[lldb][DWARFASTParserClang] Check DW_AT_declaration to determine static data members

**Background**

Prior to DWARFv4, there was no clear normative text on how to handle
static data members. Non-normative text suggested we compilers should
use `DW_AT_external` to mark static data members of structrues/unions.
Clang does this consistently. However, GCC doesn't, e.g., when the
structure/union is in an anonymous namespace (which is C++ standard
conformant). Additionally, GCC never emits `DW_AT_data_member_location`s
for union members (regardless of storage linkage and storage duration).

Since DWARFv5 (issue 161118.1), static data members get emitted as
`DW_TAG_variable`.

LLDB used to differentiate between static and non-static members by
checking the `DW_AT_external` flag and the absence of
`DW_AT_data_member_location`. With D18008 LLDB started to pretend
that union members always have a `0` `DW_AT_data_member_location`
by default (because GCC never emits these locations).

In D124409 LLDB stopped checking the `DW_AT_external` flag to account
for the case where GCC doesn't emit the flag for types in anonymous
namespaces; instead we only check for presence of
`DW_AT_data_member_location`s.

The combination of these changes then meant that LLDB would never
correctly detect that a union has static data members.

**Solution**

Instead of unconditionally initializing the `member_byte_offset` to `0`
specifically for union members, this patch proposes to check for both
the absence of `DW_AT_data_member_location` and `DW_AT_declaration`,
which consistently gets emitted for static data members on GCC and
Clang.

We initialize the `member_byte_offset` to `0` anyway if we determine
it wasn't a static. So removing the special case for unions makes this
code simpler to reason about.

Long-term, we should just use DWARFv5's new representation for static
data members.

Fixes llvm#68135

Author: Michael137

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

@@ -2482,8 +2482,9 @@ struct MemberAttributes {
   DWARFFormValue encoding_form;
   /// Indicates the byte offset of the word from the base address of the
   /// structure.
-  uint32_t member_byte_offset;
+  uint32_t member_byte_offset = UINT32_MAX;
   bool is_artificial = false;
+  bool is_declaration = false;
 };
 
 /// Parsed form of all attributes that are relevant for parsing Objective-C
@@ -2656,8 +2657,6 @@ DiscriminantValue &VariantPart::discriminant() { return this->_discriminant; }
 MemberAttributes::MemberAttributes(const DWARFDIE &die,
                                    const DWARFDIE &parent_die,
                                    ModuleSP module_sp) {
-  member_byte_offset = (parent_die.Tag() == DW_TAG_union_type) ? 0 : UINT32_MAX;
-
   DWARFAttributes attributes = die.GetAttributes();
   for (size_t i = 0; i < attributes.Size(); ++i) {
     const dw_attr_t attr = attributes.AttributeAtIndex(i);
@@ -2717,6 +2716,9 @@ MemberAttributes::MemberAttributes(const DWARFDIE &die,
       case DW_AT_artificial:
         is_artificial = form_value.Boolean();
         break;
+      case DW_AT_declaration:
+        is_declaration = form_value.Boolean();
+        break;
       default:
         break;
       }
@@ -2923,10 +2925,18 @@ void DWARFASTParserClang::ParseSingleMember(
   if (class_is_objc_object_or_interface)
     attrs.accessibility = eAccessNone;
 
-  // Handle static members, which is any member that doesn't have a bit or a
-  // byte member offset.
+  // Handle static members, which are typically members without
+  // locations. However, GCC *never* emits DW_AT_data_member_location
+  // for static data members of unions.
+  // Non-normative text pre-DWARFv5 recommends marking static
+  // data members with an DW_AT_external flag. Clang emits this consistently
+  // whereas GCC emits it only for static data members if not part of an
+  // anonymous namespace. The flag that is consistently emitted for static
+  // data members is DW_AT_declaration, so we check it instead.
+  // FIXME: Since DWARFv5, static data members are marked DW_AT_variable so we can
+  // consistently detect them on both GCC and Clang without below heuristic.
   if (attrs.member_byte_offset == UINT32_MAX &&
-      attrs.data_bit_offset == UINT64_MAX) {
+      attrs.data_bit_offset == UINT64_MAX && attrs.is_declaration) {
     Type *var_type = die.ResolveTypeUID(attrs.encoding_form.Reference());
 
     if (var_type) {

lldb/test/API/lang/cpp/union-static-data-members/Makefile

@@ -0,0 +1,3 @@
+CXX_SOURCES := main.cpp
+
+include Makefile.rules

lldb/test/API/lang/cpp/union-static-data-members/TestCppUnionStaticMembers.py

@@ -0,0 +1,43 @@
+"""
+Tests that frame variable and expr work for
+C++ unions and their static data members.
+"""
+import lldb
+from lldbsuite.test.lldbtest import *
+from lldbsuite.test.decorators import *
+import lldbsuite.test.lldbutil as lldbutil
+
+class CppUnionStaticMembersTestCase(TestBase):
+    def test(self):
+        """Tests that frame variable and expr work
+           for union static data members"""
+        self.build()
+
+        (target, process, main_thread, _) = lldbutil.run_to_source_breakpoint(
+            self, "return 0", lldb.SBFileSpec("main.cpp")
+        )                                                                     
+
+        self.expect("frame variable foo", substrs=["val = 42"])
+        self.expect("frame variable bar", substrs=["val = 137"])
+
+        self.expect_expr("foo", result_type="Foo", result_children=[ValueCheck(
+                name="val", value="42"
+            )])
+        self.expect_expr("bar", result_type="Bar", result_children=[ValueCheck(
+                name="val", value="137"
+            )])
+
+        self.expect_expr("Foo::sVal1", result_type="const int", result_value="-42")
+        self.expect_expr("Foo::sVal2", result_type="Foo", result_children=[ValueCheck(
+                name="val", value="42"
+            )])
+
+    @expectedFailureAll
+    def test_union_in_anon_namespace(self):
+        """Tests that frame variable and expr work
+           for union static data members in anonymous
+           namespaces"""
+        self.expect_expr("Bar::sVal1", result_type="const int", result_value="-137")
+        self.expect_expr("Bar::sVal2", result_type="Bar", result_children=[ValueCheck(
+                name="val", value="137"
+            )])

lldb/test/API/lang/cpp/union-static-data-members/main.cpp

@@ -0,0 +1,25 @@
+union Foo {
+  int val = 42;
+  static const int sVal1 = -42;
+  static Foo sVal2;
+};
+
+Foo Foo::sVal2{};
+
+namespace {
+union Bar {
+  int val = 137;
+  static const int sVal1 = -137;
+  static Bar sVal2;
+};
+
+Bar Bar::sVal2{};
+} // namespace
+
+int main() {
+  Foo foo;
+  Bar bar;
+  auto sum = Bar::sVal1 + Foo::sVal1 + Foo::sVal2.val + Bar::sVal2.val;
+
+  return 0;
+}