> All of those look pretty straightforward to me -- again, what assembly would you expect to be emitted in those cases?

It is very straightforward indeed, but it is still not mapping primitive operations to direct machine code, but it is forwarding to out-of-line code. Same as operator overloading in other languages.

> And here's assembly for C++: https://godbolt.org/z/dedcof9x5

That's just a symptom of allowing the compiler to inline the add code, otherwise the generated code is as straightforward:

   addOne(Int):
    push   rax
    mov    esi,0x1
    call   4010c0 <add_safe(int, int)>

Ref: https://godbolt.org/z/xo1es9TcW