GHC panic when using DeriveAnyClass with functor-like class and datatype with a type variable in a contravariant position
The following code
{-# LANGUAGE DeriveAnyClass #-}
{-# LANGUAGE KindSignatures #-}
module Example where
class C (a :: * -> *)
data T a = MkT (a -> Int) deriving C
fails to compile with the following GHC panic:
$ /opt/ghc/8.0.1/bin/ghc Example.hs
[1 of 1] Compiling Example ( Example.hs, Example.o )
ghc: panic! (the 'impossible' happened)
(GHC version 8.0.1 for x86_64-unknown-linux):
contravariant
Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
The culprit is that when DeriveAnyClass
comes up with the constraints for typeclass instances whose argument is of kind * -> *
, it calls deepSubtypesContaining, which immediately bails out when it sees a type variable in a contravariant or "bad" position. I'm less sure of what the fix should be.
Trac metadata
Trac field | Value |
---|---|
Version | 8.0.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
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Blocking | |
CC | |
Operating system | |
Architecture |