Panic when wrongly using a type family as return types for GADTs

The following code causes the following panic:

{-# LANGUAGE TypeFamilies, GADTs #-}

type family F a

data G1 a where
  G1C :: F Int

data G2 a where
  G2C :: F Int

ghc: panic! (the 'impossible' happened)
  (GHC version 7.6.0.20120810 for x86_64-apple-darwin):
	compiler/typecheck/TcTyClsDecls.lhs:1081:5-62: Irrefutable pattern failed for pattern Data.Maybe.Just subst

Admittedly, the code causing the panic is silly, but it happened to me as I was refactoring. Having two separate GADTs with the wrong return types seems necessary to cause the panic.

Trac metadata

Trac field	Value
Version	7.6.1-rc1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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