Regression from 7.6.3 to 7.8.3: could not coerce because argument "has role Nominal"

This file works fine with 7.6.3: <https://github.com/plaimi/plailude/blob/25d2a811a5d1b9b48ae8534bca10cc3798307cbc/src/Data/Time/Units.hs>

But with 7.8.3, every newtype has an error message like:

Could not coerce from ‘TimeUnit Integer’ to ‘TimeUnit (Year a)’
||   because the first type argument of ‘TimeUnit’ has role Nominal,
||   but the arguments ‘Integer’ and ‘Year a’ differ
||   arising from the coercion of the method ‘timeVal’ from type
||                ‘forall n. (TimeUnit Integer, Num n) => Integer -> n’ to type
||                ‘forall n. (TimeUnit (Year a), Num n) => Year a -> n’

To make the code work again, I need to remove a redundant type constraint from:

class TimeUnit t where timeVal :: (TimeUnit t, Num n) => t -> n

Turning it into:

class TimeUnit t where timeVal :: Num n => t -> n

The constraint is redundant because it is already implied by 'class TimeUnit t'.

I suspect this is a regression. I would be happy to work on it, but I don't have time right now, so I'm putting it here for visibility, feedback, and to remember it in the first place.