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Due to #12088, you can't define a data family instance and promote it in the same module. One could, up until GHC 8.2, work around this using (somewhat obscure) wisdom: define the data family instance in a separate module from where it's promoted. For example, Bug typechecks in GHC 8.0.1 and 8.0.2:

{-# LANGUAGE TypeFamilies #-}
module Foo where

data family T a
data instance T Int = MkT

{-# LANGUAGE TypeInType #-}
module Bug where

import Foo

data Proxy (a :: k)
data S = MkS (Proxy 'MkT)

However, this stopped typechecking in GHC 8.2:

$ /opt/ghc/8.2.1/bin/ghci Bug.hs
GHCi, version 8.2.0.20170623: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 2] Compiling Foo              ( Foo.hs, interpreted )
[2 of 2] Compiling Bug              ( Bug.hs, interpreted )

Bug.hs:1:1: error:
    Can't find interface-file declaration for variable Foo.$tc'MkT
      Probable cause: bug in .hi-boot file, or inconsistent .hi file
      Use -ddump-if-trace to get an idea of which file caused the error
  |
1 | {-# LANGUAGE TypeInType #-}
  | ^

Trac metadata

Trac field	Value
Version	8.2.1-rc2
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler
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