Disallow users to write instances for KnownNat and KnownSymbol

Consider the following patent nonsense:

{-# LANGUAGE FlexibleInstances #-}
import GHC.TypeLits
instance KnownNat n

Originally this was accepted (albeit with a warning about the missing natSing method). It is now rejected, as with instances of KnownSymbol, in the same way as instances of Typeable, which says:

    Class ‘Typeable’ does not support user-specified instances

changed weight to 5

added Tbug Trac import newcomer labels

I'm a total newbie so can't comment on whether this is a desired change. However in case it is - would extending the code that was added for Typeable (#8132 (closed)) to account for additional class names be the way to go here?

assigned to @trac-sean

Also a newbie but I've managed to get this working in the way you described. I'll start following the submission instructions by making myself the owner, apologies if I'm mistaken in doing this.

No problems so far Sean! If you need any help then feel free to ask here or on the IRC channel.

Trac metadata

Trac field	Value
Differential revisions	- → D2898

changed title from KnownNat and KnownSymbol should be abstract to Disallow users to write instances for KnownNat and KnownSymbol

changed the description

mentioned in commit c5609577

closed

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

added Pnormal label