[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.

1 files changed, 46 insertions, 0 deletions

diff --git a/lldb/source/Plugins/Language/CPlusPlus/LibStdcpp.cpp b/lldb/source/Plugins/Language/CPlusPlus/LibStdcpp.cpp
index c80a52d..595e835 100644
--- a/lldb/source/Plugins/Language/CPlusPlus/LibStdcpp.cpp
+++ b/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;
+}