[lldb] Fix dynamic type resolution for types parsed from declarations

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

@@ -11,6 +11,22 @@
 
 using namespace lldb_private;
 
+std::optional<bool> ClangASTMetadata::GetIsDynamicCXXType() const {
+  switch (m_is_dynamic_cxx) {
+  case 0:
+    return std::nullopt;
+  case 1:
+    return false;
+  case 2:
+    return true;
+  }
+  llvm_unreachable("Invalid m_is_dynamic_cxx value");
+}
+
+void ClangASTMetadata::SetIsDynamicCXXType(std::optional<bool> b) {
+  m_is_dynamic_cxx = b ? *b + 1 : 0;
+}
+
 void ClangASTMetadata::Dump(Stream *s) {
   lldb::user_id_t uid = GetUserID();
 

lldb/source/Plugins/ExpressionParser/Clang/ClangASTMetadata.h

@@ -12,19 +12,22 @@
 #include "lldb/Core/dwarf.h"
 #include "lldb/lldb-defines.h"
 #include "lldb/lldb-enumerations.h"
+#include "lldb/lldb-private-enumerations.h"
 
 namespace lldb_private {
 
 class ClangASTMetadata {
 public:
   ClangASTMetadata()
       : m_user_id(0), m_union_is_user_id(false), m_union_is_isa_ptr(false),
-        m_has_object_ptr(false), m_is_self(false), m_is_dynamic_cxx(true),
-        m_is_forcefully_completed(false) {}
+        m_has_object_ptr(false), m_is_self(false),
+        m_is_forcefully_completed(false) {
+    SetIsDynamicCXXType(std::nullopt);
+  }
 
-  bool GetIsDynamicCXXType() const { return m_is_dynamic_cxx; }
+  std::optional<bool> GetIsDynamicCXXType() const;
 
-  void SetIsDynamicCXXType(bool b) { m_is_dynamic_cxx = b; }
+  void SetIsDynamicCXXType(std::optional<bool> b);
 
   void SetUserID(lldb::user_id_t user_id) {
     m_user_id = user_id;
@@ -101,8 +104,8 @@ class ClangASTMetadata {
     uint64_t m_isa_ptr;
   };
 
-  bool m_union_is_user_id : 1, m_union_is_isa_ptr : 1, m_has_object_ptr : 1,
-      m_is_self : 1, m_is_dynamic_cxx : 1, m_is_forcefully_completed : 1;
+  unsigned m_union_is_user_id : 1, m_union_is_isa_ptr : 1, m_has_object_ptr : 1,
+      m_is_self : 1, m_is_dynamic_cxx : 2, m_is_forcefully_completed : 1;
 };
 
 } // namespace lldb_private

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

@@ -1837,7 +1837,8 @@ DWARFASTParserClang::ParseStructureLikeDIE(const SymbolContext &sc,
 
   ClangASTMetadata metadata;
   metadata.SetUserID(die.GetID());
-  metadata.SetIsDynamicCXXType(dwarf->ClassOrStructIsVirtual(die));
+  if (!attrs.is_forward_declaration)
+    metadata.SetIsDynamicCXXType(dwarf->ClassOrStructIsVirtual(die));
 
   TypeSystemClang::TemplateParameterInfos template_param_infos;
   if (ParseTemplateParameterInfos(die, template_param_infos)) {

lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp

@@ -3657,13 +3657,17 @@ bool TypeSystemClang::IsPossibleDynamicType(lldb::opaque_compiler_type_t type,
     bool success;
     if (cxx_record_decl->isCompleteDefinition())
       success = cxx_record_decl->isDynamicClass();
-    else if (std::optional<ClangASTMetadata> metadata =
-                 GetMetadata(cxx_record_decl))
-      success = metadata->GetIsDynamicCXXType();
-    else if (GetType(pointee_qual_type).GetCompleteType())
-      success = cxx_record_decl->isDynamicClass();
-    else
-      success = false;
+    else {
+      std::optional<ClangASTMetadata> metadata = GetMetadata(cxx_record_decl);
+      std::optional<bool> is_dynamic =
+          metadata ? metadata->GetIsDynamicCXXType() : std::nullopt;
+      if (is_dynamic)
+        success = *is_dynamic;
+      else if (GetType(pointee_qual_type).GetCompleteType())
+        success = cxx_record_decl->isDynamicClass();
+      else
+        success = false;
+    }
 
     if (success)
       set_dynamic_pointee_type(pointee_qual_type);

lldb/test/API/lang/cpp/dynamic-value/Makefile

@@ -1,3 +1,3 @@
-CXX_SOURCES := pass-to-base.cpp anonymous-b.cpp
+CXX_SOURCES := pass-to-base.cpp anonymous-b.cpp forward-a.cpp
 
 include Makefile.rules

lldb/test/API/lang/cpp/dynamic-value/TestDynamicValue.py

@@ -266,3 +266,15 @@ def examine_value_object_of_this_ptr(
         contained_b_static_addr = int(contained_b_static.GetValue(), 16)
 
         self.assertLess(contained_b_addr, contained_b_static_addr)
+
+    @no_debug_info_test
+    def test_from_forward_decl(self):
+        """Test fetching C++ dynamic values forward-declared types. It's
+        imperative that this is a separate test so that we don't end up parsing
+        a definition of A from somewhere else."""
+        self.build()
+        lldbutil.run_to_name_breakpoint(self, "take_A")
+        self.expect(
+            "frame var -d run-target --ptr-depth=1 --show-types a",
+            substrs=["(B *) a", "m_b_value = 10"],
+        )

lldb/test/API/lang/cpp/dynamic-value/a.h

@@ -22,4 +22,6 @@ class A {
 
 A *make_anonymous_B();
 
+A *take_A(A *a);
+
 #endif

lldb/test/API/lang/cpp/dynamic-value/forward-a.cpp

@@ -0,0 +1,3 @@
+class A;
+
+A *take_A(A *a) { return a; }

lldb/test/API/lang/cpp/dynamic-value/pass-to-base.cpp

@@ -45,5 +45,7 @@ main (int argc, char **argv)
 
   myB.doSomething(*make_anonymous_B());
 
+  take_A(&myB);
+
   return 0;
 }