[cxx-interop] Clang member lookup should not look into Swift extensions

Calling `NominalTypeDecl::lookupDirect` triggers deserialization of Swift extensions for the type. `ClangRecordMemberLookup` shouldn't assume it is allowed to deserialize Swift extensions for the given C++ type: there might be extensions which reference the module that is currently being imported, which causes circular request dependency errors.

include/swift/AST/ClangModuleLoader.h

@@ -184,6 +184,11 @@ public:
   /// Imports a clang decl directly, rather than looking up its name.
   virtual Decl *importDeclDirectly(const clang::NamedDecl *decl) = 0;
 
+  /// Imports a clang decl from a base class, cloning it for \param newContext
+  /// if it wasn't cloned for this specific context before.
+  virtual ValueDecl *importBaseMemberDecl(ValueDecl *decl,
+                                          DeclContext *newContext) = 0;
+
   /// Emits diagnostics for any declarations named name
   /// whose direct declaration context is a TU.
   virtual void diagnoseTopLevelValue(const DeclName &name) = 0;

include/swift/ClangImporter/ClangImporter.h

@@ -531,6 +531,9 @@ public:
   /// Imports a clang decl directly, rather than looking up it's name.
   Decl *importDeclDirectly(const clang::NamedDecl *decl) override;
 
+  ValueDecl *importBaseMemberDecl(ValueDecl *decl,
+                                  DeclContext *newContext) override;
+
   /// Emits diagnostics for any declarations named name
   /// whose direct declaration context is a TU.
   void diagnoseTopLevelValue(const DeclName &name) override;

lib/ClangImporter/ClangImporter.cpp

@@ -4935,11 +4935,12 @@ TinyPtrVector<ValueDecl *> ClangRecordMemberLookup::evaluate(
 
   // Find the results that are actually a member of "recordDecl".
   TinyPtrVector<ValueDecl *> result;
+  ClangModuleLoader *clangModuleLoader = ctx.getClangModuleLoader();
   for (auto found : allResults) {
     auto named = found.get<clang::NamedDecl *>();
     if (dyn_cast<clang::Decl>(named->getDeclContext()) ==
         recordDecl->getClangDecl()) {
-      if (auto import = ctx.getClangModuleLoader()->importDeclDirectly(named))
+      if (auto import = clangModuleLoader->importDeclDirectly(named))
         result.push_back(cast<ValueDecl>(import));
     }
   }
@@ -4955,16 +4956,32 @@ TinyPtrVector<ValueDecl *> ClangRecordMemberLookup::evaluate(
         continue;
 
       auto *baseRecord = baseType->getAs<clang::RecordType>()->getDecl();
-      if (auto import =
-              ctx.getClangModuleLoader()->importDeclDirectly(baseRecord)) {
+      if (auto import = clangModuleLoader->importDeclDirectly(baseRecord)) {
         // If we are looking up the base class, go no further. We will have
         // already found it during the other lookup.
         if (cast<ValueDecl>(import)->getName() == name)
           continue;
 
-        auto baseResults = cast<NominalTypeDecl>(import)->lookupDirect(name);
+        // Add Clang members that are imported lazily.
+        auto baseResults = evaluateOrDefault(
+            ctx.evaluator,
+            ClangRecordMemberLookup({cast<NominalTypeDecl>(import), name}), {});
+        // Add members that are synthesized eagerly, such as subscripts.
+        for (auto member :
+             cast<NominalTypeDecl>(import)->getCurrentMembersWithoutLoading()) {
+          if (auto namedMember = dyn_cast<ValueDecl>(member)) {
+            if (namedMember->hasName() &&
+                namedMember->getName().getBaseName() == name &&
+                // Make sure we don't add duplicate entries, as that would
+                // wrongly imply that lookup is ambiguous.
+                !llvm::is_contained(baseResults, namedMember)) {
+              baseResults.push_back(namedMember);
+            }
+          }
+        }
         for (auto foundInBase : baseResults) {
-          if (auto newDecl = cloneBaseMemberDecl(foundInBase, recordDecl)) {
+          if (auto newDecl = clangModuleLoader->importBaseMemberDecl(
+                  foundInBase, recordDecl)) {
             result.push_back(newDecl);
           }
         }
@@ -5761,6 +5778,18 @@ Decl *ClangImporter::importDeclDirectly(const clang::NamedDecl *decl) {
   return Impl.importDecl(decl, Impl.CurrentVersion);
 }
 
+ValueDecl *ClangImporter::importBaseMemberDecl(ValueDecl *decl,
+                                               DeclContext *newContext) {
+  // Make sure we don't clone the decl again for this class, as that would
+  // result in multiple definitions of the same symbol.
+  std::pair<ValueDecl *, DeclContext *> key = {decl, newContext};
+  if (!Impl.clonedBaseMembers.count(key)) {
+    ValueDecl *cloned = cloneBaseMemberDecl(decl, newContext);
+    Impl.clonedBaseMembers[key] = cloned;
+  }
+  return Impl.clonedBaseMembers[key];
+}
+
 void ClangImporter::diagnoseTopLevelValue(const DeclName &name) {
   Impl.diagnoseTopLevelValue(name);
 }

lib/ClangImporter/ImporterImpl.h

@@ -621,6 +621,10 @@ public:
                      llvm::DenseSet<clang::FunctionDecl *>>>>
       cxxMethods;
 
+  // Keep track of the decls that were already cloned for this specific class.
+  llvm::DenseMap<std::pair<ValueDecl *, DeclContext *>, ValueDecl *>
+      clonedBaseMembers;
+
   // Cache for already-specialized function templates and any thunks they may
   // have.
   llvm::DenseMap<clang::FunctionDecl *, ValueDecl *>

test/Interop/Cxx/class/inheritance/extensions-typechecker.swift

@@ -0,0 +1,9 @@
+// RUN: %target-typecheck-verify-swift -verify-ignore-unknown -I %S/Inputs -enable-experimental-cxx-interop
+
+import Functions
+
+extension Base {
+  public func swiftFunc() {}
+}
+
+Derived().swiftFunc() // expected-error {{value of type 'Derived' has no member 'swiftFunc'}}