Ah yeah, "unspecified behaviour" is my known phrase, but I can see it would be confusing to be arguing about whether something should be UB or UB :D

Unspecified behavior and implementation defined behavior are distinct in theory. The latter requires the behavior to be documented by the implementer, while the former does not.

In practice I dare you to find compiler documentation for each implementation defined behavior.

https://timsong-cpp.github.io/cppwp/n4868/intro.defs#defns.u...

https://timsong-cpp.github.io/cppwp/n4868/intro.defs#defns.i...

edit:

C wording:

https://cigix.me/c17#J.1

https://cigix.me/c17#J.3

> In practice I dare you to find compiler documentation for each implementation defined behavior.

For gcc there is this:

https://gcc.gnu.org/onlinedocs/gcc/C-Implementation.html

No idea how complete it is. Also a lot of stuff is architecture/platform specific, not compiler specific, so you won't find it in the general compiler docs but you have to look at the psABI.