[GHC] #14207: Levity polymorphic GADT requires extra type signatures

9 Sep 2017

      #14207: Levity polymorphic GADT requires extra type signatures
-------------------------------------+-------------------------------------
           Reporter:  andrewthad     |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  low            |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           Keywords:                 |  Operating System:  Unknown/Multiple
  LevityPolymorphism                 |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 This does not compile:

 {{{#!hs
 data Foo s (a :: TYPE k) where
   FooC :: Foo s Int#
 }}}

 However, this does:

 {{{#!hs
 data Foo (s :: Type) (a :: TYPE k) where
   FooC :: Foo s Int#
 }}}

 This also works:

 {{{#!hs
 data Foo (s :: j) (a :: TYPE k) where
   FooC :: Foo s Int#
 }}}

 These are all of the language pragmas I have enabled:

 {{{#!hs
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE BangPatterns #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE UnboxedSums #-}
 {-# LANGUAGE UnboxedTuples #-}
 {-# LANGUAGE MagicHash #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE UndecidableInstances #-}
 }}}

 This is pretty easy to work around, but I think it's an incorrect
 behavior.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14207
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