This is in context with backpack signatures and their instantiation with concrete implementation. Consider the signature Abstract which contains a data NatType of kind Nat.

{- skipped relevant language extensions -}

signature Abstract where
     import GHC.TypeLits
     data NatType :: Nat  -- comment this out
     

Concrete implementations are unable to instantiate this abstract class during linking

   module Concrete where
     type NatType = 42

This is neither working with explicit .bkp files nor actual cabal package. I have isolated a minimum example into a repository a cabal packages as well as a single bkp file. The urls above also have the associated ghc log messages.

The version of ghc I have tested with is 8.4.1