C.21 Exception for copy-and-swap idiom

C.21 is missing an exception for the copy-and-swap idiom. It's perfectly safe (and considered idiomatic by many) for resource management to write:

T& operator=(T) noexcept { /* ... */ }

This would violate C.21 since technically, the user has only defined the copy assignment operator, and the move assignment operator is implicitly not declared.

The lack of an exception to this rule also manifests itself in clang-tidy false positives. See:

• https://github.com/llvm/llvm-project/issues/76064
• https://stackoverflow.com/questions/77692186/clang-tidy-does-not-recognize-unifying-assignment-operator-as-move-assigment

Start with the C.21 guideline text and compare its special-member-function requirements with the copy-and-swap example in the issue. Review the linked clang-tidy issue and Stack Overflow discussion for the false-positive context. Done means C.21 clearly addresses this idiom without weakening its intended rule.