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This typechecks:

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE TypeOperators #-}

import Data.Kind
import Data.Type.Equality

type family Cast (e :: a :~: b) (x :: a) :: b where
  Cast Refl x = x

However, if you try to make the kinds a and b explicit arguments to Cast:

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE TypeOperators #-}

import Data.Kind
import Data.Type.Equality

type family Cast (a :: Type) (b :: Type) (e :: a :~: b) (x :: a) :: b where
  Cast _ _ Refl x = x

Then GHC gets confused:

GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/ryanglscott/.ghci
[1 of 1] Compiling Main             ( Bug.hs, interpreted )

Bug.hs:9:12: error:
    • Expected kind ‘_ :~: _1’, but ‘'Refl’ has kind ‘_ :~: _’
    • In the third argument of ‘Cast’, namely ‘Refl’
      In the type family declaration for ‘Cast’
  |
9 |   Cast _ _ Refl x = x
  |            ^^^^

A workaround is to explicitly write out what should be inferred by the underscores:

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE TypeOperators #-}

import Data.Kind
import Data.Type.Equality

type family Cast (a :: Type) (b :: Type) (e :: a :~: b) (x :: a) :: b where
  Cast a a Refl x = x

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
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Operating system
Architecture