relative line numbers and folding: how to make they play along?

[Filipe Silva <filipe.silva@gmail.com>]

Hi good people,

there was some discussion in spacemacs about the relative line number
behaviour in emacs:
https://github.com/syl20bnr/spacemacs/issues/6536

In essence, currently relative line numbers in emacs are rendered useless
if folding is applied to the buffer.
Here's a nice example: left is emacs, right is vim:

https://cloud.githubusercontent.com/assets/8352747/16707876/4bd64c22-45b5-11e6-8d13-ae9c994cbb02.png

One of the spacemacs maintainers specifically said: "I should note that due
to how folding and line numbers work in Emacs this would be nigh impossible
to fix."
I find that difficult to believe given how people in emacs community praise
about emacs infinite extensibility.

So I ask you, do we have tools to fix this behaviour or do we need a patch
in emacs itself?

Thanks in advance,

Filipe Silva