[LLDB] Convert libstdc++ std::variant summary to C++ (#148929)

This PR converts the `std::variant` summary from Python to C++.

Split from #148554. MSVC's STL and libstdc++ use the same type name for
`std::variant`, thus they need one "dispatcher" function that checks the
type and calls the appropriate summary. For summaries, both need to be
implemented in C++.

This is mostly a 1:1 translation. The main difference is that in C++,
the summary returns `false` if it can't inspect the object properly
(e.g. a member could not be found). In Python, this wasn't possible.

Author: Nerixyz

lldb/examples/synthetic/gnu_libstdcpp.py

@@ -882,38 +882,6 @@ def update(self):
         return False
 
 
-def VariantSummaryProvider(valobj, dict):
-    raw_obj = valobj.GetNonSyntheticValue()
-    index_obj = raw_obj.GetChildMemberWithName("_M_index")
-    data_obj = raw_obj.GetChildMemberWithName("_M_u")
-    if not (index_obj and index_obj.IsValid() and data_obj and data_obj.IsValid()):
-        return "<Can't find _M_index or _M_u>"
-
-    def get_variant_npos_value(index_byte_size):
-        if index_byte_size == 1:
-            return 0xFF
-        elif index_byte_size == 2:
-            return 0xFFFF
-        else:
-            return 0xFFFFFFFF
-
-    npos_value = get_variant_npos_value(index_obj.GetByteSize())
-    index = index_obj.GetValueAsUnsigned(0)
-    if index == npos_value:
-        return " No Value"
-
-    # Strip references and typedefs.
-    variant_type = raw_obj.GetType().GetCanonicalType().GetDereferencedType()
-    template_arg_count = variant_type.GetNumberOfTemplateArguments()
-
-    # Invalid index can happen when the variant is not initialized yet.
-    if index >= template_arg_count:
-        return " <Invalid>"
-
-    active_type = variant_type.GetTemplateArgumentType(index)
-    return f" Active Type = {active_type.GetDisplayTypeName()} "
-
-
 class VariantSynthProvider:
     def __init__(self, valobj, dict):
         self.raw_obj = valobj.GetNonSyntheticValue()

lldb/source/Plugins/Language/CPlusPlus/CPlusPlusLanguage.cpp

@@ -1513,11 +1513,9 @@ static void LoadLibStdcppFormatters(lldb::TypeCategoryImplSP cpp_category_sp) {
       TypeSummaryImplSP(new ScriptSummaryFormat(
           stl_summary_flags,
           "lldb.formatters.cpp.gnu_libstdcpp.ForwardListSummaryProvider")));
-  cpp_category_sp->AddTypeSummary(
-      "^std::variant<.+>$", eFormatterMatchRegex,
-      TypeSummaryImplSP(new ScriptSummaryFormat(
-          stl_summary_flags,
-          "lldb.formatters.cpp.gnu_libstdcpp.VariantSummaryProvider")));
+  AddCXXSummary(cpp_category_sp, LibStdcppVariantSummaryProvider,
+                "libstdc++ std::variant summary provider", "^std::variant<.+>$",
+                stl_summary_flags, true);
 
   AddCXXSynthetic(
       cpp_category_sp,

lldb/source/Plugins/Language/CPlusPlus/LibStdcpp.cpp

@@ -366,3 +366,49 @@ bool lldb_private::formatters::LibStdcppSmartPointerSummaryProvider(
 
   return true;
 }
+
+static uint64_t LibStdcppVariantNposValue(size_t index_byte_size) {
+  switch (index_byte_size) {
+  case 1:
+    return 0xff;
+  case 2:
+    return 0xffff;
+  default:
+    return 0xffff'ffff;
+  }
+}
+
+bool formatters::LibStdcppVariantSummaryProvider(
+    ValueObject &valobj, Stream &stream, const TypeSummaryOptions &options) {
+  ValueObjectSP valobj_sp = valobj.GetNonSyntheticValue();
+  if (!valobj_sp)
+    return false;
+
+  ValueObjectSP index_obj = valobj_sp->GetChildMemberWithName("_M_index");
+  ValueObjectSP data_obj = valobj_sp->GetChildMemberWithName("_M_u");
+  if (!index_obj || !data_obj)
+    return false;
+
+  auto index_bytes = index_obj->GetByteSize();
+  if (!index_bytes)
+    return false;
+  auto npos_value = LibStdcppVariantNposValue(*index_bytes);
+  auto index = index_obj->GetValueAsUnsigned(0);
+  if (index == npos_value) {
+    stream.Printf(" No Value");
+    return true;
+  }
+
+  auto variant_type =
+      valobj_sp->GetCompilerType().GetCanonicalType().GetNonReferenceType();
+  if (!variant_type)
+    return false;
+  if (index >= variant_type.GetNumTemplateArguments(true)) {
+    stream.Printf(" <Invalid>");
+    return true;
+  }
+
+  auto active_type = variant_type.GetTypeTemplateArgument(index, true);
+  stream << " Active Type = " << active_type.GetDisplayTypeName() << " ";
+  return true;
+}

lldb/source/Plugins/Language/CPlusPlus/LibStdcpp.h

@@ -29,6 +29,10 @@ bool LibStdcppUniquePointerSummaryProvider(
     ValueObject &valobj, Stream &stream,
     const TypeSummaryOptions &options); // libstdc++ std::unique_ptr<>
 
+bool LibStdcppVariantSummaryProvider(
+    ValueObject &valobj, Stream &stream,
+    const TypeSummaryOptions &options); // libstdc++ std::variant<>
+
 SyntheticChildrenFrontEnd *
 LibstdcppMapIteratorSyntheticFrontEndCreator(CXXSyntheticChildren *,
                                              lldb::ValueObjectSP);