[cxx-interop] Import using decls that expose methods from private base classes

If a C++ type `Derived` inherits from `Base` privately, the public methods from `Base` should not be callable on an instance of `Derived`. However, C++ supports exposing such methods via a using declaration: `using MyPrivateBase::myPublicMethod;`.

MSVC started using this feature for `std::optional` which means Swift doesn't correctly import `var pointee: Pointee` for instantiations of `std::optional` on Windows. This prevents the automatic conformance to `CxxOptional` from being synthesized.

 rdar://114282353 / resolves #68068

Author: egorzhdan

lib/ClangImporter/ClangImporter.cpp

@@ -7074,6 +7074,11 @@ static bool isSufficientlyTrivial(const clang::CXXRecordDecl *decl) {
 /// Checks if a record provides the required value type lifetime operations
 /// (copy and destroy).
 static bool hasCopyTypeOperations(const clang::CXXRecordDecl *decl) {
+  // Hack for a base type of std::optional from the Microsoft standard library.
+  if (decl->isInStdNamespace() && decl->getIdentifier() &&
+      decl->getName() == "_Optional_construct_base")
+    return true;
+
   // If we have no way of copying the type we can't import the class
   // at all because we cannot express the correct semantics as a swift
   // struct.

lib/ClangImporter/ImportDecl.cpp

@@ -2697,6 +2697,31 @@ namespace {
       // SemaLookup.cpp).
       if (!decl->isBeingDefined() && !decl->isDependentContext() &&
           areRecordFieldsComplete(decl)) {
+        if (decl->hasInheritedConstructor() &&
+            Impl.isCxxInteropCompatVersionAtLeast(
+                version::getUpcomingCxxInteropCompatVersion())) {
+          for (auto member : decl->decls()) {
+            if (auto usingDecl = dyn_cast<clang::UsingDecl>(member)) {
+              for (auto usingShadowDecl : usingDecl->shadows()) {
+                if (auto ctorUsingShadowDecl =
+                        dyn_cast<clang::ConstructorUsingShadowDecl>(
+                            usingShadowDecl)) {
+                  auto baseCtorDecl = dyn_cast<clang::CXXConstructorDecl>(
+                      ctorUsingShadowDecl->getTargetDecl());
+                  if (!baseCtorDecl || baseCtorDecl->isDeleted())
+                    continue;
+                  auto derivedCtorDecl = clangSema.findInheritingConstructor(
+                      clang::SourceLocation(), baseCtorDecl,
+                      ctorUsingShadowDecl);
+                  if (!derivedCtorDecl->isDefined() &&
+                      !derivedCtorDecl->isDeleted())
+                    clangSema.DefineInheritingConstructor(
+                        clang::SourceLocation(), derivedCtorDecl);
+                }
+              }
+            }
+          }
+        }
         if (decl->needsImplicitDefaultConstructor()) {
           clang::CXXConstructorDecl *ctor =
               clangSema.DeclareImplicitDefaultConstructor(
@@ -3237,6 +3262,65 @@ namespace {
                                 llvm::None);
     }
 
+    /// Handles special functions such as subscripts and dereference operators.
+    bool processSpecialImportedFunc(FuncDecl *func, ImportedName importedName) {
+      auto dc = func->getDeclContext();
+
+      if (importedName.isSubscriptAccessor()) {
+        assert(func->getParameters()->size() == 1);
+        auto typeDecl = dc->getSelfNominalTypeDecl();
+        auto parameter = func->getParameters()->get(0);
+        auto parameterType = parameter->getTypeInContext();
+        if (!typeDecl || !parameterType)
+          return false;
+        if (parameter->isInOut())
+          // Subscripts with inout parameters are not allowed in Swift.
+          return false;
+        // Subscript setter is marked as mutating in Swift even if the
+        // C++ `operator []` is `const`.
+        if (importedName.getAccessorKind() ==
+                ImportedAccessorKind::SubscriptSetter &&
+            !dc->isModuleScopeContext() &&
+            !typeDecl->getDeclaredType()->isForeignReferenceType())
+          func->setSelfAccessKind(SelfAccessKind::Mutating);
+
+        auto &getterAndSetter = Impl.cxxSubscripts[{typeDecl, parameterType}];
+
+        switch (importedName.getAccessorKind()) {
+        case ImportedAccessorKind::SubscriptGetter:
+          getterAndSetter.first = func;
+          break;
+        case ImportedAccessorKind::SubscriptSetter:
+          getterAndSetter.second = func;
+          break;
+        default:
+          llvm_unreachable("invalid subscript kind");
+        }
+
+        Impl.markUnavailable(func, "use subscript");
+      }
+
+      if (importedName.isDereferenceAccessor()) {
+        auto typeDecl = dc->getSelfNominalTypeDecl();
+        auto &getterAndSetter = Impl.cxxDereferenceOperators[typeDecl];
+
+        switch (importedName.getAccessorKind()) {
+        case ImportedAccessorKind::DereferenceGetter:
+          getterAndSetter.first = func;
+          break;
+        case ImportedAccessorKind::DereferenceSetter:
+          getterAndSetter.second = func;
+          break;
+        default:
+          llvm_unreachable("invalid dereference operator kind");
+        }
+
+        Impl.markUnavailable(func, "use .pointee property");
+      }
+
+      return true;
+    }
+
     Decl *importFunctionDecl(
         const clang::FunctionDecl *decl, ImportedName importedName,
         llvm::Optional<ImportedName> correctSwiftName,
@@ -3556,69 +3640,14 @@ namespace {
             func->setImportAsStaticMember();
           }
         }
+        // Someday, maybe this will need to be 'open' for C++ virtual methods.
+        func->setAccess(AccessLevel::Public);
 
-        bool makePrivate = false;
-
-        if (importedName.isSubscriptAccessor() && !importFuncWithoutSignature) {
-          assert(func->getParameters()->size() == 1);
-          auto typeDecl = dc->getSelfNominalTypeDecl();
-          auto parameter = func->getParameters()->get(0);
-          auto parameterType = parameter->getTypeInContext();
-          if (!typeDecl || !parameterType)
+        if (!importFuncWithoutSignature) {
+          bool success = processSpecialImportedFunc(func, importedName);
+          if (!success)
             return nullptr;
-          if (parameter->isInOut())
-            // Subscripts with inout parameters are not allowed in Swift.
-            return nullptr;
-          // Subscript setter is marked as mutating in Swift even if the
-          // C++ `operator []` is `const`.
-          if (importedName.getAccessorKind() ==
-                  ImportedAccessorKind::SubscriptSetter &&
-              !dc->isModuleScopeContext() &&
-              !typeDecl->getDeclaredType()->isForeignReferenceType())
-            func->setSelfAccessKind(SelfAccessKind::Mutating);
-
-          auto &getterAndSetter = Impl.cxxSubscripts[{ typeDecl,
-                                                       parameterType }];
-
-          switch (importedName.getAccessorKind()) {
-          case ImportedAccessorKind::SubscriptGetter:
-            getterAndSetter.first = func;
-            break;
-          case ImportedAccessorKind::SubscriptSetter:
-            getterAndSetter.second = func;
-            break;
-          default:
-            llvm_unreachable("invalid subscript kind");
-          }
-
-          Impl.markUnavailable(func, "use subscript");
         }
-
-        if (importedName.isDereferenceAccessor() &&
-            !importFuncWithoutSignature) {
-          auto typeDecl = dc->getSelfNominalTypeDecl();
-          auto &getterAndSetter = Impl.cxxDereferenceOperators[typeDecl];
-
-          switch (importedName.getAccessorKind()) {
-          case ImportedAccessorKind::DereferenceGetter:
-            getterAndSetter.first = func;
-            break;
-          case ImportedAccessorKind::DereferenceSetter:
-            getterAndSetter.second = func;
-            break;
-          default:
-            llvm_unreachable("invalid dereference operator kind");
-          }
-
-          Impl.markUnavailable(func, "use .pointee property");
-          makePrivate = true;
-        }
-
-        if (makePrivate)
-          func->setAccess(AccessLevel::Private);
-        else
-          // Someday, maybe this will need to be 'open' for C++ virtual methods.
-          func->setAccess(AccessLevel::Public);
       }
 
       result->setIsObjC(false);
@@ -3931,18 +3960,31 @@ namespace {
     }
 
     Decl *VisitUsingDecl(const clang::UsingDecl *decl) {
-      // Using declarations are not imported.
+      // See VisitUsingShadowDecl below.
       return nullptr;
     }
 
     Decl *VisitUsingShadowDecl(const clang::UsingShadowDecl *decl) {
-      // Only import types for now.
-      if (!isa<clang::TypeDecl>(decl->getUnderlyingDecl()))
+      // Only import:
+      //   1. Types
+      //   2. C++ methods from privately inherited base classes
+      if (!isa<clang::TypeDecl>(decl->getTargetDecl()) &&
+          !(isa<clang::CXXMethodDecl>(decl->getTargetDecl()) &&
+            Impl.isCxxInteropCompatVersionAtLeast(
+                version::getUpcomingCxxInteropCompatVersion())))
+        return nullptr;
+      // Constructors (e.g. `using BaseClass::BaseClass`) are handled in
+      // VisitCXXRecordDecl, since we need them to determine whether a struct
+      // can be imported into Swift.
+      if (isa<clang::CXXConstructorDecl>(decl->getTargetDecl()))
         return nullptr;
 
       ImportedName importedName;
       llvm::Optional<ImportedName> correctSwiftName;
       std::tie(importedName, correctSwiftName) = importFullName(decl);
+      // Don't import something that doesn't have a name.
+      if (importedName.getDeclName().isSpecial())
+        return nullptr;
       auto Name = importedName.getDeclName().getBaseIdentifier();
       if (Name.empty())
         return nullptr;
@@ -3953,30 +3995,66 @@ namespace {
         return importCompatibilityTypeAlias(decl, importedName,
                                             *correctSwiftName);
 
-      auto DC =
+      auto importedDC =
           Impl.importDeclContextOf(decl, importedName.getEffectiveContext());
-      if (!DC)
+      if (!importedDC)
         return nullptr;
 
-      Decl *SwiftDecl = Impl.importDecl(decl->getUnderlyingDecl(), getActiveSwiftVersion());
-      if (!SwiftDecl)
-        return nullptr;
+      if (isa<clang::TypeDecl>(decl->getTargetDecl())) {
+        Decl *SwiftDecl = Impl.importDecl(decl->getUnderlyingDecl(), getActiveSwiftVersion());
+        if (!SwiftDecl)
+          return nullptr;
 
-      const TypeDecl *SwiftTypeDecl = dyn_cast<TypeDecl>(SwiftDecl);
-      if (!SwiftTypeDecl)
-        return nullptr;
+        const TypeDecl *SwiftTypeDecl = dyn_cast<TypeDecl>(SwiftDecl);
+        if (!SwiftTypeDecl)
+          return nullptr;
 
-      auto Loc = Impl.importSourceLoc(decl->getLocation());
-      auto Result = Impl.createDeclWithClangNode<TypeAliasDecl>(
-          decl,
-          AccessLevel::Public,
-          Impl.importSourceLoc(decl->getBeginLoc()),
-          SourceLoc(), Name,
-          Loc,
-          /*genericparams*/nullptr, DC);
-      Result->setUnderlyingType(SwiftTypeDecl->getDeclaredInterfaceType());
+        auto Loc = Impl.importSourceLoc(decl->getLocation());
+        auto Result = Impl.createDeclWithClangNode<TypeAliasDecl>(
+            decl,
+            AccessLevel::Public,
+            Impl.importSourceLoc(decl->getBeginLoc()),
+            SourceLoc(), Name,
+            Loc,
+            /*genericparams*/nullptr, importedDC);
+        Result->setUnderlyingType(SwiftTypeDecl->getDeclaredInterfaceType());
+
+        return Result;
+      }
+      if (auto targetMethod =
+              dyn_cast<clang::CXXMethodDecl>(decl->getTargetDecl())) {
+        auto dc = dyn_cast<clang::CXXRecordDecl>(decl->getDeclContext());
+
+        auto targetDC = targetMethod->getDeclContext();
+        auto targetRecord = dyn_cast<clang::CXXRecordDecl>(targetDC);
+        if (!targetRecord)
+          return nullptr;
 
-      return Result;
+        // If this struct is not inherited from the struct where the method is
+        // defined, bail.
+        if (!dc->isDerivedFrom(targetRecord))
+          return nullptr;
+
+        auto importedBaseMethod = dyn_cast_or_null<FuncDecl>(
+            Impl.importDecl(targetMethod, getActiveSwiftVersion()));
+        // This will be nullptr for a protected method of base class that is
+        // made public with a using declaration in a derived class. This is
+        // valid in C++ but we do not import such using declarations now.
+        // TODO: make this work for protected base methods.
+        if (!importedBaseMethod)
+          return nullptr;
+        auto clonedMethod = dyn_cast_or_null<FuncDecl>(
+            Impl.importBaseMemberDecl(importedBaseMethod, importedDC));
+        if (!clonedMethod)
+          return nullptr;
+
+        bool success = processSpecialImportedFunc(clonedMethod, importedName);
+        if (!success)
+          return nullptr;
+
+        return clonedMethod;
+      }
+      return nullptr;
     }
 
     /// Add an @objc(name) attribute with the given, optional name expressed as
@@ -8360,11 +8438,17 @@ ClangImporter::Implementation::importDeclImpl(const clang::NamedDecl *ClangDecl,
   if (Result &&
       (!Result->getDeclContext()->isModuleScopeContext() ||
        isa<ClangModuleUnit>(Result->getDeclContext()))) {
+    // For using declarations that expose a method of a base class, the Clang
+    // decl is synthesized lazily when the method is actually used from Swift.
+    bool hasSynthesizedClangNode =
+        isa<clang::UsingShadowDecl>(ClangDecl) && isa<FuncDecl>(Result);
+
     // Either the Swift declaration was from stdlib,
     // or we imported the underlying decl of the typedef,
     // or we imported the decl itself.
     bool ImportedCorrectly =
         !Result->getClangDecl() || SkippedOverTypedef ||
+        hasSynthesizedClangNode ||
         Result->getClangDecl()->getCanonicalDecl() == Canon;
 
     // Or the other type is a typedef,
@@ -8387,7 +8471,7 @@ ClangImporter::Implementation::importDeclImpl(const clang::NamedDecl *ClangDecl,
       }
       assert(ImportedCorrectly);
     }
-    assert(Result->hasClangNode());
+    assert(Result->hasClangNode() || hasSynthesizedClangNode);
   }
 #else
   (void)SkippedOverTypedef;

lib/ClangImporter/ImportName.cpp

@@ -1522,6 +1522,17 @@ ImportedName NameImporter::importNameImpl(const clang::NamedDecl *D,
     return ImportedName();
   result.effectiveContext = effectiveCtx;
 
+  // If this is a using declaration, import the name of the shadowed decl and
+  // adjust the context.
+  if (auto usingShadowDecl = dyn_cast<clang::UsingShadowDecl>(D)) {
+    auto targetDecl = usingShadowDecl->getTargetDecl();
+    if (isa<clang::CXXMethodDecl>(targetDecl)) {
+      ImportedName baseName = importName(targetDecl, version, givenName);
+      baseName.effectiveContext = effectiveCtx;
+      return baseName;
+    }
+  }
+
   // Gather information from the swift_async attribute, if there is one.
   llvm::Optional<unsigned> completionHandlerParamIndex;
   bool completionHandlerFlagIsZeroOnError = false;

lib/ClangImporter/ImporterImpl.h

@@ -663,9 +663,9 @@ class LLVM_LIBRARY_VISIBILITY ClangImporter::Implementation
   llvm::DenseMap<std::pair<ValueDecl *, DeclContext *>, ValueDecl *>
       clonedBaseMembers;
 
+public:
   ValueDecl *importBaseMemberDecl(ValueDecl *decl, DeclContext *newContext);
 
-public:
   static size_t getImportedBaseMemberDeclArity(const ValueDecl *valueDecl);
 
   // Cache for already-specialized function templates and any thunks they may

lib/ClangImporter/SwiftLookupTable.cpp

@@ -1991,6 +1991,12 @@ void importer::addEntryToLookupTable(SwiftLookupTable &table,
       }
     }
   }
+  if (auto usingDecl = dyn_cast<clang::UsingDecl>(named)) {
+    for (auto usingShadowDecl : usingDecl->shadows()) {
+      if (isa<clang::CXXMethodDecl>(usingShadowDecl->getTargetDecl()))
+        addEntryToLookupTable(table, usingShadowDecl, nameImporter);
+    }
+  }
 }
 
 /// Returns the nearest parent of \p module that is marked \c explicit in its

test/Interop/Cxx/class/Inputs/constructors.h

@@ -80,10 +80,4 @@ struct DeletedCopyConstructor {
   DeletedCopyConstructor(const DeletedCopyConstructor &) = delete;
 };
 
-// TODO: we should be able to import this constructor correctly. Until we can,
-// make sure not to crash.
-struct UsingBaseConstructor : ConstructorWithParam {
-  using ConstructorWithParam::ConstructorWithParam;
-};
-
 #endif // TEST_INTEROP_CXX_CLASS_INPUTS_CONSTRUCTORS_H

test/Interop/Cxx/class/inheritance/Inputs/module.modulemap

@@ -28,6 +28,11 @@ module TypeAliases {
   header "type-aliases.h"
 }
 
+module UsingBaseMembers {
+  header "using-base-members.h"
+  requires cplusplus
+}
+
 module VirtualMethods {
   header "virtual-methods.h"
   requires cplusplus