testsuite/tests/indexed-types/should_fail/GADTwrong1.hs


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{-# LANGUAGE TypeFamilies, GADTs, RankNTypes, ScopedTypeVariables #-}

module ShouldFail where

type family Const a
type instance Const a = ()

data T a where T :: a -> T (Const a)

coerce :: forall a b . a -> b
coerce x = case T x :: T (Const b) of
           T y -> y