DeriveAnyClass regression involving a rigid type variable
I was writing up an example to show off how DeriveAnyClass
has improved since 639e702b, and wouldn't you know it, the example doesn't actually compile anymore post-639e702b. Oopsie.
{-# LANGUAGE DefaultSignatures #-}
{-# LANGUAGE DeriveAnyClass #-}
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
module TypeName where
import GHC.Generics
class TypeName a where
typeName :: forall proxy.
proxy a -> String
default typeName :: forall proxy d f.
(Generic a, Rep a ~ D1 d f, Datatype d)
=> proxy a -> String
typeName _ = gtypeName $ from (undefined :: a)
gtypeName :: Datatype d => D1 d f p -> String
gtypeName = datatypeName
data T a = MkT a
deriving (Generic, TypeName)
This compiles before that commit. After it, however, it fails with the error:
[1 of 1] Compiling TypeName ( Bug.hs, interpreted )
Bug.hs:23:22: error:
• Couldn't match type ‘f’
with ‘C1
('MetaCons "MkT" 'PrefixI 'False)
(S1
('MetaSel
'Nothing 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy)
(Rec0 a))’
arising from the 'deriving' clause of a data type declaration
‘f’ is a rigid type variable bound by
the deriving clause for ‘TypeName (T a)’ at Bug.hs:14:38
• When deriving the instance for (TypeName (T a))
|
23 | deriving (Generic, TypeName)
| ^^^^^^^^
I'm not sure why it's complaining only about f
and not, say, d
.