[clang] [clang][bytecode] Fix initializing bases via DefaultInitExpr (PR #215499)

[Timm Baeder via cfe-commits]

https://github.com/tbaederr updated https://github.com/llvm/llvm-project/pull/215499

>From ce6fb494322f3a05da6d45a4807cf7e9f5578cfa Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Timm=20B=C3=A4der?= <tbaeder at redhat.com>
Date: Tue, 11 Aug 2026 10:49:53 +0200
Subject: [PATCH] [clang][bytecode] Fix initializing bases via DefaultInitExpr

---
 clang/lib/AST/ByteCode/Compiler.cpp | 10 ++++++++++
 clang/lib/AST/ByteCode/Compiler.h   | 18 ++++++++++++------
 clang/test/AST/ByteCode/records.cpp | 10 ++++++++++
 3 files changed, 32 insertions(+), 6 deletions(-)

diff --git a/clang/lib/AST/ByteCode/Compiler.cpp b/clang/lib/AST/ByteCode/Compiler.cpp
index 316b2a4f092f9..9b2c9191f729a 100644
--- a/clang/lib/AST/ByteCode/Compiler.cpp
+++ b/clang/lib/AST/ByteCode/Compiler.cpp
@@ -340,6 +340,8 @@ bool InitLink::emit(Compiler<Emitter> *Ctx, const Expr *E) const {
   case K_Field:
     // We're assuming there's a base pointer on the stack already.
     return Ctx->emitGetPtrFieldPop(Offset, E);
+  case K_Base:
+    return Ctx->emitGetPtrBasePop(Offset, false, E);
   case K_Temp:
     return Ctx->emitGetPtrLocal(Offset, E);
   case K_Decl:
@@ -2398,6 +2400,8 @@ bool Compiler<Emitter>::visitInitList(ArrayRef<const Expr *> Inits,
     for (unsigned BI = 0; BI != R->getNumBases(); ++BI) {
       const Expr *Init = Inits[BI];
       const Record::Base *B = R->getBase(BI);
+      InitStackScope<Emitter> ISS(this, isa<CXXDefaultInitExpr>(Init));
+      InitLinkScope<Emitter> ILS(this, InitLink::Base(B->Offset));
       if (!this->emitGetPtrBase(B->Offset, Init))
         return false;
       if (!this->visitInitializerPop(Init))
@@ -6468,6 +6472,7 @@ bool Compiler<Emitter>::VisitCXXThisExpr(const CXXThisExpr *E) {
 
     if (InitStack[StartIndex].Kind != InitLink::K_Field &&
         InitStack[StartIndex].Kind != InitLink::K_Elem &&
+        InitStack[StartIndex].Kind != InitLink::K_Base &&
         InitStack[StartIndex].Kind != InitLink::K_DIE)
       break;
   }
@@ -6475,6 +6480,11 @@ bool Compiler<Emitter>::VisitCXXThisExpr(const CXXThisExpr *E) {
   if (StartIndex == 0 && EndIndex == 0)
     EndIndex = InitStack.size() - 1;
 
+  assert(InitStack[StartIndex].Kind == InitLink::K_Decl ||
+         InitStack[StartIndex].Kind == InitLink::K_This ||
+         InitStack[StartIndex].Kind == InitLink::K_Temp ||
+         InitStack[StartIndex].Kind == InitLink::K_RVO);
+
   // NOTE: This could be StartIndex < EndIndex, but we're also abusing the
   // InitStack mechanism in visitWithSubstitutions to have the This pointer
   // _just_ be a local variable.
diff --git a/clang/lib/AST/ByteCode/Compiler.h b/clang/lib/AST/ByteCode/Compiler.h
index 46d6b1a49c001..40589b45fa051 100644
--- a/clang/lib/AST/ByteCode/Compiler.h
+++ b/clang/lib/AST/ByteCode/Compiler.h
@@ -49,12 +49,13 @@ struct InitLink {
   enum {
     K_This = 0,
     K_Field = 1,
-    K_Temp = 2,
-    K_Decl = 3,
-    K_Elem = 5,
-    K_RVO = 6,
-    K_InitList = 7,
-    K_DIE = 8,
+    K_Base = 2,
+    K_Temp = 3,
+    K_Decl = 4,
+    K_Elem = 6,
+    K_RVO = 7,
+    K_InitList = 8,
+    K_DIE = 9,
   };
 
   static InitLink This() { return InitLink{K_This}; }
@@ -66,6 +67,11 @@ struct InitLink {
     IL.Offset = Offset;
     return IL;
   }
+  static InitLink Base(unsigned Offset) {
+    InitLink IL{K_Base};
+    IL.Offset = Offset;
+    return IL;
+  }
   static InitLink Temp(unsigned Offset) {
     InitLink IL{K_Temp};
     IL.Offset = Offset;
diff --git a/clang/test/AST/ByteCode/records.cpp b/clang/test/AST/ByteCode/records.cpp
index b792b56563b2f..36b5cb62fe95f 100644
--- a/clang/test/AST/ByteCode/records.cpp
+++ b/clang/test/AST/ByteCode/records.cpp
@@ -2044,3 +2044,13 @@ namespace VariadicCtorStartsLifetime {
   /// Used to not start the lifetime of 's'.
   constexpr C c;
 }
+
+namespace BaseInitViaDIE {
+  struct S {
+    int a = 42, b = a;
+  };
+
+  struct SS : S {};
+  constexpr SS ss {};
+  static_assert(ss.b == 42, "");
+}