> Yes, I recognize the issue. This can be resolved by stating that implementations may refuse to relocate function parameters, and providing a mechanism (e.g. an attribute) for the author to switch ABI to callee-destroy.

It's more complicated than that. If relocation also destructs objects, then you allow for a new category of unmovable objects to support relocation. For example, std::lock_guard could be relocatable with no change.

Then how do you deal with this?

    void foo(lock_guard<mutex> lg);
    void fwd_to_foo(lock_guard<mutex> lg) { foo(reloc lg); }
    void bar(mutex& m)
    {
        std::lock_guard lg{m};
        fwd_to_foo(reloc lg);
    }

This code will work fine if reloc avoids the destructor call. But it will not if "implementations refuse to relocate", as lock_guard is not movable.

We cannot have code optionally compile depending on which ABI we pick, can we?