[lldb][ClangExpressionParser] Don't by default enable Objecitve-C support when evaluating C++ expressions

This patch attempts to decouple C++ expression evaluation from
Objective-C support. We've previously enabled it by default
(if a runtime existed), but that meant we're opting into extra
work we only need to do for Objective-C, which complicates/slows down
C++ expression evaluation. Of course there's a valid use-case for this,
which is calling Objective-C APIs when stopped in C++ frames (which
Objective-C++ developers might want to do). In those case we should
really prompt the user to add the `expr --language objc++` flag.
To accomodate a likely frequent use-case where a user breaks in a system
C++ library (without debug-symbols) but their application is actually
an Objective-C app, we allow Objective-C support in C++ expressions
if the current frame doesn't have debug-info.

This fixes llvm#75443 and
allows us to add more `LangOpts.ObjC` guards around the expression
evaluator in the future (e.g., we could avoid looking into the
Objective-C runtime during C++ expression evaluation, which we
currently do unconditionally).

Depends on llvm#87657

Author: Michael137

lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionParser.cpp

@@ -526,7 +526,10 @@ ClangExpressionParser::ClangExpressionParser(
     [[fallthrough]];
   case lldb::eLanguageTypeC_plus_plus_03:
     lang_opts.CPlusPlus = true;
-    if (process_sp)
+    if (process_sp
+        // We're stopped in a frame without debug-info. The user probably
+        // intends to make global queries (which should include Objective-C).
+        && !(frame_sp && frame_sp->HasDebugInformation()))
       lang_opts.ObjC =
           process_sp->GetLanguageRuntime(lldb::eLanguageTypeObjC) != nullptr;
     break;

lldb/test/API/commands/expression/diagnostics/TestExprDiagnostics.py

@@ -172,7 +172,7 @@ def test_source_locations_from_objc_modules(self):
 
         # Import foundation so that the Obj-C module is loaded (which contains source locations
         # that can be used by LLDB).
-        self.runCmd("expr @import Foundation")
+        self.runCmd("expr --language objective-c++ -- @import Foundation")
         value = frame.EvaluateExpression("NSLog(1);")
         self.assertFalse(value.GetError().Success())
         # LLDB should print the source line that defines NSLog. To not rely on any

lldb/test/API/commands/expression/options/TestExprOptions.py

@@ -59,7 +59,6 @@ def test_expr_options(self):
         self.assertTrue(val.IsValid())
         self.assertFalse(val.GetError().Success())
 
-    @skipIfDarwin
     def test_expr_options_lang(self):
         """These expression language options should work as expected."""
         self.build()

lldb/test/API/lang/objcxx/objc-builtin-types/TestObjCBuiltinTypes.py

@@ -53,5 +53,5 @@ def test_with_python_api(self):
         )
         self.expect(
             "expr --language C++ -- id my_id = 0; my_id",
-            patterns=["\(id\) \$.* = nullptr"],
+            error=True
         )

lldb/test/API/lang/objcxx/objc-from-cpp-frames-without-debuginfo/Makefile

@@ -0,0 +1,4 @@
+CXX_SOURCES := main.cpp
+CXXFLAGS_EXTRAS := -g0
+
+include Makefile.rules

lldb/test/API/lang/objcxx/objc-from-cpp-frames-without-debuginfo/TestObjCFromCppFramesWithoutDebugInfo.py

@@ -0,0 +1,17 @@
+"""
+Test that the the expression parser enables ObjC support
+when stopped in a C++ frame without debug-info.
+"""
+
+import lldb
+from lldbsuite.test.decorators import *
+from lldbsuite.test.lldbtest import *
+from lldbsuite.test import lldbutil
+
+class TestObjCFromCppFramesWithoutDebugInfo(TestBase):
+    def test(self):
+        self.build()
+        (_, process, _, _) = lldbutil.run_to_name_breakpoint(self, "main")
+
+        self.assertState(process.GetState(), lldb.eStateStopped)
+        self.expect("expr id", error=False)

lldb/test/API/lang/objcxx/objc-from-cpp-frames-without-debuginfo/main.cpp

@@ -0,0 +1 @@
+int main() { return 0; }