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{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeInType #-}

import Data.Kind
import Data.Proxy

type family TrivialFamily t :: Type
type instance TrivialFamily (t :: Type) = Bool

data R where
    R :: Proxy Bool -> R

type ProblemType t = 'R ('Proxy :: Proxy (TrivialFamily t))

Compiling this program as-is, GHC rejects it!

error:
    • Expected kind ‘Proxy Bool’,
        but ‘'Proxy’ has kind ‘Proxy (TrivialFamily t)’
    • In the first argument of ‘R’, namely
        ‘(Proxy :: Proxy (TrivialFamily t))’
      In the type ‘R (Proxy :: Proxy (TrivialFamily t))’
      In the type declaration for ‘ProblemType’

But if we move TrivialFamily to another module and import it, GHC discovers that TrivialFamily t = Bool and the program is accepted.

When compiling the rejected program (with the local family instance) I observe that the instance environments given by FamInst.tcGetFamInstEnvs contain no instances! The renamer processes the local instance, but no FamInst is created for it, and nothing enters the TcGblEnv's family instance record.

Trac metadata

Trac field	Value
Version	8.0.1-rc1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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