[kazinator]

ELF shared libraries like Glibc do this at the binary level. If some function changes in a way that breaks backward binary compatibility, then it gets versioned; so that existing compiled programs use the compatibility version.

E.g. suppose that there is a new version of pthread_mutex_lock(&mutex) which relies on a larger structure with new members in it. Problem is that compiled programs have pthread_mutex_lock(&mutex) which pass a pointer to the older, smaller structure. If the library were to work with the structure using the new definition, it would access out of bounds. Versioning take care of this; the old clients call a backwards compatible function. It might work with the new definition, but avoids touching the new members that didn't exist in the old library.

But this is a very low-level motivation; this same problem of low-level layout information being baked into the contract shouldn't exist in a higher level language.