[GHC] #14047: "Illegal instance for type synonym" while deriving Typeable1 for data type

[GHC]

#14047: "Illegal instance for type synonym" while deriving Typeable1 for data type
---------------------------------+--------------------------------------
        Reporter:  alexbiehl     |                Owner:  (none)
            Type:  bug           |               Status:  new
        Priority:  normal        |            Milestone:
       Component:  Compiler      |              Version:  8.2.1
      Resolution:                |             Keywords:
Operating System:  Windows       |         Architecture:  x86_64 (amd64)
 Type of failure:  None/Unknown  |            Test Case:
      Blocked By:                |             Blocking:
 Related Tickets:                |  Differential Rev(s):
       Wiki Page:                |
---------------------------------+--------------------------------------
Description changed by alexbiehl:

Old description:

> This occured to me in `network-ip` package today:
>
> {{{#!hs
> {-# LANGUAGE StandaloneDeriving #-}
> module Repro where
>
> import Data.Typeable (Typeable1)
>
> data InetAddr a = InetAddr
> deriving instance Typeable1 InetAddr
> }}}
>
> Results in GHC complaining with:
>
> {{{#!hs
> repro.hs:7:19: error:
>     * Illegal instance for a type synonym
>       A class instance must be for a class
>     * In the stand-alone deriving instance for `Typeable1 InetAddr'
>   |
> 7 | deriving instance Typeable1 InetAddr
>   |                   ^^^^^^^^^^^^^^^^^^
> }}}
>
> This is due to `Typeable1` defined as type synonym for `Typeable` (see
> here http://hackage.haskell.org/package/base-4.10.0.0/docs/Data-
> Typeable.html#t:Typeable1).
>
> I can see it is deprecated but I would still expect it to work correctly
> still.

New description:

 This occured to me in `network-ip` package today:

 {{{#!hs
 {-# LANGUAGE StandaloneDeriving #-}
 module Repro where

 import Data.Typeable (Typeable1)

 data InetAddr a = InetAddr
 deriving instance Typeable1 InetAddr
 }}}

 Results in GHC complaining with:

 {{{#!hs
 repro.hs:7:19: error:
     * Illegal instance for a type synonym
       A class instance must be for a class
     * In the stand-alone deriving instance for `Typeable1 InetAddr'
   |
 7 | deriving instance Typeable1 InetAddr
   |                   ^^^^^^^^^^^^^^^^^^
 }}}

 This is due to `Typeable1` defined as type synonym for `Typeable` (see
 here http://hackage.haskell.org/package/base-4.10.0.0/docs/Data-
 Typeable.html#t:Typeable1).

 I see it is deprecated but I would still expect it to work correctly
 still.

--

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14047#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler