[lldb] Reactivate Objective-C++ plugin

Summary:
Since commit 7b3ef05a37 the Objective-C++ plugin is dead code.
That commit added Objective-C++ to the list of languages for which `Language::LanguageIsCPlusPlus`
returns true. As the C++ language plugin also uses that method to figure out if it is responsible for a
given language, the C++ plugin since then also became the plugin that we found when looking for
a language plugin for Objective-C++. The only real fallout from that is that the source highlighting
for Objective-C++ files never worked as we always found the C++ plugin which refuses to highlight
files with Objective-C++ extensions.

This patch just adds a special exception for Objective-C++ to the list of languages that are governed
by the C++ plugin. Also adds a test that makes sure that we find the right plugin for all C language
types and that the highlighting for `.mm` (Objective-C++) and `.m` (Objective-C) files works.

I didn't revert 7b3ef05a37 as it does make sense to return
true for Objective-C++ from `Language::LanguageIsCPlusPlus` (e.g., we currently check if we care about
ODR violations by doing `if (Language::LanguageIsCPlusPlus(...))` and this should also work for
Objective-C++).

Fixes rdar://64420183

Reviewers: aprantl

Reviewed By: aprantl

Subscribers: mgorny, abidh, JDevlieghere

Differential Revision: https://reviews.llvm.org/D82109

lldb/source/Plugins/Language/CPlusPlus/CPlusPlusLanguage.cpp

@@ -70,7 +70,9 @@ uint32_t CPlusPlusLanguage::GetPluginVersion() { return 1; }
 // Static Functions
 
 Language *CPlusPlusLanguage::CreateInstance(lldb::LanguageType language) {
-  if (Language::LanguageIsCPlusPlus(language))
+  // Use plugin for C++ but not for Objective-C++ (which has its own plugin).
+  if (Language::LanguageIsCPlusPlus(language) &&
+      language != eLanguageTypeObjC_plus_plus)
     return new CPlusPlusLanguage();
   return nullptr;
 }

lldb/unittests/Language/CLanguages/CLanguagesTest.cpp

@@ -0,0 +1,46 @@
+//===-- CLanguagesTest.cpp ------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Plugins/Language/CPlusPlus/CPlusPlusLanguage.h"
+#include "Plugins/Language/ObjC/ObjCLanguage.h"
+#include "Plugins/Language/ObjCPlusPlus/ObjCPlusPlusLanguage.h"
+#include "TestingSupport/SubsystemRAII.h"
+#include "lldb/lldb-enumerations.h"
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+
+using namespace lldb_private;
+
+/// Returns the name of the LLDB plugin for the given language or a null
+/// ConstString if there is no fitting plugin.
+static ConstString GetPluginName(lldb::LanguageType language) {
+  Language *language_plugin = Language::FindPlugin(language);
+  if (language_plugin)
+    return language_plugin->GetPluginName();
+  return ConstString();
+}
+
+TEST(CLanguages, LookupCLanguagesByLanguageType) {
+  SubsystemRAII<CPlusPlusLanguage, ObjCPlusPlusLanguage, ObjCLanguage> langs;
+
+  // There is no plugin to find for C.
+  EXPECT_EQ(Language::FindPlugin(lldb::eLanguageTypeC), nullptr);
+  EXPECT_EQ(Language::FindPlugin(lldb::eLanguageTypeC89), nullptr);
+  EXPECT_EQ(Language::FindPlugin(lldb::eLanguageTypeC99), nullptr);
+  EXPECT_EQ(Language::FindPlugin(lldb::eLanguageTypeC11), nullptr);
+
+  EXPECT_EQ(GetPluginName(lldb::eLanguageTypeC_plus_plus), "cplusplus");
+  EXPECT_EQ(GetPluginName(lldb::eLanguageTypeC_plus_plus_03), "cplusplus");
+  EXPECT_EQ(GetPluginName(lldb::eLanguageTypeC_plus_plus_11), "cplusplus");
+  EXPECT_EQ(GetPluginName(lldb::eLanguageTypeC_plus_plus_14), "cplusplus");
+
+  EXPECT_EQ(GetPluginName(lldb::eLanguageTypeObjC), "objc");
+
+  EXPECT_EQ(GetPluginName(lldb::eLanguageTypeObjC_plus_plus), "objcplusplus");
+}

lldb/unittests/Language/CLanguages/CMakeLists.txt

@@ -0,0 +1,8 @@
+add_lldb_unittest(LanguageCLanguagesTests
+  CLanguagesTest.cpp
+
+  LINK_LIBS
+    lldbPluginCPlusPlusLanguage
+    lldbPluginObjCLanguage
+    lldbPluginObjCPlusPlusLanguage
+)

lldb/unittests/Language/CMakeLists.txt

@@ -1,2 +1,3 @@
 add_subdirectory(CPlusPlus)
+add_subdirectory(CLanguages)
 add_subdirectory(Highlighting)

lldb/unittests/Language/Highlighting/HighlighterTest.cpp

@@ -58,6 +58,8 @@ TEST_F(HighlighterTest, HighlighterSelectionPath) {
   EXPECT_EQ(getName("a/dir.CC"), "clang");
   EXPECT_EQ(getName("/a/dir.hpp"), "clang");
   EXPECT_EQ(getName("header.h"), "clang");
+  EXPECT_EQ(getName("foo.m"), "clang");
+  EXPECT_EQ(getName("foo.mm"), "clang");
 
   EXPECT_EQ(getName(""), "none");
   EXPECT_EQ(getName("/dev/null"), "none");