[clang][bytecode] Mark IndirectFieldDecl chain links as initialized (llvm#125869)

We only initialize the final field above, so make sure we're marking the
links in the chain on the way there as initialized as well.

Author: tbaederr

clang/lib/AST/ByteCode/Compiler.cpp

@@ -5604,6 +5604,22 @@ bool Compiler<Emitter>::compileConstructor(const CXXConstructorDecl *Ctor) {
 
       if (!emitFieldInitializer(NestedField, NestedFieldOffset, InitExpr))
         return false;
+
+      // Mark all chain links as initialized.
+      unsigned InitFieldOffset = 0;
+      for (const NamedDecl *ND : IFD->chain().drop_back()) {
+        const auto *FD = cast<FieldDecl>(ND);
+        const Record *FieldRecord = this->P.getOrCreateRecord(FD->getParent());
+        assert(FieldRecord);
+        NestedField = FieldRecord->getField(FD);
+        InitFieldOffset += NestedField->Offset;
+        assert(NestedField);
+        if (!this->emitGetPtrThisField(InitFieldOffset, InitExpr))
+          return false;
+        if (!this->emitFinishInitPop(InitExpr))
+          return false;
+      }
+
     } else {
       assert(Init->isDelegatingInitializer());
       if (!this->emitThis(InitExpr))

clang/test/AST/ByteCode/unions.cpp

@@ -463,4 +463,26 @@ namespace MoveAssign {
   }
   static_assert(f()== 12);
 }
+
+namespace IFD {
+  template <class T>
+  struct Optional {
+    struct {
+      union {
+        char null_state;
+        T val;
+      };
+    };
+    constexpr Optional() : null_state(){}
+  };
+
+  constexpr bool test()
+  {
+    Optional<int> opt{};
+    Optional<int> opt2{};
+    opt = opt2;
+    return true;
+  }
+  static_assert(test());
+}
 #endif