Type inference regression between GHC 8.0.2 and 8.2.2

I observed this when debugging a test case from the HList library that works in GHC 8.0.2, but not in GHC 8.2.2 or later. Consider the following minimized example:

{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE TypeFamilies #-}
module Bug where

import Data.Coerce
import Data.Proxy

type family TagR a

class TypeIndexed r tr | r -> tr, tr -> r where
    typeIndexed ::
       (Coercible (TagR a) s, Functor f) =>
         Proxy a
      -> (tr (TagR a) -> f (tr (TagR a))) -> r s -> f (r s)

typeIndexed' pa x = typeIndexed pa x

In GHC 8.0.2, the type of typeIndexed' is correctly inferred as:

$ /opt/ghc/8.0.2/bin/ghci Bug.hs
GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )
Ok, modules loaded: Bug.
λ> :t typeIndexed'
typeIndexed'
  :: (Coercible s (TagR a), TypeIndexed r tr, Functor f) =>
     Proxy a -> (tr (TagR a) -> f (tr (TagR a))) -> r s -> f (r s)

In GHC 8.2.2 and later, however, the inferred type is less general:

$ /opt/ghc/8.4.2/bin/ghci Bug.hs
GHCi, version 8.4.2: 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.
λ> :t typeIndexed'
typeIndexed'
  :: (TypeIndexed r tr, Functor f) =>
     Proxy a
     -> (tr (TagR a) -> f (tr (TagR a))) -> r (TagR a) -> f (r (TagR a))

Notice how the Coercible s (TagR a) constraint is no longer inferred. Instead, it seems that GHC is inferring the less general constraint s ~ TagR a, since s has been substituted for TagR a in the type r (TagR a) -> f (r (TagR a)) (whereas in 8.0.2, it was r s -> f (r s)).

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture