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:

• [cppcoreguidelines-special-member-functions] Spiritually false positive for operator=(T) llvm/llvm-project#76064
• https://stackoverflow.com/questions/77692186/clang-tidy-does-not-recognize-unifying-assignment-operator-as-move-assigment