[llvm-branch-commits] Add pointer field protection feature. (PR #133538)

[Mark de Wever via llvm-branch-commits]

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@@ -1215,6 +1215,29 @@ typedef __char32_t char32_t;
 #    define _LIBCPP_HAS_EXPLICIT_THIS_PARAMETER 0
 #  endif
 
+#  if __has_feature(pointer_field_protection)
+// Force a class to be non-standard layout by giving it two bases with the same
+// type. This is useful when structure protection is enabled because structure
+// protection cannot be applied to standard layout classes. We may use this in
+// cases where the standard does not specify whether a standard library class is
+// standard layout. See C++2a [class]p7:
+//   A class S is a standard-layout class if it:
+//     -- has at most one base class subobject of any given type
+_LIBCPP_BEGIN_NAMESPACE_STD
+_LIBCPP_DIAGNOSTIC_PUSH
+_LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Winaccessible-base")
+class __force_nonstandard_layout_base1 {};
+class __force_nonstandard_layout_base2 : __force_nonstandard_layout_base1 {};
+class __force_nonstandard_layout : __force_nonstandard_layout_base1, __force_nonstandard_layout_base2 {};
+_LIBCPP_DIAGNOSTIC_POP
+_LIBCPP_END_NAMESPACE_STD
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mordante wrote:

This should be done (conditionally?) is a different header. If we add `_LIBCPP_BEGIN_NAMESPACE_STD` to this header it suddenly breaks this.

https://github.com/llvm/llvm-project/pull/133538