Add unittest for external interpreter use

unittests/CppInterOp/InterpreterTest.cpp

@@ -1,6 +1,15 @@
 #include "Utils.h"
 
 #include "clang/Interpreter/CppInterOp.h"
+
+#ifdef USE_CLING
+#include "cling/Interpreter/Interpreter.h"
+#endif // USE_CLING
+
+#ifdef USE_REPL
+#include "clang/Interpreter/Interpreter.h"
+#endif // USE_REPL
+
 #include "clang/Basic/Version.h"
 
 #include "llvm/ADT/SmallString.h"
@@ -162,3 +171,33 @@ TEST(InterpreterTest, CodeCompletion) {
   GTEST_SKIP();
 #endif
 }
+
+TEST(InterpreterTest, ExternalInterpreterTest) {
+
+#ifdef USE_REPL
+  llvm::ExitOnError ExitOnErr;
+  clang::IncrementalCompilerBuilder CB;
+  CB.SetCompilerArgs({"-std=c++20"}); // pass `-xc` for a C REPL.
+
+  // Create the incremental compiler instance.
+  std::unique_ptr<clang::CompilerInstance> CI;
+  CI = ExitOnErr(CB.CreateCpp());
+
+  // Create the interpreter instance.
+  std::unique_ptr<Interpreter> I =
+      ExitOnErr(Interpreter::create(std::move(CI)));
+
+  auto ExtInterp = I.get();
+#endif
+
+#ifdef USE_CLING
+  const char* LLVMRESDIR = "/usr/local/"; // path to llvm resource directory
+  cling::Interpreter interp(argc, argv, LLVMRESDIR);
+  compat::Interpreter ExtInterp = &interp;
+#endif
+
+  std::cout << "External Interpreter:" << ExtInterp;
+  EXPECT_NE(ExtInterp, nullptr);
+  Cpp::UseExternalInterpreter(ExtInterp);
+  EXPECT_FALSE(Cpp::OwnsInterpreter());
+}