[fuhsnn]

>the proposal is tailored so that other than simple syntax support no new semantics need to be implemented within GCC

Not just GCC, but you're right it's tailored, to the same "unwinding" queue that C++ destructor, stack-VLA de-allocation and __attribute__((cleanup)) shared, won't fit into the current state of language otherwise.

Clang share more frontend between C and C++ so I imagine they can implement it as hidden C++ lambda scope-guards, the nested scenario is just full-capturing lambdas inside another.