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  • Glasgow Haskell CompilerGlasgow Haskell Compiler
  • GHCGHC
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  • #14209
Closed
Open
Issue created Sep 09, 2017 by Ryan Scott@RyanGlScottMaintainer

GHC 8.2.1 regression involving telescoping kind signature

The following program typechecks in GHC 8.0.1 and 8.0.2:

{-# LANGUAGE GADTs #-}
{-# LANGUAGE TypeInType #-}
module Bug where

data MyProxy k (a :: k) = MyProxy
data Foo (z :: MyProxy k (a :: k))

But in GHC 8.2.1, it's rejected:

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

Bug.hs:6:1: error:
    Kind variable ‘k’ is implicitly bound in datatype
    ‘Foo’, but does not appear as the kind of any
    of its type variables. Perhaps you meant
    to bind it explicitly somewhere?
    Type variables with inferred kinds:
      (k :: *) (a :: k) (z :: MyProxy k a)
  |
6 | data Foo (z :: MyProxy k (a :: k))
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Trac metadata
Trac field Value
Version 8.2.1
Type Bug
TypeOfFailure OtherFailure
Priority high
Resolution Unresolved
Component Compiler (Type checker)
Test case
Differential revisions
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