[clang] [clang][SYCL] Diagnose reference kernel arguments (PR #192957)

[Mariya Podchishchaeva via cfe-commits]

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@@ -0,0 +1,140 @@
+//===---------- 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 {
+template <typename Derived> class SubobjectVisitor {
+  ASTContext &Ctx;
+
+  public:
+  SubobjectVisitor(ASTContext &Ctx) : Ctx(Ctx) {}
+  /// Return a reference to the derived class.
+  Derived &getDerived() { return *static_cast<Derived *>(this); }
+
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Fznamznon wrote:

Hi @tahonermann , thanks. I've updated the PR to reflect the changes. 

> If we later find that it would be useful to provide more specific visitors for different kinds of records or types, then we can implement the fallback approach used in CommentVisitor.h, DeclVisitor.h, StmtVisitor.h, and TemplateArgumentVisitor.h; they provide specialized visitors (e.g., VisitNullTemplateArgument()) that, if not implemented by the derived class, fall back to a generic visitor (e.g., VisitTemplateArgument()). 

Well I had to do that already for the basic kernel argument checking visitor I'm adding in this PR. Otherwise I don't see how it would find a reference type for me. If you think I would need to move all this dispatching to the derived class, please let me know. I don't think that, for example, a visitor that will find special types and build a chain of member accesses for decomposition support will need to know how to visit a specific type.

Also, note that in DPC++ we added possibility to visit each element of the array for decomposition support, since we had to call `__init` for each element of the array of accessors (if it appears within kernel arguments). I suppose it might be needed here in the future as well.

> Most visitors allow the node pointer/reference type to be customized to support non-const and const visitation. I think that is a useful feature to provide.

This is slightly problematic. 1. I don't know an example where it would be useful. 2. I added visitation for different kinds of records or types and since visit interface accepts a `QualType`, I only could do `getTypePtr` to dispatch to different types and `getTypePtr` only returns `const Type*` . So there is no non-const visitation for all kinds of types which I find weird.

I also had to apply a few tweaks here because `getAsArrayType` only returns const qualified `ArrayType*` and `CXXRecordDecl::bases` returns an array of `CXXBaseQualifier` objects, not array of pointers.

Please let me know if this is moving in the right direction.

https://github.com/llvm/llvm-project/pull/192957