[GHC] #11237: Type synonyms are not expanded in the data type declaration return kind

[GHC]

#11237: Type synonyms are not expanded in the data type declaration return kind
-------------------------------------+-------------------------------------
           Reporter:  thomasw        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.11
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I was playing around with the cool new `-XTypeInType` stuff when I
 encountered the following issue:

 {{{#!hs
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE GADTs #-}
 module TypeInTypeBug where

 import qualified Data.Kind


 -- This works, using Data.Kind.Type as return kind
 --------------vvvvvvvvvvvvvv
 data Works :: Data.Kind.Type where
   WorksConstr :: Works

 type Set = Data.Kind.Type

 -- This doesn't work, using a type synonym for Data.Kind.Type as return
 kind
 ---------------vvv
 data Doesnt :: Set where
   DoesntConstr :: Doesnt
 }}}
 {{{
 TypeInTypeBug.hs:17:1: error:
     • Kind signature on data type declaration has non-* return kind Set
     • In the data declaration for ‘Doesnt’
 }}}
 I suppose type synonyms should be expanded in the return kind of a data
 type declaration before checking it is `*`.

 I also think the error message is not totally correct, take the following
 '''valid''' data declaration:

 {{{#!hs
 data Foo :: Bool -> Data.Kind.Type where
     Tru :: Foo True
     Fal :: Foo False
 }}}

 The return kind of the data declaration is actually `Bool ->
 Data.Kind.Type` (or `Bool -> *`), which is not `*`. I assume the error
 message is talking about the return kind (`*`) of the return kind (`Bool
 -> *`)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11237>
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