Panic on a context with additional variable

File:

{-# LANGUAGE GADTs #-}
data X a where
  N :: (a ~ b) => X a

k :: X a -> X b
k N = N

Result with HEAD:

X.hs:6:7:
    Couldn't match type ‛b’ with ‛b1’
      ‛b’ is untouchable
        inside the constraints (a ~ b)
        bound by a pattern with constructor
                   N :: forall a. a ~ b => X a,
                 in an equation for ‛k’
        at X.hs:6:3ghc: panic! (the 'impossible' happened)
  (GHC version 7.7.20130608 for x86_64-unknown-linux):
	No skolem info: b{tv ae1} [sk]

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

I got it by attempting to compile data-category with HEAD.

Trac metadata

Trac field	Value
Version	7.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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