[Reland] Detect against invalid variant index for LibStdC++ std::variant data formatters (#69614)

This is relanding of #69253.
`TestTemplatePackArgs.py` is passing now.

https://github.com/llvm/llvm-project/pull/68012/files added new data
formatters for LibStdC++ std::variant.

However, this formatter can crash if std::variant's index field has
invalid value (exceeds the number of template arguments).
This can happen if the current IP stops at a place std::variant is not
initialized yet.

This patch fixes the crash by ensuring the index is a valid value and
fix GetNthTemplateArgument() to make sure it is not crashing.

Co-authored-by: jeffreytan81 <[email protected]>

Author: jeffreytan81

lldb/examples/synthetic/gnu_libstdcpp.py

@@ -914,6 +914,11 @@ def get_variant_npos_value(index_byte_size):
     if index == npos_value:
         return " No Value"
 
+    # Invalid index can happen when the variant is not initialized yet.
+    template_arg_count = data_obj.GetType().GetNumberOfTemplateArguments()
+    if index >= template_arg_count:
+        return " <Invalid>"
+
     active_type = data_obj.GetType().GetTemplateArgumentType(index)
     return f" Active Type = {active_type.GetDisplayTypeName()} "
 

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

@@ -7183,7 +7183,8 @@ GetNthTemplateArgument(const clang::ClassTemplateSpecializationDecl *decl,
   // (including the ones preceding the parameter pack).
   const auto &pack = args[last_idx];
   const size_t pack_idx = idx - last_idx;
-  assert(pack_idx < pack.pack_size() && "parameter pack index out-of-bounds");
+  if (pack_idx >= pack.pack_size())
+    return nullptr;
   return &pack.pack_elements()[pack_idx];
 }
 

lldb/test/API/functionalities/data-formatter/data-formatter-stl/libstdcpp/variant/TestDataFormatterLibStdcxxVariant.py

@@ -71,3 +71,29 @@ def test_with_run_command(self):
             substrs=["v_many_types_no_value =  No Value"],
         )
         """
+
+    @add_test_categories(["libstdcxx"])
+    def test_invalid_variant_index(self):
+        """Test LibStdC++ data formatter for std::variant with invalid index."""
+        self.build()
+
+        (self.target, self.process, thread, bkpt) = lldbutil.run_to_source_breakpoint(
+            self, "// break here", lldb.SBFileSpec("main.cpp", False)
+        )
+
+        lldbutil.continue_to_breakpoint(self.process, bkpt)
+
+        self.expect(
+            "frame variable v1",
+            substrs=["v1 =  Active Type = int  {", "Value = 12", "}"],
+        )
+
+        var_v1 = thread.frames[0].FindVariable("v1")
+        var_v1_raw_obj = var_v1.GetNonSyntheticValue()
+        index_obj = var_v1_raw_obj.GetChildMemberWithName("_M_index")
+        self.assertTrue(index_obj and index_obj.IsValid())
+
+        INVALID_INDEX = "100"
+        index_obj.SetValueFromCString(INVALID_INDEX)
+
+        self.expect("frame variable v1", substrs=["v1 =  <Invalid>"])