Data family instances aren't eta-reduced correctly
(I originally discovered this when debugging #17296 (closed)/!1877 (closed), but this doesn't rely on !1877 (closed) to reproduce.)
Take this code:
{-# LANGUAGE GADTs #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeFamilies #-}
module Bug where
import Data.Kind
import Language.Haskell.TH hiding (Type)
import System.IO
data family Foo a
data instance Foo :: Type -> Type where
MkFoo :: Foo a
$(do i <- reify ''Foo
runIO $ hPutStrLn stderr $ pprint i
pure [])
And compile it with a devel2
-flavoured compiler. (I'm on commit d0924b15.) You'll get the following panic:
$ ~/Software/ghc3/inplace/bin/ghc-stage2 Bug.hs
[1 of 1] Compiling Bug ( Bug.hs, Bug.o, Bug.dyn_o )
Bug.hs:1:1: error:
Exception when trying to run compile-time code:
ghc-stage2: panic! (the 'impossible' happened)
(GHC version 8.9.0.20191003:
zipEqual: unequal lists:zipTyEnv
Please report this as a GHC bug: https://www.haskell.org/ghc/reportabug
Code: do i <- reify ''Foo
runIO $ hPutStrLn stderr $ pprint i
pure []
|
1 | {-# LANGUAGE GADTs #-}
| ^