It wouldn’t work on capability-based architectures where pointer validity is enforced by tag bits. Cast the pointer to an integer, lose the tag; cast it back, tag is missing and pointer is invalid (except for privileged system software which has the authority to enable the tag bit on an arbitrary pointer.)
Do such architectures still exist? Do/did they support C? Well, 128-bit MI pointers on IBM i (fka AS/400) are like this - a hardware tag bit protects them against forgery - and ILE C lets you manipulate such pointers (it calls them “system pointers”, _SYSPTR), so that would be a real world example of a pointer in C which can be cast to an integer but cannot be cast back. (IBM i also has 64-bit pointers which aren’t capabilities and hence aren’t tag-protected and can be cast to/from integers - but they don’t point into the main operating system address space, which is a single-level store single address space shared by all non-Unix processes, they only point into per-process private address spaces, so-called “teraspaces”.)
I think some UB in C is motivated by allowing C to be used on these kinds of architectures, even if they are now exceptionally rare. When C was initially being standardised in the 1980s, many people thought these kinds of architectures were the future, I think they were surprised by the fact they’ve never gone mainstream
ARM Morello (on the front page earlier this week: https://news.ycombinator.com/item?id=30007474) is a capability-based architecture, with 129-bit pointers. Compilers for it provide a uintptr_t that is appropriate, but it is far stricter about the kinds of operations that can be done in the reverse direction.
Do such architectures still exist? Do/did they support C? Well, 128-bit MI pointers on IBM i (fka AS/400) are like this - a hardware tag bit protects them against forgery - and ILE C lets you manipulate such pointers (it calls them “system pointers”, _SYSPTR), so that would be a real world example of a pointer in C which can be cast to an integer but cannot be cast back. (IBM i also has 64-bit pointers which aren’t capabilities and hence aren’t tag-protected and can be cast to/from integers - but they don’t point into the main operating system address space, which is a single-level store single address space shared by all non-Unix processes, they only point into per-process private address spaces, so-called “teraspaces”.)
I think some UB in C is motivated by allowing C to be used on these kinds of architectures, even if they are now exceptionally rare. When C was initially being standardised in the 1980s, many people thought these kinds of architectures were the future, I think they were surprised by the fact they’ve never gone mainstream