var x = 42.narrow doesn't compile in scala 2.12

[JoeCordingley]

This line just blows up the compiler in scala 2.12.x. In https://github.com/underscoreio/shapeless-guide/blob/master/src/pages/labelled-generic/literal-types.md . Issue here: milessabin/shapeless#674 . Perhaps just a note not to try it with scala 2.12.x?

[JoeCordingley]

I should also point out, it's fine with val. But that obviously ruins the point that you make afterwards about not being able to change it.