================
@@ -0,0 +1,109 @@
+//===---------- SubobjectVisitor.h - Subobject Visitor ----------*- C++
-*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM
Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file defines the SubobjectVisitor interface, which recursively
+// traverses subobjects within a type.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_SUBOBJECTVISITOR_H
+#define LLVM_CLANG_AST_SUBOBJECTVISITOR_H
+
+#include "clang/AST/Type.h"
+
+namespace clang {
+
+/// Given a type, subobject visitors visit all subobjects of the type in depth
+/// first order. Both pre-order and post-order visitation are performed so that
+/// derived classes can maintain an access path to the visited elements.
+/// Subobjects include all base classes and non-static data members, including
+/// those that are not subobjects according to the C++standard like data
+/// members with a reference type. Virtual base classes are visited each time
+/// they appear in a class hierarchy despite there being only one actual
+/// subobject present in an object of a most derived type. Array elements are
+/// not individually visited; only their containing array is.
+template <template <typename> class Ptr, typename Derived>
+class SubobjectVisitorBase {
+ ASTContext &Ctx;
+ template <typename Class> using ptr_t = typename Ptr<Class>::type;
+
+public:
+ SubobjectVisitorBase(ASTContext &Ctx) : Ctx(Ctx) {}
+
+ /// Return a reference to the derived class.
+ Derived &getDerived() { return *static_cast<Derived *>(this); }
+
+ void visit(QualType QT) {
+ assert(!QT->isDependentType());
+ QT = QT.getDesugaredType(Ctx);
+
----------------
tahonermann wrote:
The following types are neither always-dependent nor sugared types, but are
types that can be ignored for other reasons. I think an explicit comment is
worthwhile.
```suggestion
// FunctionType, FunctionProtoType, and FunctionNoProtoType are never
// the type of a subobject.
// ObjCObjectType and ObjCInterfaceType are never the type of a
// subobject due to the Objective-C object allocation model.
// PointerType, BlockPointerType, ReferenceType, LValueReferenceType,
// RValueReferenceType, MemberPointerType, and ObjCObjectPointerType
// may all be the type of a subobject, but they do not contain other
// subobjects; the pointee type should not be visited.
// ComplexType, VectorType, ExtVectorType, MatrixType, PipeType,
// EnumType, and OverflowBehaviorType all have an element or
// underlying type that could be visited. However, in each of these
// cases, the lower type is constrained to a fundamental type and
// therefore doesn't contain any fields or base classes.
```
https://github.com/llvm/llvm-project/pull/192957
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