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

[GHC]

#10348: HEAD: `KnownNat` does not imply `Typeable` any more
-------------------------------------+-------------------------------------
        Reporter:  heisenbug         |                   Owner:  diatchki
            Type:  bug               |                  Status:  new
        Priority:  highest           |               Milestone:  7.12.1
       Component:  Compiler (Type    |                 Version:  7.11
  checker)                           |                Keywords:
      Resolution:                    |            Architecture:
Operating System:  Unknown/Multiple  |  Unknown/Multiple
 Type of failure:  GHC rejects       |               Test Case:
  valid program                      |                Blocking:
      Blocked By:                    |  Differential Revisions:
 Related Tickets:                    |
-------------------------------------+-------------------------------------

Comment (by heisenbug):

 I changed the test case thus:

 {{{
 diff --git a/testsuite/tests/typecheck/should_compile/T10348.hs
 b/testsuite/tests/typecheck/should_compile/T10348.hs
 index 213079b..14fc085 100644
 --- a/testsuite/tests/typecheck/should_compile/T10348.hs
 +++ b/testsuite/tests/typecheck/should_compile/T10348.hs
 @@ -15,8 +15,7 @@ data T t where

  deriving instance Show (T n)

 -hey :: (Typeable n, KnownNat n) => T (Foo n)
 --- SHOULD BE: hey :: KnownNat n => T (Foo n)
 +hey :: KnownNat n => T (Foo n)
  hey = T Hey

  ho :: T (Foo 42)
 }}}

 Then this code (I just added a panic for visibility) is triggered:


 {{{
 diff --git a/compiler/typecheck/TcInteract.hs
 b/compiler/typecheck/TcInteract.hs
 index 01b0ba1..bf5f237 100644
 --- a/compiler/typecheck/TcInteract.hs
 +++ b/compiler/typecheck/TcInteract.hs
 @@ -48,6 +48,7 @@ import Pair (Pair(..))
  import Unique( hasKey )
  import DynFlags
  import Util
 +import TypeRep( Type(..) )

  {-
  **********************************************************************
 @@ -1852,6 +1853,8 @@ matchTypeableClass clas _k t
    | Just (f,kt)       <- splitAppTy_maybe t    = doTyApp f kt
    | Just _            <- isNumLitTy t          = mkSimpEv
 (EvTypeableTyLit t)
    | Just _            <- isStrLitTy t          = mkSimpEv
 (EvTypeableTyLit t)
 +  | TyConApp con []   <- _k
 +  , Just tcon         <- isPromotedTyCon_maybe con = pprPanic
 "matchTypeableClass" $ ppr tcon
    | otherwise                                  = return NoInstance

    where
 }}}

 I guess code must be generated that creates the Typeable hash on the fly
 (at runtime) by appealing to the KnownNat's method.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10348#comment:5>
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