GHC mistakenly believes datatype with type synonym in its type can't be eta-reduced

I uncovered this when playing around with -XTypeInType:

{-# LANGUAGE DeriveFunctor, TypeInType #-}
module CantEtaReduce1 where

import Data.Kind

type ConstantT a b = a
newtype T (f :: * -> *) (a :: ConstantT * f) = T (f a) deriving Functor

This fails because it thinks that you can't reduce the last type variable of T, since it mentions another type variable (f):

    • Cannot eta-reduce to an instance of form
        instance (...) => Functor (T f)
    • In the newtype declaration for ‘T

But it should be able to, since ConstantT * f reduces to *, and the equivalent declaration newtype T (f :: * -> *) (a :: *) = ... eta-reduces just fine.

I marked this as appearing in GHC 8.1 since you need -XTypeInType to have kind synonyms, but this can also happen in earlier GHC versions with data families:

{-# LANGUAGE DeriveFunctor, PolyKinds, TypeFamilies #-}
module CantEtaReduce2 where

type ConstantT a b = a
data family T (f :: * -> *) (a :: *)
newtype instance T f (ConstantT a f) = T (f a) deriving Functor

I believe the fix will be to apply coreView with precision to parts of the code which typecheck deriving statements so that these type synonyms are peeled off.

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	goldfire
Operating system
Architecture