[libc++] Take the ABI break for std::list's pointer UB unconditionally (#100585)

This ABI break only affects fancy pointer which have a different value
representation when pointing to a base of T instead of T itself. This
seems like a rather small set of fancy pointers, which themselves
already represent a very small niche. This patch swaps a pointer to T
with a pointer to base of T in a few library-internal types.

Author: philnik777

libcxx/docs/ReleaseNotes/20.rst

@@ -102,6 +102,12 @@ LLVM 21
 ABI Affecting Changes
 ---------------------
 
+- The ABI breaks for removing undefined behaviour in ``std::forward_list``, ``std::list``, ``std::map``, ``std::set``,
+  ``std::multimap``, ``std::multiset``, ``std::unordered_map``, ``std::unordered_set``, ``std::unordered_multimap`` and
+  ``std::unordered_multiset`` are now applied unconditionally. This only affects fancy pointers which have a different
+  value representation when pointing at the base of an internal node type instead of the type itself. A size or
+  alignment difference is diagnosed, but more subtle ABI breaks may result in unexpected behaviour.
+
 - The internal structure ``__compressed_pair`` has been replaced with ``[[no_unique_address]]``. The ABI impact is:
 
   - When using the Itanium ABI (most non-MSVC platforms), empty types are now placed at the beginning of the enclosing

libcxx/include/__hash_table

@@ -77,11 +77,18 @@ struct __hash_node_base {
   typedef __hash_node_base __first_node;
   typedef __rebind_pointer_t<_NodePtr, __first_node> __node_base_pointer;
   typedef _NodePtr __node_pointer;
-
-#if defined(_LIBCPP_ABI_FIX_UNORDERED_NODE_POINTER_UB)
   typedef __node_base_pointer __next_pointer;
-#else
-  typedef __conditional_t<is_pointer<__node_pointer>::value, __node_base_pointer, __node_pointer> __next_pointer;
+
+// TODO(LLVM 22): Remove this check
+#ifndef _LIBCPP_ABI_FIX_UNORDERED_NODE_POINTER_UB
+  static_assert(sizeof(__node_base_pointer) == sizeof(__node_pointer) && _LIBCPP_ALIGNOF(__node_base_pointer) ==
+                    _LIBCPP_ALIGNOF(__node_pointer),
+                "It looks like you are using std::__hash_table (an implementation detail for the unordered containers) "
+                "with a fancy pointer type that thas a different representation depending on whether it points to a "
+                "__hash_table base pointer or a __hash_table node pointer (both of which are implementation details of "
+                "the standard library). This means that your ABI is being broken between LLVM 19 and LLVM 20. If you "
+                "don't care about your ABI being broken, define the _LIBCPP_ABI_TREE_REMOVE_NODE_POINTER_UB macro to "
+                "silence this diagnostic.");
 #endif
 
   __next_pointer __next_;

libcxx/include/__tree

@@ -566,11 +566,18 @@ struct __tree_node_base_types {
 
   typedef __tree_end_node<__node_base_pointer> __end_node_type;
   typedef __rebind_pointer_t<_VoidPtr, __end_node_type> __end_node_pointer;
-#if defined(_LIBCPP_ABI_TREE_REMOVE_NODE_POINTER_UB)
   typedef __end_node_pointer __parent_pointer;
-#else
-  typedef __conditional_t< is_pointer<__end_node_pointer>::value, __end_node_pointer, __node_base_pointer>
-      __parent_pointer;
+
+// TODO(LLVM 22): Remove this check
+#ifndef _LIBCPP_ABI_TREE_REMOVE_NODE_POINTER_UB
+  static_assert(sizeof(__node_base_pointer) == sizeof(__end_node_pointer) && _LIBCPP_ALIGNOF(__node_base_pointer) ==
+                    _LIBCPP_ALIGNOF(__end_node_pointer),
+                "It looks like you are using std::__tree (an implementation detail for (multi)map/set) with a fancy "
+                "pointer type that thas a different representation depending on whether it points to a __tree base "
+                "pointer or a __tree node pointer (both of which are implementation details of the standard library). "
+                "This means that your ABI is being broken between LLVM 19 and LLVM 20. If you don't care about your "
+                "ABI being broken, define the _LIBCPP_ABI_TREE_REMOVE_NODE_POINTER_UB macro to silence this "
+                "diagnostic.");
 #endif
 
 private:
@@ -605,12 +612,7 @@ public:
   typedef _Tp __node_value_type;
   typedef __rebind_pointer_t<_VoidPtr, __node_value_type> __node_value_type_pointer;
   typedef __rebind_pointer_t<_VoidPtr, const __node_value_type> __const_node_value_type_pointer;
-#if defined(_LIBCPP_ABI_TREE_REMOVE_NODE_POINTER_UB)
   typedef typename __base::__end_node_pointer __iter_pointer;
-#else
-  typedef __conditional_t< is_pointer<__node_pointer>::value, typename __base::__end_node_pointer, __node_pointer>
-      __iter_pointer;
-#endif
 
 private:
   static_assert(!is_const<__node_type>::value, "_NodePtr should never be a pointer to const");

libcxx/include/forward_list

@@ -278,18 +278,20 @@ struct __forward_node_traits {
   typedef __rebind_pointer_t<_NodePtr, __begin_node> __begin_node_pointer;
   typedef __rebind_pointer_t<_NodePtr, void> __void_pointer;
 
-#if defined(_LIBCPP_ABI_FORWARD_LIST_REMOVE_NODE_POINTER_UB)
-  typedef __begin_node_pointer __iter_node_pointer;
-#else
-  typedef __conditional_t<is_pointer<__void_pointer>::value, __begin_node_pointer, __node_pointer> __iter_node_pointer;
+// TODO(LLVM 22): Remove this check
+#ifndef _LIBCPP_ABI_FORWARD_LIST_REMOVE_NODE_POINTER_UB
+  static_assert(sizeof(__begin_node_pointer) == sizeof(__node_pointer) && _LIBCPP_ALIGNOF(__begin_node_pointer) ==
+                    _LIBCPP_ALIGNOF(__node_pointer),
+                "It looks like you are using std::forward_list with a fancy pointer type that thas a different "
+                "representation depending on whether it points to a forward_list base pointer or a forward_list node "
+                "pointer (both of which are implementation details of the standard library). This means that your ABI "
+                "is being broken between LLVM 19 and LLVM 20. If you don't care about your ABI being broken, define "
+                "the _LIBCPP_ABI_FORWARD_LIST_REMOVE_NODE_POINTER_UB macro to silence this diagnostic.");
 #endif
 
-  typedef __conditional_t<is_same<__iter_node_pointer, __node_pointer>::value, __begin_node_pointer, __node_pointer>
-      __non_iter_node_pointer;
-
-  _LIBCPP_HIDE_FROM_ABI static __iter_node_pointer __as_iter_node(__iter_node_pointer __p) { return __p; }
-  _LIBCPP_HIDE_FROM_ABI static __iter_node_pointer __as_iter_node(__non_iter_node_pointer __p) {
-    return static_cast<__iter_node_pointer>(static_cast<__void_pointer>(__p));
+  _LIBCPP_HIDE_FROM_ABI static __begin_node_pointer __as_iter_node(__begin_node_pointer __p) { return __p; }
+  _LIBCPP_HIDE_FROM_ABI static __begin_node_pointer __as_iter_node(__node_pointer __p) {
+    return static_cast<__begin_node_pointer>(static_cast<__void_pointer>(__p));
   }
 };
 
@@ -351,10 +353,9 @@ class _LIBCPP_TEMPLATE_VIS __forward_list_iterator {
   typedef __forward_node_traits<_NodePtr> __traits;
   typedef typename __traits::__node_pointer __node_pointer;
   typedef typename __traits::__begin_node_pointer __begin_node_pointer;
-  typedef typename __traits::__iter_node_pointer __iter_node_pointer;
   typedef typename __traits::__void_pointer __void_pointer;
 
-  __iter_node_pointer __ptr_;
+  __begin_node_pointer __ptr_;
 
   _LIBCPP_HIDE_FROM_ABI __begin_node_pointer __get_begin() const {
     return static_cast<__begin_node_pointer>(static_cast<__void_pointer>(__ptr_));
@@ -417,10 +418,9 @@ class _LIBCPP_TEMPLATE_VIS __forward_list_const_iterator {
   typedef typename __traits::__node_type __node_type;
   typedef typename __traits::__node_pointer __node_pointer;
   typedef typename __traits::__begin_node_pointer __begin_node_pointer;
-  typedef typename __traits::__iter_node_pointer __iter_node_pointer;
   typedef typename __traits::__void_pointer __void_pointer;
 
-  __iter_node_pointer __ptr_;
+  __begin_node_pointer __ptr_;
 
   _LIBCPP_HIDE_FROM_ABI __begin_node_pointer __get_begin() const {
     return static_cast<__begin_node_pointer>(static_cast<__void_pointer>(__ptr_));