[clang] [clang][bytecode] Mark CXXDefaultInitExprs in InitLink chain (PR #166395)

[Timm Baeder via cfe-commits]

https://github.com/tbaederr created https://github.com/llvm/llvm-project/pull/166395

So we know before _what_ entry in the chain we need to look for the InitList.

Fixes https://github.com/llvm/llvm-project/issues/166171

>From 7e589cbff35a0e3c5184932eaf7d04a549bab227 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Timm=20B=C3=A4der?= <tbaeder at redhat.com>
Date: Tue, 4 Nov 2025 16:48:51 +0100
Subject: [PATCH] [clang][bytecode] Mark CXXDefaultInitExprs in InitLink chain

So we know before _what_ entry in the chain we need to look for the
InitList.

Fixes https://github.com/llvm/llvm-project/issues/166171
---
 clang/lib/AST/ByteCode/Compiler.cpp | 11 ++++++-----
 clang/lib/AST/ByteCode/Compiler.h   | 17 +++++++++++++----
 clang/test/AST/ByteCode/cxx14.cpp   | 21 +++++++++++++++++++++
 3 files changed, 40 insertions(+), 9 deletions(-)

diff --git a/clang/lib/AST/ByteCode/Compiler.cpp b/clang/lib/AST/ByteCode/Compiler.cpp
index 6c088469a3ca2..09d22e3d79e5a 100644
--- a/clang/lib/AST/ByteCode/Compiler.cpp
+++ b/clang/lib/AST/ByteCode/Compiler.cpp
@@ -5412,8 +5412,7 @@ bool Compiler<Emitter>::VisitCXXThisExpr(const CXXThisExpr *E) {
   unsigned EndIndex = 0;
   // Find the init list.
   for (StartIndex = InitStack.size() - 1; StartIndex > 0; --StartIndex) {
-    if (InitStack[StartIndex].Kind == InitLink::K_InitList ||
-        InitStack[StartIndex].Kind == InitLink::K_This) {
+    if (InitStack[StartIndex].Kind == InitLink::K_DIE) {
       EndIndex = StartIndex;
       --StartIndex;
       break;
@@ -5426,7 +5425,8 @@ bool Compiler<Emitter>::VisitCXXThisExpr(const CXXThisExpr *E) {
       continue;
 
     if (InitStack[StartIndex].Kind != InitLink::K_Field &&
-        InitStack[StartIndex].Kind != InitLink::K_Elem)
+        InitStack[StartIndex].Kind != InitLink::K_Elem &&
+        InitStack[StartIndex].Kind != InitLink::K_DIE)
       break;
   }
 
@@ -5437,7 +5437,8 @@ bool Compiler<Emitter>::VisitCXXThisExpr(const CXXThisExpr *E) {
 
   // Emit the instructions.
   for (unsigned I = StartIndex; I != (EndIndex + 1); ++I) {
-    if (InitStack[I].Kind == InitLink::K_InitList)
+    if (InitStack[I].Kind == InitLink::K_InitList ||
+        InitStack[I].Kind == InitLink::K_DIE)
       continue;
     if (!InitStack[I].template emit<Emitter>(this, E))
       return false;
@@ -6306,8 +6307,8 @@ bool Compiler<Emitter>::compileConstructor(const CXXConstructorDecl *Ctor) {
 
       unsigned FirstLinkOffset =
           R->getField(cast<FieldDecl>(IFD->chain()[0]))->Offset;
-      InitStackScope<Emitter> ISS(this, isa<CXXDefaultInitExpr>(InitExpr));
       InitLinkScope<Emitter> ILS(this, InitLink::Field(FirstLinkOffset));
+      InitStackScope<Emitter> ISS(this, isa<CXXDefaultInitExpr>(InitExpr));
       if (!emitFieldInitializer(NestedField, NestedFieldOffset, InitExpr,
                                 IsUnion))
         return false;
diff --git a/clang/lib/AST/ByteCode/Compiler.h b/clang/lib/AST/ByteCode/Compiler.h
index 5c46f75af4da3..0c6cab9276531 100644
--- a/clang/lib/AST/ByteCode/Compiler.h
+++ b/clang/lib/AST/ByteCode/Compiler.h
@@ -52,12 +52,14 @@ struct InitLink {
     K_Decl = 3,
     K_Elem = 5,
     K_RVO = 6,
-    K_InitList = 7
+    K_InitList = 7,
+    K_DIE = 8,
   };
 
   static InitLink This() { return InitLink{K_This}; }
   static InitLink InitList() { return InitLink{K_InitList}; }
   static InitLink RVO() { return InitLink{K_RVO}; }
+  static InitLink DIE() { return InitLink{K_DIE}; }
   static InitLink Field(unsigned Offset) {
     InitLink IL{K_Field};
     IL.Offset = Offset;
@@ -668,22 +670,29 @@ template <class Emitter> class InitLinkScope final {
 
   ~InitLinkScope() { this->Ctx->InitStack.pop_back(); }
 
-private:
+public:
   Compiler<Emitter> *Ctx;
 };
 
 template <class Emitter> class InitStackScope final {
 public:
   InitStackScope(Compiler<Emitter> *Ctx, bool Active)
-      : Ctx(Ctx), OldValue(Ctx->InitStackActive) {
+      : Ctx(Ctx), OldValue(Ctx->InitStackActive), Active(Active) {
     Ctx->InitStackActive = Active;
+    if (Active)
+      Ctx->InitStack.push_back(InitLink::DIE());
   }
 
-  ~InitStackScope() { this->Ctx->InitStackActive = OldValue; }
+  ~InitStackScope() {
+    this->Ctx->InitStackActive = OldValue;
+    if (Active)
+      Ctx->InitStack.pop_back();
+  }
 
 private:
   Compiler<Emitter> *Ctx;
   bool OldValue;
+  bool Active;
 };
 
 } // namespace interp
diff --git a/clang/test/AST/ByteCode/cxx14.cpp b/clang/test/AST/ByteCode/cxx14.cpp
index 9622311e100cb..57cb42ea4a98b 100644
--- a/clang/test/AST/ByteCode/cxx14.cpp
+++ b/clang/test/AST/ByteCode/cxx14.cpp
@@ -7,3 +7,24 @@ constexpr int(*null_ptr)() = nullptr;
 constexpr int test4 = (*null_ptr)(); // both-error {{must be initialized by a constant expression}} \
                                      // both-note {{evaluates to a null function pointer}}
 
+struct E {
+  int n = 0;
+  struct {
+    void *x = this;
+  };
+  void *y = this;
+};
+constexpr E e1 = E();
+static_assert(e1.x != e1.y, "");
+constexpr E e2 = E{0};
+static_assert(e2.x != e2.y, "");
+
+struct S {
+  int &&a = 2;
+  int b[1]{a};
+};
+constexpr int foo() {
+  S s{12};
+  return s.b[0];
+}
+static_assert(foo() == 12, "");