Handle instantiating an imported class/vtable (#7792)

Usual LoadImportRef, plus some generalization of importing entities.

toolchain/check/cpp/generate_ast.cpp

@@ -501,10 +501,14 @@ auto CarbonExternalASTSource::CompleteType(clang::TagDecl* tag_decl) -> void {
   llvm::SmallVector<PendingVirtualFunction> pending_virtual_functions;
 
   if (class_info.vtable_decl_id.has_value()) {
+    LoadImportRef(*context_, class_info.vtable_decl_id);
+    auto canonical_vtable_decl_id =
+        context_->constant_values().GetConstantInstId(
+            class_info.vtable_decl_id);
     auto vtable_inst_block = context_->inst_blocks().Get(
         context_->vtables()
             .Get(context_->insts()
-                     .GetAs<SemIR::VtableDecl>(class_info.vtable_decl_id)
+                     .GetAs<SemIR::VtableDecl>(canonical_vtable_decl_id)
                      .vtable_id)
             .virtual_functions_id);
     for (auto vtable_entry_id : vtable_inst_block) {

toolchain/check/cpp/import.cpp

@@ -362,28 +362,13 @@ auto ImportCppConstantFromFile(Context& context, SemIR::LocId loc_id,
     return SemIR::ErrorInst::ConstantId;
   }
 
-  auto const_inst_id = file.constant_values().GetConstantInstId(inst_id);
-  CARBON_KIND_SWITCH(file.insts().Get(const_inst_id)) {
-    case CARBON_KIND(SemIR::ClassType class_type): {
-      const auto& class_info = file.classes().Get(class_type.class_id);
-      CARBON_CHECK(class_info.scope_id.has_value());
-      return ImportCppDeclFromFile(
-          context, loc_id, file,
-          file.name_scopes().Get(class_info.scope_id).clang_decl_context_id());
-    }
-
-    case CARBON_KIND(SemIR::Namespace namespace_decl): {
-      return ImportCppDeclFromFile(context, loc_id, file,
-                                   file.name_scopes()
-                                       .Get(namespace_decl.name_scope_id)
-                                       .clang_decl_context_id());
-    }
-
-    default: {
-      context.TODO(loc_id, "indirect import of unsupported C++ declaration");
-      return SemIR::ErrorInst::ConstantId;
-    }
+  if (const auto* clang_decl = file.clang_decls().Lookup(inst_id)) {
+    auto clang_decl_id = file.clang_decls().LookupId(clang_decl->key);
+    return ImportCppDeclFromFile(context, loc_id, file, clang_decl_id);
   }
+
+  context.TODO(loc_id, "indirect import of unsupported C++ declaration");
+  return SemIR::ErrorInst::ConstantId;
 }
 
 // Returns the Clang `DeclContext` for the given name scope. Return the

toolchain/check/import_ref.cpp

@@ -2612,6 +2612,11 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
   auto thunk_specific_data = GetLocalSpecificData(
       resolver, import_thunk_info ? import_thunk_info->specific_id
                                   : SemIR::SpecificId::None);
+  auto thunk_override_self_type_const_id = SemIR::ConstantId::None;
+  if (import_thunk_info) {
+    thunk_override_self_type_const_id =
+        GetLocalConstantId(resolver, import_thunk_info->override_self_type_id);
+  }
 
   auto& new_function = resolver.local_functions().Get(function_id);
   if (resolver.HasNewWork()) {
@@ -2680,6 +2685,11 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
         local_thunk_info.specific_id = GetOrAddLocalSpecific(
             resolver, import_thunk_info->specific_id, thunk_specific_data);
       }
+      if (thunk_override_self_type_const_id.has_value()) {
+        local_thunk_info.override_self_type_id =
+            resolver.local_types().GetTypeIdForTypeConstantId(
+                thunk_override_self_type_const_id);
+      }
       new_function.SetThunk(resolver.local_ir().thunks().Add(local_thunk_info));
       break;
     }

toolchain/check/testdata/interop/cpp/class/export/override.carbon

@@ -3,6 +3,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 // INCLUDE-FILE: toolchain/testing/testdata/min_prelude/destroy.carbon
+// EXTRA-ARGS: --share-cpp-ast
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
@@ -30,3 +31,29 @@ class B {
 inline Cpp '''
 Carbon::B b;
 ''';
+
+// --- base.h
+struct Base {
+  virtual ~Base();
+  virtual void F() = 0;
+};
+
+// --- derived.carbon
+library "derived";
+import Cpp library "base.h";
+
+class Derived {
+  extend base: Cpp.Base;
+  override fn F(unused self) {}
+}
+
+// --- imported.carbon
+library "imported";
+import Cpp library "base.h";
+import library "derived";
+
+inline Cpp '''
+void Test() {
+  Carbon::Derived d;
+}
+''';