[lldb][DWARFASTParserClang] Don't complete conflicting Objective-C++ types

This upstreams swiftlang#10313.

If we detect a situation where a forward declaration is C++ and the
definition DIE is Objective-C, then just don't try to complete the type
(it would crash otherwise). In the long term we might want to add
support for completing such types.

We've seen real world crashes when debugging WebKit and wxWidgets because of this.
Both projects forward declare ObjC++ decls in the way shown in the test.

rdar://145959981

Author: Michael137

lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.cpp

@@ -2230,6 +2230,18 @@ bool DWARFASTParserClang::CompleteRecordType(const DWARFDIE &die,
       for (DelayedAddObjCClassProperty &property : delayed_properties)
         property.Finalize();
     }
+  } else if (Language::LanguageIsObjC(
+                 static_cast<LanguageType>(die.GetAttributeValueAsUnsigned(
+                     DW_AT_APPLE_runtime_class, eLanguageTypeUnknown)))) {
+    /// The forward declaration was C++ but the definition is Objective-C.
+    /// We currently don't handle such situations. In such cases, keep the
+    /// forward declaration without a definition to avoid violating Clang AST
+    /// invariants.
+    LLDB_LOG(GetLog(LLDBLog::Expressions),
+             "WARNING: Type completion aborted because forward declaration for "
+             "'{0}' is C++ while definition is Objective-C.",
+             llvm::StringRef(die.GetName()));
+    return {};
   }
 
   if (!bases.empty()) {

lldb/test/Shell/SymbolFile/DWARF/objcxx-forward-decls.test

@@ -0,0 +1,70 @@
+# REQUIRES: system-darwin
+
+# In this test we have two CUs with conflicting forward declaration
+# depending on the CU language (one is C++ and the other is Objective-C++).
+# We are then stopped in the C++ CU and try to print the type, at which
+# point LLDB will try to make it into an Clang AST node. If LLDB were to
+# interpret the type as C++ instead of Objective-C, we'd violate Clang
+# invariants and crash.
+#
+# RUN: split-file %s %t
+# RUN: %clangxx_host -c -g -x objective-c++ %t/request.m -o %t/request_objc.o
+# RUN: %clangxx_host -c -g %t/main.cpp -o %t/main.o
+# RUN: %clangxx_host %t/main.o %t/request_objc.o -framework Foundation -o %t/a.out
+#
+# RUN: %lldb %t/a.out \
+# RUN:    -o "breakpoint set -p return -X main" \
+# RUN:    -o run \
+# RUN:    -o "frame variable r" \
+# RUN:    -o exit | FileCheck %s
+#
+# RUN: dsymutil %t/a.out
+#
+# RUN: %lldb %t/a.out \
+# RUN:    -o "breakpoint set -p return -X main" \
+# RUN:    -o run \
+# RUN:    -o "frame variable r" \
+# RUN:    -o exit | FileCheck %s --check-prefix=CHECK-DSYM
+
+# CHECK:      (lldb) frame variable r
+# CHECK-NEXT: (Request) ::r = (m_request = "Hello, World!")
+
+# CHECK-DSYM:      (lldb) frame variable r
+# CHECK-DSYM-NEXT: (Request) ::r = (m_request = "Hello, World!")
+
+#--- lib.h
+#ifndef LIB_H_IN
+#define LIB_H_IN
+
+#ifdef __OBJC__
+@class NSString;                                               
+#else
+class NSString;
+#endif
+
+struct Request {
+  NSString * m_request = nullptr;
+};
+
+#endif // _H_IN
+
+#--- main.cpp
+#include "lib.h"
+
+void process(Request *);
+
+Request r;
+
+int main() {
+    process(&r);
+    return 0;
+}
+
+#--- request.m
+#import <Foundation/Foundation.h>
+
+#include "lib.h"
+
+void process(Request * r) {
+  r->m_request = @"Hello, World!";
+}