Implementation defined is quite different from undefined... But I agree, making this bit implementation defined is not cool. I don't have strong opinions on w

Implementation defined is quite different from undefined... But I agree, making this bit implementation defined is not cool. I don't have strong opinions on whether defer should be in C or not. It does seem like a potentially useful feature. Whatever they do though, I think they should do it right or not do it at all. The last thing I want is to bloat the language with half-assed features that you can't use half the time because they're not defined well enough. If people want implementation defined features, geez, just keep using compiler extensions until the WG can define a proper standard. Apart from this bit, the proposal seems pretty decent to me. I kinda like the lambda syntax and the control it gives you over the value you capture. Just make it block scoped and don't leave it implementation defined.