec->getType()->getAs<RecordType>()->getDecl());

	// Either look at the base class type or look into the base class
	// type to see if we've found a member that meets the search
	// criteria.
	bool FoundPathToThisBase = false;
	switch (Criteria.Kind) {
	case MemberLookupCriteria::LK_Base:
	FoundPathToThisBase
	= (Context.getCanonicalType(BaseSpec->getType()) == Criteria.Base);
	break;
	case MemberLookupCriteria::LK_NamedMember:
	Paths.ScratchPath.Decls = BaseRecord->lookup(Criteria.Name);
	while (Paths.ScratchPath.Decls.first != Paths.ScratchPath.Decls.second) {
	if (isAcceptableLookupResult(*Paths.ScratchPath.Decls.first,
	Criteria.NameKind, Criteria.IDNS)) {
	FoundPathToThisBase = true;
	break;
	}
	++Paths.ScratchPath.Decls.first;
	}
	break;
	case MemberLookupCriteria::LK_OverriddenMember:
	Paths.ScratchPath.Decls =
	BaseRecord->lookup(Criteria.Method->getDeclName());
	while (Paths.ScratchPath.Decls.first != Paths.ScratchPath.Decls.second) {
	if (CXXMethodDecl *MD =
	dyn_cast<CXXMethodDecl>(*Paths.ScratchPath.Decls.first)) {
	OverloadedFunctionDecl::function_iterator MatchedDecl;
	if (MD->isVirtual() &&
	!IsOverload(Criteria.Method, MD, MatchedDecl)) {
	FoundPathToThisBase = true;
	break;
	}
	}

	++Paths.ScratchPath.Decls.first;
	}
	break;
	}

	if (FoundPathToThisBase) {
	// We've found a path that terminates that this base.
	FoundPath = true;
	if (Paths.isRecordingPaths()) {
	// We have a path. Make a copy of it before moving on.
	Paths.Paths.push_back(Paths.ScratchPath);
	} else if (!Paths.isFindingAmbiguities()) {
	// We found a path and we don't care about ambiguities;
	// return immediately.
	return FoundPath;
	}
	} else if (VisitBase && LookupInBases(BaseRecord, Criteria, Paths)) {
	// C++ [class.member.lookup]p2:
	// A member name f in one sub-object B hides a member name f in
	// a sub-object A if A is a base class sub-object of B. Any
	// declarations that are so hidden are eliminated from
	// consideration.

	// There is a path to a base class that meets the criteria. If we're not
	// collecting paths or finding ambiguities, we're done.
	FoundPath = true;
	if (!Paths.isFindingAmbiguities())
	return FoundPath;
	}

	// Pop this base specifier off the current path (if we're
	// collecting paths).
	if (Paths.isRecordingPaths())
	Paths.ScratchPath.pop_back();
	// If we set a virtual earlier, and this isn't a path, forget it again.
	if (SetVirtual && !FoundPath) {
	Paths.DetectedVirtual = 0;
	}
	}

	return FoundPath;
	}

	/// CheckDerivedToBaseConversion - Check whether the Derived-to-Base
	/// conversion (where Derived and Base are class types) is
	/// well-formed, meaning that the conversion is unambiguous (and
	/// that all of the base classes are accessible). Returns true
	/// and emits a diagnostic if the code is ill-formed, returns false
	/// otherwise. Loc is the location where this routine should point to
	/// if there is an error, and Range is the source range to highlight
	/// if there is an error.
	bool
	Sema::CheckDerivedToBaseConversion(QualType Derived, QualType Base,
	unsigned InaccessibleBaseID,
	unsigned AmbigiousBaseConvID,
	SourceLocation Loc, SourceRange Range,
	DeclarationName Name) {
	// First, determine whether the path from Derived to Base is
	// ambiguous. This is slightly more expensive than checking whether
	// the Derived to Base conversion exists, because here we need to
	// explore multiple paths to determine if there is an ambiguity.
	BasePaths Paths(/FindAmbiguities=/true, /RecordPaths=/true,
	/DetectVirtual=/false);
	bool DerivationOkay = IsDerivedFrom(Derived, Base, Paths);
	assert(DerivationOkay &&
	"Can only be used with a derived-to-base conversion");
	(void)DerivationOkay;

	if (!Paths.isAmbiguous(Context.getCanonicalType(Base).getUnqualifiedType())) {
	// Check that the base class can be accessed.
	return CheckBaseClassAccess(Derived, Base, InaccessibleBaseID, Paths, Loc,
	Name);
	}

	// We know that the derived-to-base conversion is ambiguous, and
	// we're going to produce a diagnostic. Perform the derived-to-base
	// search just one more time to compute all of the possible paths so
	// that we can print them out. This is more expensive than any of
	// the previous derived-to-base checks we've done, but at this point
	// performance isn't as much of an issue.
	Paths.clear();
	Paths.setRecordingPaths(true);
	bool StillOkay = IsDerivedFrom(Derived, Base, Paths);
	assert(StillOkay && "Can only be used with a derived-to-base conversion");
	(void)StillOkay;

	// Build up a textual representation of the ambiguous paths, e.g.,
	// D -> B -> A, that will be used to illustrate the ambiguous
	// conversions in the diagnostic. We only print one of the paths
	// to each base class subobject.
	std::string PathDisplayStr = getAmbiguousPathsDisplayString(Paths);

	Diag(Loc, AmbigiousBaseConvID)
	<< Derived << Base << PathDisplayStr << Range << Name;
	return true;
	}

	bool
	Sema::CheckDerivedToBaseConversion(QualType Derived, QualType Base,
	SourceLocation Loc, SourceRange Range) {
	return CheckDerivedToBaseConversion(Derived, Base,
	diag::err_conv_to_inaccessible_base,
	diag::err_ambiguous_derived_to_base_conv,
	Loc, Range, DeclarationName());
	}


	/// @brief Builds a string representing ambiguous paths from a
	/// specific derived class to different subobjects of the same base
	/// class.
	///
	/// This function builds a string that can be used in error messages
	/// to show the different paths that one can take through the
	/// inheritance hierarchy to go from the derived class to different
	/// subobjects of a base class. The result looks something like this:
	/// @code
	/// struct D -> struct B -> struct A
	/// struct D -> struct C -> struct A
	/// @endcode
	std::string Sema::getAmbiguousPathsDisplayString(BasePaths &Paths) {
	std::string PathDisplayStr;
	std::set<unsigned> DisplayedPaths;
	for (BasePaths::paths_iterator Path = Paths.begin();
	Path != Paths.end(); ++Path) {
	if (DisplayedPaths.insert(Path->back().SubobjectNumber).second) {
	// We haven't displayed a path to this particular base
	// class subobject yet.
	PathDisplayStr += "\n ";
	PathDisplayStr += Paths.getOrigin().getAsString();
	for (BasePath::const_iterator Element = Path->begin();
	Element != Path->end(); ++Element)
	PathDisplayStr += " -> " + Element->Base->getType().getAsString();
	}
	}

	return PathDisplayStr;
	}