[GHC] #10348: HEAD: `KnownNat` does not imply `Typeable` any more

23 Apr 2015


      #10348: HEAD: `KnownNat` does not imply `Typeable` any more
-------------------------------------+-------------------------------------
              Reporter:  heisenbug   |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.11
  (Type checker)                     |  Operating System:  Unknown/Multiple
              Keywords:              |   Type of failure:  GHC rejects
          Architecture:              |  valid program
  Unknown/Multiple                   |        Blocked By:
             Test Case:              |   Related Tickets:
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------
 As Iavor confirmed (https://mail.haskell.org/pipermail/ghc-
 devs/2015-April/008885.html) there is a regression on HEAD relative to
 `ghc-7.11.20150215`. Here is a reproduction snippet:


 {{{#!hs
 {-# LANGUAGE AutoDeriveTypeable, GADTs, DataKinds, KindSignatures,
 StandaloneDeriving #-}

 import GHC.TypeLits
 import Data.Typeable

 data Foo (n :: Nat) where
   Hey :: KnownNat n => Foo n

 deriving instance Show (Foo n)

 data T t where
   T :: (Show t, Typeable t) => t -> T t

 deriving instance Show (T n)
 }}}
 With ghci-7.11.20150407 I now see more constraints
 {{{
 *Main> :t T Hey
 T Hey :: (Typeable n, KnownNat n) => T (Foo n)
 }}}

 OTOH ghci-7.11.20150215 only wanted `KnownNat`:
 {{{
 *Main> :t T Hey
 T Hey :: KnownNat n => T (Foo n)
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10348
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