GHC 8.4.1-alpha allows the use of kind polymorphism without PolyKinds
This program:
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE ImpredicativeTypes #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}
{-# OPTIONS_GHC -ddump-deriv #-}
module Bug where
import Data.Coerce
import Data.Proxy
class C a b where
c :: Proxy (x :: a) -> b
newtype I a = MkI a
instance C x a => C x (Bug.I a) where
c = coerce @(forall (z :: x). Proxy z -> a)
@(forall (z :: x). Proxy z -> I a)
c
is rightfully rejected by GHC 8.2.2:
$ /opt/ghc/8.2.2/bin/ghc Bug.hs
[1 of 1] Compiling Bug ( Bug.hs, Bug.o )
Bug.hs:20:16: error:
Unexpected kind variables Perhaps you intended to use PolyKinds
In a type argument
|
20 | c = coerce @(forall (z :: x). Proxy z -> a)
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Bug.hs:21:16: error:
Unexpected kind variables Perhaps you intended to use PolyKinds
In a type argument
|
21 | @(forall (z :: x). Proxy z -> I a)
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
But GHC 8.4.1-alpha2 actually //accepts// this program!
$ /opt/ghc/8.4.1/bin/ghci Bug.hs
GHCi, version 8.4.0.20180118: http://www.haskell.org/ghc/ :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug ( Bug.hs, interpreted )
Ok, one module loaded.
This is almost certainly bogus.
Trac metadata
Trac field | Value |
---|---|
Version | 8.4.1-alpha1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | high |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |