From 14396552be75d0fcb9c9e43d4e256076f2467b85 Mon Sep 17 00:00:00 2001
From: David Blaikie <dblaikie@gmail.com>
Date: Tue, 12 Nov 2024 20:55:54 +0000
Subject: [PATCH] WIP

---
 MODULE.bazel.lock                | 4 ++--
 common/array_stack.h             | 7 +++++++
 toolchain/check/handle_class.cpp | 9 +++++----
 3 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/MODULE.bazel.lock b/MODULE.bazel.lock
index aa644d8bf9053..9d4b7f434bada 100644
--- a/MODULE.bazel.lock
+++ b/MODULE.bazel.lock
@@ -130,7 +130,7 @@
     "//bazel/cc_toolchains:clang_configuration.bzl%clang_toolchain_extension": {
       "general": {
         "bzlTransitiveDigest": "YfAlFgFWuxAcofKFZTG3JR3DeQD3yED9F1mzTVz6xoA=",
-        "usagesDigest": "FiqDwj5QoiCFb1PRYvajLeyuRjRXbsy2CVC+VsEEt7Q=",
+        "usagesDigest": "gfR4YaqF67HjpCEGrL0eyE8UHB6u6nYKukxqkDtpUnA=",
         "recordedFileInputs": {},
         "recordedDirentsInputs": {},
         "envVariables": {},
@@ -147,7 +147,7 @@
     "//bazel/llvm_project:llvm_project.bzl%llvm_project": {
       "general": {
         "bzlTransitiveDigest": "QG2kkxgw5yvNHGJYbsdwlidT0uFXu5uZMW6KftuXDsI=",
-        "usagesDigest": "0qlzYSob/rZ/yVQnGrAnMBlAhKszo5qDlauf4tO9tgY=",
+        "usagesDigest": "xAFaBH1/4TOjTzJ1M7AQoJOUT6PulRTi3jh5H7fQmno=",
         "recordedFileInputs": {},
         "recordedDirentsInputs": {},
         "envVariables": {},
diff --git a/common/array_stack.h b/common/array_stack.h
index 9021c8a4bdb53..1e6cab296d042 100644
--- a/common/array_stack.h
+++ b/common/array_stack.h
@@ -47,6 +47,13 @@ class ArrayStack {
     return llvm::ArrayRef(values_).slice(array_offsets_.back());
   }
 
+  // Returns the top array from the stack in such a way that the array's
+  // contents can be modified by the caller.
+  auto PeekMutableArray() -> llvm::MutableArrayRef<ValueT> {
+    CARBON_CHECK(!array_offsets_.empty());
+    return llvm::MutableArrayRef(values_).slice(array_offsets_.back());
+  }
+
   // Returns the array at a specific index.
   auto PeekArrayAt(int index) const -> llvm::ArrayRef<ValueT> {
     auto ref = llvm::ArrayRef(values_).slice(array_offsets_[index]);
diff --git a/toolchain/check/handle_class.cpp b/toolchain/check/handle_class.cpp
index f0ef56f0f9a5e..a0dba32a2b009 100644
--- a/toolchain/check/handle_class.cpp
+++ b/toolchain/check/handle_class.cpp
@@ -296,6 +296,8 @@ auto HandleParseNode(Context& context, Parse::ClassDefinitionStartId node_id)
   context.inst_block_stack().Push();
   context.node_stack().Push(node_id, class_id);
   context.struct_type_fields_stack().PushArray();
+  context.struct_type_fields_stack().AppendToTop(
+      {.name_id = SemIR::NameId::Vptr, .type_id = context.GetTupleType({})});
 
   // TODO: Handle the case where there's control flow in the class body. For
   // example:
@@ -668,10 +670,9 @@ static auto CheckCompleteClassType(Context& context, Parse::NodeId node_id,
   }
 
   if (defining_vtable_ptr) {
-    context.struct_type_fields_stack().PrependToTop(
-        {.name_id = SemIR::NameId::Vptr,
-         .type_id = context.GetPointerType(
-             context.GetBuiltinType(SemIR::BuiltinInstKind::VtableType))});
+    context.struct_type_fields_stack().PeekMutableArray().front().type_id =
+        context.GetPointerType(
+            context.GetBuiltinType(SemIR::BuiltinInstKind::VtableType));
   }
 
   auto fields_id = context.struct_type_fields().AddCanonical(