[clang] [clang] Implement a bitwise_copyable builtin type trait. (PR #86512)

[Ilya Biryukov via cfe-commits]

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@@ -1120,6 +1120,22 @@ class QualType {
   /// Return true if this is a trivially copyable type (C++0x [basic.types]p9)
   bool isTriviallyCopyableType(const ASTContext &Context) const;
 
+  /// Return true if the type is safe to bitwise copy using memcpy/memmove.
+  ///
+  /// This is an extension in clang: bitwise clonable types act as trivially
+  /// copyable types, meaning their underlying bytes can be safely copied by
+  /// memcpy or memmove. After the copy, the destination object has the same
+  /// object representation.
+  ///
+  /// However, there are cases where it is not safe to copy:
+  ///  - When sanitizers, such as AddressSanitizer, add padding with poison,
+  ///    which can cause issues if those poisoned padding bits are accessed.
+  ///  - Types with Objective-C lifetimes, where specific runtime
+  ///    semantics may not be preserved during a bitwise copy.
+  ///
+  // FIXME: each call will trigger a full computation, cache the result.
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ilya-biryukov wrote:

NIT: move this FIXME into the implementation. I don't think it's relevant to the public API contract of this method.

https://github.com/llvm/llvm-project/pull/86512